Introduction to Ergonomics Instructor's Manual

Introduction to Ergonomics

Instructor’s Manual

Essential Reading Ergonomics for Beginners 2nd editionJan Dul, Erasmus University, The Netherlands and B A Weerdmeester, TNOInstitute, The Netherlands Pbk 0–7484–0825–8 Taylor & Francis A Guide to Methodology in Ergonomics: Designing for Human UseNeville Stanton and Mark Young, Brunel University, UK Taylor & Francis Pbk 0– 7484–0703–0 Fitting the Task to the Human 5th editionK Kroemer and E GrandjeanTaylor & Francis Hbk 0–7484–0664–6; Pbk 0–7484–0665–4 Evaluation of Human Work 2nd editionJohn Wilson and Nigel Corlett, The University of Nottingham, UK Taylor & Francis Hbk 0–7484–0083–4; Pbk 0–7484–0084–2 Information and ordering details For price availability and ordering visit our www.ergonomicsarena.com Alternatively our books are available from all good bookshops.

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Introduction to Ergonomics Instructor’s Manual

R.S.Bridger

LONDON AND NEW YORK

First published 2003 by Taylor & Francis 11 New Fetter Lane, London EC4P 4EE Simultaneously published in the USA and Canada by Taylor & Francis Inc, 29 West 35th Street, New York, NY 10001 Taylor & Francis is an imprint of the Taylor & Francis Group This edition published in the Taylor & Francis e-Library, 2005. “To purchase your own copy of this or any of Taylor & Francis or Routledge’s collection of thousands of eBooks please go to www.eBookstore.tandf.co.uk.” © 2003 R.S.Bridger This book has been produced as camera-ready copy from text and figures supplied by the author. All rights reserved. No part of this book may be reprinted or reproduced or utilised in any form or by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying and recording, or in any information storage or retrieval system, without permission in writing from the publishers. Every effort has been made to ensure that the advice and information in this book is true and accurate at the time of going to press. However, neither the publisher nor the authors can accept any legal responsibility or liability for any errors or omissions that may be made. In the case of drug administration, any medical procedure or the use of technical equipment mentioned within this book, you are strongly advised to consult the manufacturer’s guidelines. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging in Publication Data A catalogue record has been requested ISBN 0-203-50491-7 Master e-book ISBN

ISBN 0-203-57324-2 (Adobe eReader Format) ISBN 0-415-31266-3 (Print Edition)

Contents

Introduction

vi

Chapter 1

1

Chapter 2

14

Chapter 3

24

Chapter 4

38

Chapter 5

47

Chapter 6

56

Chapter 7

63

Chapter 8

68

Chapter 9

73

Chapter 10

80

Chapter 11

87

Chapter 12

94

Chapter 13

106

Chapter 14

116

Chapter 15

125

Chapter 16

130

Introduction

This manual has been written to assist less-experienced lecturers wishing to use the book ‘Introduction to Ergonomics’ as their main course text for an introductory program in ergonomics. Instructors with more experience will be quite able to modify the material to suit their own educational objectives and instructional focus. Each chapter in the manual contains: 1. A list of objectives for the material. The list describes the main concepts students should learn from lectures and study and the rudimentary skills they should acquire from the exercises and demonstrations. 2. Commentaries on the chapter material to assist the instructor with the preparation and presentation of lectures. 3. Worked examples of problems, where appropriate 4. Demonstrations. The instructor may wish to hold laboratory sessions in which these can be shown to the students. 5. Comments on the essays and exercises which will assist the instructor in evaluating the students’ work. The exercises can also be carried out under the instructor’s supervision and the findings discussed in tutorials. In keeping with the philosophy of the book, lecturers should place equal emphasis on the teaching of each chapter in order to provide students with a balanced introduction to ergonomics. It is suggested that a minimum of two lectures be allocated to each chapter. Together with exercises, demonstrations and project work, this will result in a course of 45 sessions, although at least 60 hours would be preferable. The book and manual have a clear educational philosophy concerning the teaching of ergonomics which states that students learn the fundamental principles and basic applications of all areas of ergonomics before specialising. Students come to the study of ergonomics by a variety of routes and bring with them different skills, knowledge and experiences. It is only too easy for the student to specialise in a familiar area prematurely and never really grasp the breadth of the subject and the contribution of less familiar areas. This approach to education in ergonomics has much in common with the training of engineers and medical doctors. Most practising engineers and medical doctors are specialists and only use a part of the basic engineering or medical

vii

knowledge acquired during training. However, despite consisting of distinct groups of specialists, the disciplines of engineering and medicine are intellectually coherent because all practitioners, irrespective of their own specialist interests, retain a common, general knowledge of their subject which far exceeds that possessed by the layman or by specialists in other fields. If ergonomics is to remain a coherent discipline in the face of diverging research and market demands, the training of students must be broadly based and their general knowledge of the subject acquired at an early stage. This is particularly important given the recent debate in the US and the moves in the EC to develop some form of professional certification for ergonomists. The modern trend for ergonomists to act as self-employed consultants rather than working exclusively for one organisation in a clearly defined role means that the practitioner’s expertise must not be limited to a particular set of high-level skills. It must include the ability to recognise or “diagnose” a wide range of ergonomic problems and refer the client to an appropriate specialist and perhaps suggest an interim solution. It is hoped that a non-partisan approach to introductory ergonomics will engender future flexibility amongst its practitioners. Chapter Contents Each chapter of the manual contains a commentary on the corresponding book chapter to assist the teacher with the preparation of lectures, tutorials and demonstrations. The objectives of each book chapter are described together with the key concepts students must master before moving on to more advanced literature. In addition to the commentary are suggestions for demonstrations, laboratory work and homework to get the students working with the concepts in a more practical way. Each manual chapter also contains short reading lists containing supplementary material which should be used by students in their written course exercises and/or as study material for an end of course exam. Use of `Introduction to Ergonomics' for Advanced Study or in Specialised Curricula Introduction to Ergonomics’ was written primarily with the needs of undergraduate students embarking on their first course in mind. It is not intended to replace more advanced or specialised books—a list of these is contained in the bibliography. It is assumed that those presenting advanced or specialised courses will have developed their own teaching materials and be actively involved in the research area they are teaching. Individual chapters may be of use to teachers of advanced or specialised courses as introductory material at the beginning of the course. In addition, other parts of the book may, if suitably summarised, assist the teacher in providing an overview of the field of ergonomics by bringing together a wide range of material

viii

in a convenient form. In this sense, Introduction to Ergonomics’ may also have a place on the reading list as a companion text to more advanced and specialised volumes.

Chapter 1

After completing this chapter the student should understand:

1. The origins and present scope of ergonomics 2. The systems approach to ergonomics and the concept of a worksystem 3. The types of knowledge, fundamental and task-related, which are needed in design. 4. The relationships between ergonomics and other worksystem disciplines. 5. The multidisciplinary nature of ergonomics. The student must be able to:

1. Identify the main components of a worksystem and describe the first order interactions between the components. 2. Carry out simple task analyses of common activities using the terminology given in the chapter. 1. COMMENTARY The main goal of chapter 1 is to prepare students for the diversity of ergonomics by providing a conceptual framework for the subject and describing its historical development. A general discussion of the modern ergonomist’s role and responsibilities is also included. 1.1 Conceptual Framework The conceptual framework which has been chosen for the book consists of the work systems concept and Leamon’s human-machine model. These have the following advantages: 1. They are context-independent models appropriate to an introductory course in ergonomics. They enable ergonomics to be approached from its own foundations rather than as an offshoot of another discipline. Students may have pre-

2 INTRODUCTION TO ERGONOMICS

conceived ideas about the subject which must be removed at an early stage because they limit the perceived range of applications and can cause confusion later on. For example, students who believe ergonomics to be an offshoot of vehicle design or physical therapy may question the relevance of learning about human-computer interaction. 2. The models encourage a systems approach to the conceptualisation of ergonomics problems but avoid some of the more abstract and esoteric ideas of Systems Theory itself. 3. They provide a common starting point for the discussion of ergonomics problems. Students with different backgrounds will more easily be able to communicate with each other. 4. Ergonomics draws on the theories and findings of different scientific and professional domains. The models assist in integrating this patchwork of human, natural and social science, showing where the different domains stand in relation to one another and how they are related in the operation of real systems. An iterative approach is taken in the description of the models. First, the work systems framework is described at the level of analysis of H (human), M (machine) and E (environment). Some basic interactions between these components are described and the notion of directionality is introduced (i.e. interactions take place in a particular direction, human to machine or machine to human etc. No reference to particular human components or machines is made at this point as the attempt is to develop a context free notion of the domain of application of ergonomics. The concept of higher order goals and synthesis is also introduced at this stage. It is emphasised that work systems are purposeful systems and have higher level goals which must be analysed in terms of inputs and outputs. The system components interact to carry out the functions needed to fulfil the purpose of the system. It is emphasised that although ergonomics is focused at the level of the interactions between the human and other components, the purpose of its application is to improve the functioning of the system. Secondly, Leamon’s human-machine model is described. This is the next step lower in the analysis stage. The H, M and E components are broken down to the basic level they are usually discussed in ergonomics. These are briefly discussed to serve as an introduction to the more technical chapters which follow. 1.2 History of Ergonomics The history of ergonomics is discussed in some detail. It is emphasised that ergonomics owes it development to industrialisation. The historical processes which gave rise to ergonomics are the same as those which gave rise to many other work system disciplines. It should be emphasised that there is nothing particularly “special” about ergonomics—practitioners work with other

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specialists to design systems which can be cost-effectively and reliably operated by humans. It is emphasised that labour is a major cost in many organisations and therefore people’s need to work efficiently and safely must be met. Although labour cost as a percentage of total costs has decreased in the “blue collar” sector (as evidenced by a steady increase in labour productivity in manufacturing) the same improvements have not occurred in the “white collar” or service sector. This explains why there is so much attention being paid nowadays to health in the automated office. 1.3 Modern Ergonomics An attempt is made to characterise the role(s) playable by the ergonomist in modern work system design. The emphasis is on not constraining the set of contributions that an ergonomist might make in an organisation. Students may have pre-conceived ideas about what ergonomists actually do and these will undoubtedly depend on their previous educational experiences and on the course of study in which the ergonomics program is embedded. By sketching out, in general terms, the different types of contributions an “ideal” ergonomist might make in an organisation, it is intended to avoid the tendency for engineers to propose hardware solutions to problems and for psychologists and physiologists to propose psychological and physiological solutions to the same problems. 1. Standard format. This is the starting point for the analysis of ergonomic problems. Students need to practice identifying and describing the various elements of example work systems. This can be done interactively during lectures at first The lecturer might have slides made of various work systems and lead the students through the application of the standard format. Interactions between the various elements can be identified and tabulated as in the book chapter. This can later be extended to field work as students develop expertise in describing systems according to the standard format. The format can be used to help students write reports of site visits, essays and field work. 2. Identification/Classification of Issues. This section is concerned basically with the interpretation of field data. Students must not only learn to identify ergonomic design problems, badly designed jobs etc. but must be able to classify them in an appropriate way before embarking on a course of intervention. Interpretation is particularly important when dealing with subjective data such as is obtained from questionnaires, self-reports and interviews. Personal factors such as dissatisfaction with the job or supervisors can manifest themselves in other ways such as health complaints, dissatisfaction with equipment etc. 3. Task and Human Machine Interaction Analysis. In order to design an interface to support optimal task performance, the ergonomist must first understand the task and the needs of the users. Task analysis is a method for

4 INTRODUCTION TO ERGONOMICS

doing this. There are many different approaches to task analysis and the one used here is recommended as being appropriate for an introductory course The hierarchical approach us useful as it enables a job to be broken down into key tasks which can then be analysed in more detail using the flow diagram approach. In this approach, a task is decomposed into a sequence of operations and decisions. Each operation is represented as a box and each decision as a diamond as follows:

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1.3.1 Example Hierarchical Task Analysis for Lectures: Cheese Sandwich Task Analysis

6 INTRODUCTION TO ERGONOMICS

Basic Analysis: Make Sandwich

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Detailed Analysis of Operations: Make Sandwich 1. Take bread and place on clean breadboard with top crust towards you 2. Check bread is suitable 3. Hold handle of bread knife in preferred hand and keep cutting edge facing downwards. 4. With the other hand, hold the knife firmly in the middle of the loaf to avoid cutting the hand.

8 INTRODUCTION TO ERGONOMICS

5. Place the cutting edge of the knife firmly on top of the loaf and with even back and forth movements accompanied by pressure on the knife, cut off about 5mm of crust. 6. After crust has been removed, place cutting edge of knife 10mm back from the edge where the crust was removed. Repeat back and forth movements of the knife until you have cut right through the loaf. 7. Repeat. 8. Place the loaf back in its storage area and wipe breadboard clean of any crumbs. 9. Lay the two 10mm thick slices side by side, flat on the breadboard. 10. Take butter knife and 8gm portion of butter. Remove covering from the butter and place the butter on one of the slices of bread. Discard the cover in the bin provided 11. Cut the butter portion in half using the butter knife. Use the butter knife to place a half portion of butter on the other slice of bread. 12. Use the flat side of the butter knife to spread the butter equally thick on both slices of bread. 13. Use the cheese grater to grate 25 gm of cheese into the cheese dish. Hold the flat side of the cheese block against the roughened surface of the cheese grater and move the cheese up and down maintaining slight pressure at the cheese-cheese grater interface. 14. Scatter the 25 gm portion of grated cheese evenly on one of the slices of bread. 15. After one slice has been evenly covered with cheese, take the other slice of bread and place it on the scattered cheese with buttered side facing downwards. 16. Press the two slices lightly together with one hand. 17. With the other hand, take the bread knife and cut diagonally from one corner to another. Repeat the action at the other corner to produce four triangular slices of sandwich. Be careful not to cut your fingers! 18. Place the sandwich on a plate ready for serving. Clean the work area. 1.3.2 Supplementary Information In addition to the flow diagram, supplementary information is required which specifies how the operations carried out by the human are mapped onto the machine —in other words, how the operator and the machine interact with each other in terms of information flow (control actions and system feedback). So for every operation, the analyst must consider: Indications—When to do the task. How does the operator know when to do the task? Often this information comes from a source external to the particular subsystem, e.g. from superiors, customers, or as the output of another system. All sources of indication should be considered. They can be characterised in

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several ways, for example according to the sensory modality (verbal, visual etc.) physical location of the source with respect to the operator, skill or knowledge required to identify the indication etc. Control Object and Operation—This specifies the particular controls or instruments and how they are used to carry out the operations (e.g. push red lever forwards to position one). Precautions—Specifies any precautions which have to be made before executing an operation (e.g. check water level in kettle covers heater element before pressing ON button). Feedback Modality and Indication of Response Adequacy—Describes the feedback that the operator should receive from the system once the operation has been carried out (often forms the content of the decision box). This feedback often comes directly from the system itself (e.g. vehicle on course) or from displays on the control panel (e.g. course heading confirmed). Fault Diagnosis and Maintenance—Describes the actions that the operator should take to recognise and deal with faults at the level of each operation. The values of task analysis is as follows: because much of modern ergonomics consists of design guidelines derived from laboratory research, we cannot be sure that it will lead to better system performance when implemented. Unlike in mechanical engineering, where there are mathematical models of the behaviour of structures which enable the designer to predict how the completed design will behave, the research findings of ergonomics are not certain. They are usually associated with probability values from the statistical procedures used by the researchers. Task analysis often provides the designer with the only really firm ground on which to stand before making any recommendations. It provides a description of the sequence of operations required to carry out the task, the controls and how they are used and the feedback provided by the system. Task analyses can also be used to prepare training manuals, decide on skill requirements and personnel selection as well as interface and equipment design. 4. Specification of System Design and Human Behaviour. Specifications, standards and design guidelines are available from a variety of sources. Departments offering ergonomics training might establish a library of design data. Handbooks such as Woodsman’s can be included as well as the NIOSH work practices guide and other data from sources such as OHASA, ISO etc. The book by Pheasant (1986) is a useful compilation of anthropometric data. 5. Identification and Analysis of Core Trends. A useful teaching exercise is to examine some of the core trends in ergonomics over the last 40 years and discuss where the subject is going in the long term, for example: 1940’s—Military Ergonomics 1950’s—Primary and Secondary Manufacturing Industry 1960’s—Process Industries in Europe, US Space Program 1970’s—Complex Systems, Safety 1980’s—Human-Computer Interaction, Occupational Health, Technological Change

10 INTRODUCTION TO ERGONOMICS

1990’s—Integrated Information Processing and Communication, Intelligent Systems, Cost Benefits of Ergonomics, Developing Countries ? Generation and Implementation of New Concepts. This arises naturally out of the task analysis and specification activities. There are several levels of detail at which design can take place. It is not normally the function of the ergonomist to do detailed design of the actual system since this is usually the function of the engineer or system designer. Ergonomists often make recommendations which are then implemented by designers. Recommendations must be specific (they must based on a specific problems or design faults which has been identified in the analysis phase) and they must also be implementable (in terms of cost, practicality etc.). Some examples of recommendations which are specific and implementable are: —Increase worksurface heights in the fruit inspection area by 50 mm —Upgrade the lighting system to increase illumination in the work area by at least 100 lux (use lamps with a colour rendering index of 85 or more). —Reduce the size of the boxes so that a full carton of fruit does not weigh more than 14 kg. After the analysis phases are complete and a detailed set of recommendations has been generated, it may be appropriate to redesign the entire system. Redesign requires that the recommendations be implemented in an integrated way taking into account the current state of available hardware and software, the task requirements (including production targets) and the potential of alternative ways of doing the job to support the task operations. A taxonomy for prioritising the implementation of new ideas is presented in the text. Evaluation of Sociotechnical Implications. One of the cornerstones of the ergonomic approach to system design is to consult the users of a system at all stages of the design process and to know their needs. In addition to the basic needs for efficient operation, there are also psychosocial needs which have to do with preferred or customary ways of working and of interacting with co-workers. In most organisations, informal procedures and practices evolve in parallel with the formal procedures laid down by management. User-centred design takes account of these informal aspects of work organisation as well as the formal aspects. Participatory ergonomics is a popular term used nowadays. It consists of consulting the users early on in the design process and involving them in design decisions to ensure that designs which should work in principle, really do work in practice. By taking into account the day to day realities of system operation, the finished product should not only work well but also be acceptable to its users.

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2. ESSAYS AND EXERCISES 1. In a sense, ergonomics has always taken place anyway, as users of equipment made things appropriate for themselves. The key point to note is that with the increasing distance between designers and users and the demands of mass production, ergonomics, like so many other disciplines has become formalised and gone from being an inuititive “craft” skill to an explicit profession. 2. One of the most important skills a new ergonomist needs is the ability to find answers to design problems. Whenever a particulat problem crops up, it always seems to be the one that is not covered in any of the main textbooks. Now that most people have access to the web, it can be recommended as almost a standard search tool for advice and information. Visit the websites of the ISO, ANSI, lighting companies etc. Experiment with different keywords to drive your search. This exercise should help students to relate the contents of the textbook to international standards and guidelines and thus make links between scientific research findings and engineering solutions to design problems. 3. This exercise is a practical one, so that students get an early appreciation of how ergonomics workds as a profession and as an application. Different manufacturers will approach things differently. White goods manufacturers often subsume ergonomics under industrial design and emply designers to do the work. Computer manufacturers may have ergonomics or human factors departments or even their own usability laboratories. Car manufactuters are very conscious of ergonomics. Many of these companies do not have such an explicit commitment to the subject in the design and management of their production facilities, however. 4. Present Scope of Ergonomics The main purpose of this exercise is to get students to work out some kind of classification scheme for ergonomics using recent literature as a source of information. There are many possible ways of doing this but they all involve a “bottom-up” approach in which a model of present day ergonomics is derived from a review of published literature. Students should survey this literature and summarise the research concerns (not the detailed findings) of all published papers in at least two journals. The papers can then be classified in a variety of ways. Some sample classification schemes are given below. The classification scheme can be further elaborated by subdividing each cell in the matrix according to whether the research is mainly concerned with productivity or performance improvements, reliability, safety, accident prevention or health preservation. An alternative approach is to represent the contents of modern ergonomics using a hierarchical approach:

12 INTRODUCTION TO ERGONOMICS

Research Methods can be broken down into, Experimental Methods versus Field Work. Experimental Methods can be further subdivided into categories such as Laboratory Experiments, Simulation and Prototyping. Laboratory Experiments can be further subdivided in terms of the type of dependent and independent variables being investigated. Dependent variables may be performance, health or comfort related etc. Field Work can be broken down into categories such as prospective or retrospective studies, questionnaire survey or naturalistic observation and so on. Application areas may be the same as in the previous classification scheme. Principles can be decomposed into principles for improving performance or

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for safeguarding health. Performance improvement can be further subdivided into principles for display design, control design and task design and so on. A useful extension of this exercise is to develop a menu-driven system for accessing information about ergonomics. The first line of the above example would be the main menu of the system. 3. FURTHER READING The Encyclopaedia of Ergonomics and Human Factors is a very good place to start. It provides very readable summaries of the state of play in most areas of the subject.

Chapter 2

On completing this chapter, the student should understand:

1. The definition of posture and the use of the “tent analogy” used to describe different postures. 2. The difference between postural load and task-load. 3. The main components of the spine and pelvis 4. The role of muscle in the maintenance of posture and in work activities The student should be able to:

1. Recognise biomechanically demanding jobs/tasks. 2. Apply the concepts of mechanics, in a qualitative way, to analyse whole body work postures. 3. Use these analyses to suggest possible ergonomic risk factors. 4. Suggest simple ergonomic improvements to hazardous tasks. 5. Apply a simple biomechanical model to calculate spinal compression forces and compare the forces with the NIOSH limits or with SCTLs. 1. COMMENTARY This chapter attempts, at a very basic level, to illustrate fundamental concepts of posture and body mechanics together with related aspects of musculoskeletal functioning. It is regarded as important to get students thinking about the human body as a mechanical system at an early stage. An intuitive approach has been deliberately taken as opposed to beginning the discussion with more formal biomechanical models. It is felt that the intuitive approach is sufficient for an introductory course and to provide students with the basic ergonomic analysis skills needed to carry out simple interventions which will improve working conditions.

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1.1 Stability and Support This section aims to describe the requirements for maintaining an upright posture — something that most of us take for granted. It should be emphasised that posture depends on the interplay of several variables, tonic muscle activity maintains joint position which keeps the long bones and spine in relation to each other such that most of the load is transmitted to the floor along the axes of the bones and moments about the joints are minimised. In the erect standing position most of the muscles are relaxed apart from tonic activity, the main exceptions being the calf (gastrocnemius and soleus) muscles and the neck extensors. This can be demonstrated quite clearly in the electromyographic demonstration (see below). The tent analogy is often useful in conveying the basic concept of antagonistic muscle pairs (e.g. if you sway in one direction from a standing position the muscles in the direction of sway relax and shorten and the muscles behind the direction of sway lengthen and tighten, rather like the guy ropes of a tent pole on the wind). In this section, it is also important to emphasise the importance of foot position in determining the stability of the body. In many industrial situations, foot position analysis is an important item on the evaluation checklist. This is particularly so in evaluating manual handling tasks, evaluating the design of stairs, ramps and walkways and emergency exits and in the construction industry where workers walk on scaffolding, girders etc. Leading naturally from this, is the discussion of the different tissues which can be strained when they are subjected to postural stress. In the present book, a simplified view of stress/strain relationships is taken, similar to that used in engineering. Stress corresponds to the load placed on a part of the body and strain to the way the body part responds to the load. 1.2 Aspects of Muscle Function This section has been included to describe concepts of muscle function which are essential to the understanding of fatigue and discomfort. It is also emphasised that muscles are active tissues whose state depends not only on their intrinsic condition at any time but also on the operation of feedback loops in the central nervous system. This is important in the understanding of concepts such as muscle spasm and muscle tone. A brief discussion of electromyography is also included to describe the nature of the electrical signals which emanate from muscle tissue and the limitations of EMG as an index of assessing muscle tension.

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1.3 Anatomy of the Spine and Pelvis Related to Posture. Ergonomists seem to have a fascination with the design of seats and of workspaces for the seated worker. An clear understanding of the anatomy of the spine and pelvis and the lumbo-pelvic mechanism is absolutely essential in posture analysis and in any non-trivial discussion of the design of seats. Considerable time should be devoted to the teaching of this section so that students understand: —the basic anatomy at a structural level —the functional anatomy (relationship between body position and spinalpelvic posture) —basic dysfunctional aspects relevant to ergonomics (relationship between postural load and loading of the intervertebral discs and/or facet joints, compression of nerve roots and stretching of ligaments etc.). Similar considerations apply to the discussion of the cervical spine as to the lumbar spine but it is acceptable treat these two structures separately (despite their morphological similarity) because of the different causes of lumbar and cervical stress in the workplace. Lumbar loading often arises as a result of bad sitting postures and manual handling whereas cervical spine posture is very susceptible to the visual requirements of tasks. The pelvis is discussed in more detail than is usually the case in mainstream ergonomics texts. From a basic biomechanics perspective, the spine can be thought of as a column which transmits the weight of the upper body to the legs via the pelvis which acts like an arch. The pelvis provides a load splitting function and its main point of weakness is the sacro-iliac joint. This is susceptible to injury and to task-induced stress particularly in forward-flexed standing postures and in sitting where the ischial tuberosities are forced apart. Pain in the very low back and to one side may emanate from the sacro-iliac joint, not the spine. By making students aware of some of the weak points in the skeletal system and how they can be over stressed at work it is intended to make them more able to detect bad postures and workplace design faults in practice. 2. DEMONSTRATION A posture demonstration is included using a simple electromyographic device as a qualitative tool to display muscle activity. The minimum apparatus required for the demonstration is a one, or preferably, two-channel EMG feedback system with audio display. These systems are commercially available at low cost (they are sometimes referred to incorrectly as EMG biofeedback systems). A basic EMG monitor can be fairly easily constructed at low cost (a circuit diagram for such a system can be found at the back of this manual). More sophisticated commercial systems are available which can display several channels of EMG

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simultaneously and these, of course, can be used for the demonstrations described here. It should be noted that these demonstrations are very crude, the EMG monitor is simply being used to indicate the presence of muscle activity, i.e. as a tool to assist students in the learning of anatomical concepts. The amplitude of the EMG auditory and/or visual feedback is interpreted qualitatively (or, at most, in an ordinal way) as an indication of electrical activity in the muscle. Quantitative analyses and more sophisticated forms of signal processing are only suitable for more advanced courses. 2.1 Requirements: • • • • •

Laboratory or other open area for holding demonstrations EMG monitor with audio display (preferably 2 channels) Surface EMG (or ECG) electrodes Subject in swimming costume/gym clothes Chairs, weights, boxes, hand tools 2.2. Demonstration of Standing Posture (tent analogy) 2.2.1. Lower Leg EMG

Electrode Placement: Place electrodes over calf muscles (gastrocnemius) and the ankle flexors (tibialis anterior, located medially to the shin). Ask the subject to plantarflex the foot (i.e. extend the ankle) to shorten the gastrocnemius and disclose its location. It may be necessary to palpitate the body of the muscle to find the position of the two heads of the muscle. Place an electrode over each one. Ask subject to dorsiflex the foot (i.e. flex at the ankle) to locate the body of the tibialis anterior muscle. Place electrodes several centimetres apart over the muscle. Channel 1: Ankle Extensors (mainly Gastrocnemius) Channel 2: Ankle Flexors (mainly tibialis) Subject Position: Subject stands with weight distributed equally between both legs looking directly ahead. Guided by the demonstrator, the subject sways forwards from the ankles slightly. NOTE the increase in EMG activity from the calf muscles and the absence of activity from the tibialis anterior. Subjects then sways back slowly. NOTE, the reduction in calf muscle EMG and the balanced position in which there is minimal EMG from either muscle group (even here, more activity should be found in the calf muscles than tibialis since the former is a true postural muscle). As subject continues to sway backwards, NOTE the drop

18 INTRODUCTION TO ERGONOMICS

in calf muscle EMG and the sudden increase in tibialis muscle EMG when the subject is almost at the point of losing balance and falling backwards. This is a REFLEX contraction which is part of the ankle strategy for maintaining standing balance against mild perturbations and destabilising forces. This demonstrates the basic idea of the “tent analogy” used to describe the maintenance of the stable erect standing position. 2.2.2 Supplementary Demonstration: Effect of Joint Angle on Muscle Force and EMG With the EMG system displaying Channel 1, ask the subject to stand on tip-toe from a relaxed standing position. Note the increase in EMG activity as the plantarflexors contract. Ask the subject to sit down, bend the knee by at least 90 degrees or more to shorten the gastrocnemius (gastrocnemius is a two joint muscle which crosses the knee and ankle joints and is shortened when the knee joint is flexed) and ask the subject to plantarflex against resistance. Demonstrate to the students how much weaker the torque of plantarflexion is when the knee is flexed by resisting it with your hands (impossible when the knee is fully extended —i.e. most people can easily raise their own body weight to stand on tip-toe on one leg). 2.2.3 Supplementary Demonstration: Effect of Postural Load Increase If possible, display both channels simultaneously or repeat the exercise switching between channels, as appropriate. From a comfortable standing position, ask the subject to place one leg on a footrail or footrest. Note the reduction in EMG from both muscle groups in the raised leg and the consequent increase in EMG in the supporting leg. 2.2.4 Upper Leg EMG Electrode Placement. Place electrodes on the quadriceps muscles, just above and either side of the knee after palpating with the knee extended and the subject actively contracting the muscle. Place the other set of electrodes on the hamstring muscles at the back of the thigh. To facilitate electrode placement, ask the subject to flex the knee against resistance with the knee in an already flexed position. Subject Position: As in the previous demonstration, repeat the anterior and posterior postural sway exercises, listening or watching the activity from both muscle groups. Ask the subject to touch the toes while observing the activity

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from the hamstring muscles. Note the sudden increase in EMG activity from the quadriceps group as the person leans backwards, almost at the point of losing balance. This is a REFLEX contraction which is part of the hip strategy for maintaining balance against large perturbations. The function of the contraction is to prevent the knee from flexing further and it is sometimes accompanied by cocontraction of the hamstrings and gluteals whose function is to extend the hip joint, the net effect being to thrust the pelvis forwards to keep the COG of the body over the base of support. 2.2.5 Supplementary Demonstration: Effect of Joint Angle In relaxed standing, the knee is hyperextended (“locked”) and the quadriceps muscles relaxed. This can be confirmed using the EMG system. Ask the subject to bend the knees slightly. Immediately, an increase in quadriceps EMG will be observed. This is an antigravity reflex which is essential to maintain the erect posture against the pull of gravity. The demonstration should indicate convincingly why the erect position is an energy efficient one and why confined spaces (as are found in the mining industry, for example) and bad design can increase energy expenditure of work activities by their effect on working posture. 2.2.6. Lumbo-pelvic and Trunk EMG Electrode Placement: Place electrodes over the skin overlying the low back muscles either side of the spine at the level of the lumbar spine. Hint: use the iliac crests (tops of the iliac bones) to estimate the mid-lumbar level i.e. the highest point of the iliac bones is level, approximately, with L3. Place electrodes on the abdominal muscles (rectus abdominis) bilaterally on the upper central portion of the abdomen and below the rib cage. Place electrodes on one side of the body on the inguinal canal to detect iliopsoas muscle EMG (iliopsoas is a deep muscle and it is not always easy to get a good signal using surface electrodes). Subject Position: With the subject in a relaxed standing position repeat the anterior posterior swaying manoeuvre. Note the increase in back muscle EMG as the subject sways forward and the decrease in back muscle EMG activity as the subject leans backwards. As the subject continues to lean backwards, abdominal muscle activity can be detected. Indicate to the students that in relaxed standing both the erectores spinae and the rectus abdominis muscles are “quiet”. The lumbar lordosis minimises the flexion moments on the spine so back muscle activity is not needed to keep the spine erect in normal standing. Since the line of gravity of the upper body passes through the posterior portion of the hip joint in standing there is a net posterior pelvic tilt which enables posture to be maintained

20 INTRODUCTION TO ERGONOMICS

with the abdominal muscles relaxed. To demonstrate the action of the iliopsoas muscles, ask the subject to raise the leg on the monitored side. This shows the action of the iliopsoas as a flexor of the hip when the femur is free to move (as in walking to initiate the swing through phase of gait). 2.2.7 Supplementary Demonstration: Anterior/Posterior Pelvic Tilting Instruct the subject to execute an anterior pelvic tilt manoeuvre when standing (to arch the back as much as possible). Note the increase in erectores spinae activity and possibly some activity in the iliopsoas muscles. Ask the subject to carry out a posterior pelvic tilt (flatten the back as much as possible). Note the involvement of the abdominal muscles (it may also be possible to detect gluteal muscle activity when this manoeuvre is carried out). 2.2.8 Supplementary Demonstration: Forward Flexion of the Trunk Connect the erectores spinae electrodes to one channel and the hamstring electrodes to the other. Ask the subject to adopt a fully forward flexed position as if to touch the toes. Note how in the forward flexed position, the lumbar spine is “hanging” on its posterior ligaments so to speak—there is no activity from the back muscles at all. As the subject slowly returns to the erect position by extending the hip, it will be observed that it is the hip extensors which initiate the movement and not the back muscles—these only begin to contract when the trunk is about 60 degrees from vertical. This demonstrates that hip flexion/ extension is the primary movement in bending forward and not lumbar flexion as is often suggested in the literature. The actual range of flexion/extension of the lumbar spine is rather limited. 2.2.9 Supplementary Demonstration: Function of the Abdominal and Iliopsoas Muscles Instruct the subject to lie down in a supine position with the feet stabilised and the knees slightly bent. Have the subject execute sit-ups in a slow and controlled way. Note that most of the movement is hip flexion and that the iliopsoas muscles play a major role. Repeat the activity but with the knees bent by approximately 90 degrees and the feet to move but held flat on the floor by the subject Ask the subject to very slowly raise the shoulders off of the floor as far as possible while keeping the feet flat on the floor and the knees still. Note the reduced range of movement and the involvement on the abdominal muscles which act as trunk

INSTRUCTOR’S MANUAL 21

flexors. It can be seen that the actual amount of trunk thigh movement which is due to spinal flexion is very small compared to that due to hip flexion. 3. ESSAYS AND EXERCISES 1. A simple exercise to enable students to “visualise” the structures and develop an intuitive understanding of the physical basis of terms such as “facet” joint. In reviewing drawings, deduct marks for the ommission of any of the structures that are labelled in the chapter. Deduct marks for mislabelling. 2. Divide the marks into halves for each part of the question. Suggest 10 marks per drawing with 2 marks for correct identification of the position of the feet/shaded support, 3 marks for overall correct rendering of bod parts and posture. Of the remaining 5 marks, give 2 marks each for correct identification of static and dynamic load and leave 1 mark remaining for any additional observations or comments (e.g. including the young child in the analysis). 3. Compression due to upper body weight (assumed to be directly above L3 in the erect posture):

Downward force exerted by the sack of feathers Load moment about the lumbar spine due to carrying the sack at a distance of 70 cm (0.7m) from the spine: Back muscle extension force needed to generate a countermoment is given by the countermoment of 137.4Nm acting at a distance of 0.5 cm from the lumbar spine: Total spinal compression=Compression due to body weight+compression due to load+compression due to back muscle extensor force Comments As can be seen, the task load is much greater than the postural load— almost 10 times greater, in fact. The total compression is close to the NIOSH limit of 3500N for a manual handling task. The carrying posture is a good one and this example shows that something that is as “light as a feather” can cause problems when it is awkward to handle. There are many ways of making a task like this safer—the easiest is to carry the sack on

22 INTRODUCTION TO ERGONOMICS

the head. This eliminates the load moment altogether and therefore the need to generate a countermoment over a short lever arm. The resulting compression is below 600N—well within safe limits. Of course, we could also ask ourselves why a sack of feathers has to be carried at all—after all “sack barrows” have been around for hundreds of years. 4. Upper body weight=45 kg Distance of upper body COG from LS joint=45 cm Shovel 1 fw=downward force due to body weight=450×9.81=441.15N fsh=downward force due to loaded shovel=15×9.81=147.15N dw=horizontal distance of upper body COG and shovel blade from lumbar spine=45 cm Back muscle force to sustain equilibrium=fm

Total compression force=5294.7+(441.15 cos 90)+(147.15 cos 90)=5294.7N Total shear force=(441.15 sine 90)+(147.15 sine 90)=588.3N Shovel 2

fw=downward force due to body weight=450×9.81=441.15N fsh=downward force due to loaded shovel=15×9.81=147.15N dw=horizontal distance of upper body COG from LS joint=0.45 cos 80 m=0. 078 m dload=horizontal distance of load from LS joint =1.2 m (given in question). Back muscle force to sustain equilibrium=fm fm=[(441.15×0.078)+(147.15×1.2)]/0.05 =210.99/0.05 N =4219.8 N Total compression force=4219.8+(441.5 cos 10)+(147.15 cos 10) =4219.8+579.4 =4799.2N Total shear force=(441.15 sine 10)+(147.15×sine 10)=76.6+25.55 =102.15N Interpretation The redesigned shovel has reduced the compression at the LS joint. However, the cost of reducing the trunk flexion to eliminate the postural load has resulted in a large increase in task load. A relatively ligth load—15 kg—exerts a large flexion moment because it is lifted at a distance of 1.2 m from the spine. It seems

INSTRUCTOR’S MANUAL 23

possible that there is room for optimisation here and that a shovel with a handle of intermediate length might bring about the best compromise between task load and postural load. This example illustrates the point that more upright postures are not automatically “better” than stooped postures. The biomechanics of different lifting methods and techniques needs to be taken into account and not not just the posture of the body. 4. FURTHER READING More advanced literature in biomechanics in general can be found in the book Occupational Biomechanics by Chaffin and Anderson. The book by Singleton contains a detailed and useful chapter on biomechanics in ergonomics. A highly recommended source of anatomical drawings is volume 3 of the book by Kapandji. All of these are in the further reading section

Chapter 3

After studying this material, the student should understand:

1. The nature of anthropometric data and the need for these data in design 2. The constraints on the use and availability of data 3. The statistical terms used to describe the distribution of an anthropometric variable 4. Common applications of data to arrive at design solutions The student should be able to:

1. Calculate the mean and standard deviation of an Anthropometric variable from sample data 2. Use tables of the standard normal (“Z”) distribution to calculate percentiles 3. Use existing data to specify the main dimensions of a product or workspace for use by a defined population 4. Identify basic anthropometric mismatches between users and equipment and suggest ways of ameliorating them 5. Use the RASH technique to estimate anthropometric data for a target population of known stature, using data from a reference population 1. COMMENTARY The main purpose of this chapter is to introduce the concept of human variability and explain why and how it is necessary to consider human variability in design. In many countries, there are standards and regulations which are used to specify the dimensions of building interiors, furniture and equipment. However, for many applications, no standards are available and it is not always possible to simply take standards from one country and use them in another. A better approach, is to understand the ergonomics principles on which a particular guideline rests and then specify dimensions for the new application so that it will not violate the ergonomic principles. For example, in the US, the recommended height for a standing worker’s workspace is 850–1010 mm if heavy work is to be

INSTRUCTOR’S MANUAL 25

performed. This height is relatively low for standing work and has been selected to allow the worker to use body weight to bear down on heavy objects and to use the legs when raising them (it allows the worker to bend the knees slightly while keeping the trunk erect and the arms low while grasping the object). A US industrial engineer in charge of setting up a new factory in Mexico would be advised not to implement this as the standard height for heavy work benches in the new factory without considering the implications for the shorter Mexican workers. It might be better to consider the differences in stature, standing elbow height and leg length between Mexicans and US workers and change the bench height accordingly. 1.1 Anthropometry and Its Uses The main thrust of this section is to get the student to relate equipment and product dimensions to human dimensions. One of the most important distinctions is between static and functional data—most data are in static form. A good way of demonstrating the use of anthropometric data in design is by example. The instructor can use real products or equipment items to illustrate the design principles involved. A useful interactive exercise is to ask students to identify the important dimensions of a range of products and relate them to anthropometric variables most relevant to the design. Some examples are as follows: —Screwdriver (grip circumference, palm depth) —Power drill (grip circumference, finger length) —Office Chair (popliteal height, buttock-popliteal length, hip breath, sitting elbow height) —Mattress (stature, body mass, shoulder width) —Bath (stature, leg length, shoulder width, body volume) —Power station control panel (stature, standing eye height, sitting eye height, functional reach 2. USEFUL STATISTICAL CONCEPTS For those lecturers whose students have no formal training in statistics, some of the fundamental concepts are presented here and can be used as supplementary lecture material. Measures of Position and Variability. Measures of Position There are three main measures of central tendency, the Mean, the Median and the Mode 1. The arithmetic mean is a measure of central tendency given by the formula:

26 INTRODUCTION TO ERGONOMICS

2. The median is the middle observation when the observations are listed in increasing order. More formally, it is the observation in the ordered series. If the number of observations in the series is even, the median is the arithmetic mean of the two middle observations. 3. The mode is the most commonly occurring value. The arithmetic mean is usually the best measure of central tendency because it is based on more information from the sample than the other two. In ergonomics, when we wish to make statements about the body dimensions of people in a population of operators or users, we need data on these dimensions sampled from a large number of people. Where this is the case (and it is the case with much of the data pertaining to US and European populations) the mean is usually the best measure. Anthropometric data are used in ergonomics under the assumption that the variables in question are distributed normally (i.e. according to the Guassian distribution). For many anthropometric variables, this appears to be a valid assumption. However, for small samples, the underlying distribution may be asymmetrical and under these circumstances the mean may give a very misleading result. Consider the following measurements: 5 5 5 5 7 10 20 102 the mean value is 22. It is very unrepresentative of the observations in the data set as a whole—only one value is even close to it and it would be most unsuitable to use it as a basis for design. In this case, the median value (7) is more appropriate— special considerations would be required to design for the two outlying observations. The mode is rarely used in ergonomics. 2.1 Weighted Means The weighted mean is used when we have means from two samples and wish to combine them. For example, suppose a tractor manufacturer has data on the popliteal heights of Mexican and Brazilian tractor drivers—his two main export markets in Latin America. He wishes, if possible, to standardise on the seat heights of all tractors exported to these two countries. Given the following data: Mean Popliteal Height (Brazilians)=390 mm N (sample size)=100 Mean Popliteal Height (Mexicans)=385 mm N (sample size)=50

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2.2. Measures of Variability The range of a sample is the difference between the largest and smallest observations. For example, in the following sequence: 1, 3, 5, 6, 9, 3, 2, 16 the range is 1 to 16 i.e. 15. The disadvantages of using the range to make design decisions in ergonomics are many. 1. The range wastes data as it tells us nothing about the spread of scores between the two extremes. 2. Extreme values are often those which have been measured incorrectly. 3. The range is very dependant on the size of the sample—it tends to get bigger as sample size increases and is therefore of little value in making design decisions. 4. Small samples give very unreliable estimates of the range. For example, if we continue sampling to add observations to the above sequence of numbers, we may find, 2, 4, 6, 7, 5, 3, 1, 34, 2, and the range is now 1–34 i.e. 33. The range has doubled after adding only 9 more observations. 2.3 Variance and Standard Deviation The variance is a measure of variability which takes into account the difference between each score and the mean. It is given by: The numerator in the equation is squared to account for the fact that the normal distribution is symmetrical. If all the scores above and below the mean were subtracted from the mean, the result would be a series of positive and negative numbers which would add up to zero. By squaring the difference between each observation and the mean we end up with a series of positive numbers. The denominator in the equation is given by n−1 where n is the number of observations or difference scores. The denominator is called the “degrees of freedom” and is used to calculate the variance of samples.

28 INTRODUCTION TO ERGONOMICS

The variance is an efficient measure because it uses information from every observation in the data. Its main drawback in ergonomics is that it consists of squared units which have little meaning in relation to the design problems at hand. The solution to this problem is to use the square root of the variance. This is known as the standard deviation and is referred to by the symbol “S”. Most anthropometric calculation is done automatically nowadays. But it is worth noting that there is a formula for ease of computation of the standard deviation. The numerator in the equation above, (x−x )2, is substituted as follows: dividing by (n−1) gives the variance and taking the square root of the result, the standard deviation. 3. Sample Calculations Calculation of mean and standard deviation: Range 1–16 i.e. 15 x

x2

1 3 3 5 6 9 3 2 16 -48

1 9 9 25 36 81 9 4 256 ---430

s= ( 430−256)/8=4.66 Range 1–34 i.e. 33 x

x2

1 3

1 9

INSTRUCTOR’S MANUAL 29

x

x2

3 5 6

9 25 36

9 3 2 16 2 4 6 7 5 3 1 34 2 --112

81 9 4 256 4 16 36 49 25 9 1 1156 4 ------1730

s=7.8 We can see from these two examples how the standard deviation is much more resistant to extreme observations than the range. Under the assumption of normality, estimates of the standard deviation can be used to estimate percentiles (see table 3.11 main text or refer to tables of areas under the normal curve in any statistics book). The general formula is as follows:

For a normally distributed variable, the mean is the 50th percentile (the value of z, the standard normal variate, is zero at that point where 50% of the population lies above and 50% below z). We can also calculate Quartiles (25% of the observations lie below the lower quartile and 75% below the upper quartile). In ergonomics, we are usually interested in the 5th and 95th percentile values of an anthropometric variable. These are calculated by subtracting 1.64 standard deviations from the mean and adding 1.64 standard deviations respectively.

30 INTRODUCTION TO ERGONOMICS

Sampling Distributions and Confidence Limits. In the absence of a detailed anthropometric database, the designer may need to take sample measurements of appropriate anthropometric dimensions of the user population. From these, estimates of the means and standard deviations of the variables can be made (as can be seen from the previous discussion, it is probably not sufficient to select the largest and smallest users and design to accommodate them, since this is equivalent to using the range to estimate variability). These estimates are known as statistics (usually expressed using Roman symbols). They are estimates of the true mean and standard deviation in the population ( the population parameters which are expressed using Greek letters). The designer's problem is to decide how true an estimate of the population parameters are the sample statistics (the parameters are, of course unknown). The answer to this question requires a brief excursion into Sampling Theory. Sampling When a sample is taken, part of a group is selected to provide information about the whole group. Usually, the population is too large to be sampled in its entirety, or, it would be too expensive to sample or there is not enough time. Anthropometric surveys are very expensive to carry out. They require skilled personnel to identify anatomical landmarks and make proper measurements. In ergonomics, we normally want to know —the mean value of an anthropometric variable —the standard deviation From these we can estimate the proportion of the population which lies above and below chosen percentile values of the variable. However, sampling always entails some or other type of error such that our estimates of the mean and standard deviation are different from the real (unknown) parameter values. Sampling errors usually occur when we neglect to sample across the population as whole. Sampling error is reduced as sample size increases. For example, we might sample the body dimensions of 15 Mexican truck drivers only to find later on that there is considerable variability in different parts of the country which a sample as small as this just couldn’t detect. Non-sampling errors occur when we use an incorrect strategy to sample from the population. For example, we might forget to include self-employed truck drivers who own their vehicles themselves. Non-sampling errors do not necessarily diminish as sample size increases. Even if we do take care to ensure that our sample is fairly large and unbiased, it is clear that the estimate of the mean that we arrive at will be different from the real mean. The problem facing the investigator is to determine the likely size of this difference and this is done by finding the standard error of the mean.

INSTRUCTOR’S MANUAL 31

Standard Error and Calculation of Confidence Limits

Suppose we measure the grip diameter of a sample of 10 Canadian lumberjacks. Then we calculate the mean using the formula: If this procedure was repeated many, many times on different occasions, we would have a distribution of mean grip circumferences for the population. That is, grip circumference has a distribution which is probably approximately normal as follows: According to the central limit theorem, these estimates of the mean grip circumference (i.e. the sample means) have a normal distribution whose mean is the true mean and whose standard deviation is given by /n:

32 INTRODUCTION TO ERGONOMICS

Note: 1. The distribution of is normal even if the parent distribution from which the xs are calculated is decidedly non-normal. 2. The mean of the distribution of is µ , the true mean of the population. 3. The variance of is s2/n where n is the sample size. 4. The standard deviation of X is given by S/n. As the sample size of X is increased, the variance of X decreases. This is another way of saying that the larger the sample, the closer X is likely to be to the true mean. 5. The standard deviation of x is known as the standard error of the mean. It is usually written as SE(x). Because we know that x is normally distributed, we can apply our knowledge of the normal distribution to the analysis of sample means. Referring to Table 3. 11 in the main text we know that the 90% of the time, the sample mean will not differ from the true mean by more than 1.64 standard errors. This is extremely important since it tells us the range of variability associated with our estimate of the mean (i.e. by how much we are likely to have over or underestimated the true mean) and thus the confidence with which we can make statements about the true value of the mean. Confidence Limits Since the true mean is unknown, we can never be exactly sure how close to it is our estimate of the sample mean. However, from the theory which has been described above, we can make certain statements which are of value. For example, because the sample mean is known and its standard deviation can be estimated we can say that there is a 90% probability that the interval: +/−1.64 SE(x) contains the unknown population mean If we want to be more rigorous, we can say that there is a 95% probability that the interval: +/−1.96 SE(x) contains the unknown population mean. These are known as 90 and 95% confidence intervals for the mean. Example.

Suppose our sample of 10 Canadian lumberjacks has a mean grip diameter of 60 mm and a standard deviation of 10 mm. The standard error of the mean is given by

INSTRUCTOR’S MANUAL 33

The 90% confidence interval for the mean is:

In other words, the probability is 90% that the range 55 to 65 mm contains the true grip diameter. Estimating the Dimensions of Unknown Populations Pheasant (1982) has published a method of estimating the parameters of the distributions of unknown anthropometric variables from the parameters of the distribution of stature. The method is based on the fact that stature is one of the few anthropometric variables which is routinely measured in many different situations or can be easily measured if it is not available. Suppose we want to estimate the distribution of a variable such as knee height in a target population but have few data on its anthropometrics. First we obtain the parameters of the distribution of stature (st) in this population: Next, we obtain the distribution of stature (st) and knee height (kh) in a known population (k):

We then calculate the scale ratios for the known population: Scale ratio for the means: Scale ratio for the standard deviations: The mean knee height ( kht) of the target population is given by: The standard deviation knee height in the target population is given by: Empirical testing of this technique has shown that 95% of the predictions are within 2.5 cm of the actual values. The technique is best used when we wish to estimate unknown values of the long bones. It is known that there are high correlations between the dimensions of the long bones in populations.

34 INTRODUCTION TO ERGONOMICS

The correlations between the long bone dimensions and other dimensions such as skinfold thicknesses and limb circumferences are not nearly so strong. EXERCISES 1. VDT workstation dimensions for different populations. One way to approach this problem is to look at the ANSI/HFS national standard for VDT workplaces and compare the recommended dimensions with some US anthropometric data to determine what the designers are trying to achieve. The data (cm) are as follows:

Seat Height Elbow Rest Height Worksurface Height

95th Percentile Male

5th Percentile Female

49 29 71

41 18 58

The popliteal height of a small (5th percentile) US female is 36 cm and of a 95th percentile male, 49 cm. It can be seen that the seat height is 5cm higher than the 5th percentile popliteal height which may seem to violate ergonomics principles at first. However, the specification probably allows for a 1 to 2 cm reduction in seat height due to compression of foam and upholstery and also for shoes with heel heights of at least 2–3 cm. Clearly, females with less than 5th percentile popliteal heights will need footrests. It was probably the intention of those who drafted the standard that this would be the case. The large male has a popliteal height of 49 cm and the seat height is also 49 cm although the functional seat height is probably somewhat less than this (about 5 cm) taking into account compression of padding and the use of shoes. This seat height should allow a long-legged user to sit comfortably with the thighs approximately horizontal and not with the knees cramped above hip height as often happens when tall people are given too-low seats to use. Turning to the elbow rest height specification, it can be seen that the height of the elbow rest above the seat must be 18 cm for a 5th percentile female and 29 cm for a 95th percentile male. Thus, the furniture dimensions correspond very closely to the anthropometry. Clearly, the elbow rest should be height adjustable between these two extremes if 90% of users are to be able to rest their elbows comfortably i.e. the intention is that the elbow rests should be the same height as the seated person’s elbow when the person is sitting erect. Worksurface heights of 58cm and 71 cm are recommended for 5th percentile females and 95th percentile males respectively. For females with 5th percentile popliteal heights, the seat height must be 41cm. For such a female with 5th percentile elbow height (18.5 cm), a desk height of 58 cm would approximate the

INSTRUCTOR’S MANUAL 35

height of the elbows For females with higher elbow heights, the elbows would be above the surface of the desk but still at a reasonable height for operating the keyboard. The intention seems to be to ensure that a short female can sit comfortably on the chair with both feet on the floor and reach the keyboard without having to elevate the shoulders. For tall males (95th percentile popliteal height), the seat height is 49 cm. Once again, a worksurface height of 78 cm allows a male with 95th percentile popliteal height and 5th percentile elbow height access to the desk (i.e. 49 +195=79.5 cm). The rationale for the worksurface specification seems to be:

A wide range of workers must be able to sit comfortably (i.e. rest their feet on the floor) and not have to work at a too-high desk in relation to their elbows (alternatively, we might standardise on the 79 cm desk height and provide footrests for short females of 49.5–360=13.5 cm). Seat depth must not exceed the buttock popliteal length of a 5th percentile female i.e. 44 cm and seat width (a minimum dimension) must not be less than the hip breadth of a 95th percentile female i.e. 44 cm (Table 3.10). Probably 5 or more centimetres of extra space would be needed to allow for clothing. As an example of the specification of dimensions for different populations, we will use the data on Japanese adults. The 5th percentile Japanese popliteal height is 32.5 cm for females. The 95th percentile male popliteal height is 44 cm. If we allow about 3 cm for heels and two centimetres for compression of padding, the maximum allowable height for a seat in its lowest position must not be more than about 37 cm if footrests are not provided. As far as Japanese males are concerned, the 95th percentile male would just be able to use the seat at 49 cm because of compression padding and the wearing of shoes. However, if the range of adjustability of the seat height is fixed (e.g. because a standard design of pneumatic cylinder is used in the adjustment mechanism) a reduction of 5 cm to 44 cm will still accommodate the taller worker. The height of the elbow rest above the seat can be raised for Japanese users because the 5th percentile females elbow rest height is 21.5 cm. The elbow rest height for Japanese males is very similar to that for US males (only 0.5 cm higher). It may be sufficient only to increase the elbow rest height for females by 3 cm. If a seat height of 37 cm is needed for a female with 5th percentile popliteal and elbow heights, then a desk height of 58 cm would approximate the inner part of the elbow when the sitter was erect (i.e. from where the included angle is measured when the elbow is flexed). For 95th percentile popliteal height Japanese males a seat height of 49 cm or up to 5 cm lower could be used. If the sitter had a 5th percentile elbow rest height, of 220 cm a worksurface height of 71 cm would be usable.

36 INTRODUCTION TO ERGONOMICS

Japanese users have lower popliteal heights than US users but higher sitting elbow heights which compensate to a certain extent. It can be seen that the main modifications are to the chair height to allow short females to rest their feet on the floor and the elbow rest height. The example demonstrates the importance of analysing differences in proportion as a basis for design and why it would not be appropriate to just “make everything smaller because the people are smaller”. 2. Kitchen Survey. Compare worksurface heights with recommendations in the literature. Evaluate cupboard and shelf heights according to requirements for reaching and stooping. Interpret data using information on common tasks. Comment on desirability of redesigning the kitchen to better support common tasks. 3. The key concept here is that good design depends on a knowledge of human variability rather than a mythical “Mr Average”. If you design for an “average” person the only certainty is that the product will not be suitable for most users. 4ii. Using reference data from the USA, we will use the RASH technique to estimate the following variables: Cape Town Company Employees Stature

Males Females 1500

Mean

SD

1600 70

80 1385

5th Percentile

95th Percentile 1631

Popliteal height—5th percentile female Buttock-popliteal length—5th percentile female Elbow rest height—95th percentile male Scaling factor for 5th percentile female popliteal height=360/1520=0.24 Scaling factor for 5th percentile female buttock popliteal length=440/1520=0. 29 Scaling factor for 95th percentile male elbow rest height=295/1870=0.16 Estimated 5th percentile popliteal height Cape Town females=1500×0.24= Estimated 5th percentile buttock-popliteal length Cape Town females=1500×0. 29 Estimated 95th percentile elbow rest height Cape Town males=1600×0.16 5th percentile popliteal height Cape Town females=5th percentile female stature× scaling factor=1385.2×0.24=333 mm 5th percentile buttock popliteal length Cape Town females=5th percentile female stature×scaling factor=1385.2×0.29=402 mm 95th percentile elbow rest height Cape Town males=95th percentile male stature× scaling factor=1631×0.16=261 mm 4iii. The key to this question is that a user will be able to sit on the seat if his or her buttock popliteal length is greater than the seat depth. So, we need to find out what percentile female buttock popliteal length is just longer than the seat depth.

INSTRUCTOR’S MANUAL 37

This percdentile will give the percentage of females that will have to sit on the edge of the seat. 450 420 400 390 370 Another way of looking at this is to regard the seat depths as buttock-popliteal lengths and calculate the percentile. We need to know the mean and sd buttock politeal length of Cape Town females. Standard deviation of stature US females=95th−5th percentile stature/ (1.64×2) = 1730–1520/3.28=210/3.28=64 Standard deviation of buttock popliteal lenght US females=95th−5th percentile Standard deviation of buttock p buttock popliteal length/3.28=100/3.28=30.5 Scaling factor for standard deviation of buttock popliteal length=30.5/64=0.48 Estimated standard deviation of buttock popliteal length (from 4.ii) =70×0. 48=33.6 Mean and SD of Cape Town Female Buttock Popliteal length are 457mm and 33.6mm Seat Depths

Distance from mean (in SDs)

% fitted*

370 mm 390 mm 400 mm 420 mm 450 mm

87/33.6=2.6 67/33.6=2.0 57/33.6=1.69 37/33.6=1.1 7/33.6=0.2

99.5% 97.7% 95.4% 86.4% 53.9%

So, the maximum allowable dimension for seat depth is around 400 mm. *From tables of Z, the standard normal distribution FURTHER READING The book by Pheasant (1986) is a readable introduction to anthropometry and its uses in ergonomics. It contains a useful collection of anthropometric data for several different populations. Panero and Zelnick (1979) have compiled a very well-illustrated collection of standards for the design of interiors. Woodson’s (1981) handbook is also a good source of information. For UK readers, ADULTDATA (see Further Reading section) is the recommended source of data.

Chapter 4

On completing this chapter, the student should understand:

1. Standing and sitting postures, the anatomical differences between them and their relative advantages and disadvantages as working postures. 2. Spinal problems in standing and sitting and how to avoid them (together with Chapter 2). 3. The requirements for good standing and sitting postures 4. The workspace design options available. The student should be able to: 1. Use the “postural triangle” to carry out a workspace analysis. 2. Relate postural problems to workspace design, task requirements or user characteristics. 3. Use appropriate anthropometric data to specify workspace dimensions 4. Carry out simple task analyses and use them to evaluate workers’ furniture and accessories. 5. Use ISO 1226 to evaluate static work postures of the trunk 6. Estimate postural angles from 2-Dimensional photos taken at the workplace. 1. COMMENTARY The chapter is concerned with the essential ergonomic aspects of design for standing and seated workers. It is based around a simple conceptual model which emphasises the importance of a good working posture for the efficient execution of work tasks (chapter 2 introduces the basic requirements for a good working posture). If the ergonomist’s goal for a well-designed workplace is a good working posture, then it is necessary to describe to students the variables, which determine working posture. The conceptual model stresses the importance of: Task requirements Workplace design

INSTRUCTOR’S MANUAL 39

Personal characteristics — and how these variables interact to determine posture. A holistic approach is needed and material from other chapters in the book can be included or referenced to augment the discussion as follows: 1.1 Task Requirements Task analysis is of obvious relevance here. The minimal data for determining a worker’s furniture needs would be some consideration of the main assignments and segments of the job (i.e. a basic job analysis should be at least one level of detail more descriptive than the person’s job title). For secretaries, typists and other workers, some quantitative indication of work output (e.g. number of letters and reports typed per day) would be relevant as well as a qualitative indication of the type of tasks performed. This is useful in determining the degree of postural constraint of the worker and the main elements of hardware around which key design or purchasing decisions have to be made. In industrial or military environments where people are working with more complex interfaces, more detailed task analyses may be required. Of particular importance, is the identification of sequences of tasks or movements to assist in the layout of displays and controls. 1.2 Workplace Design Apart from basic furniture considerations, illumination levels, noise levels and the ambient temperature are also important. 1.3 Personal Characteristics These are discussed in the text. Two key considerations are the anthropometry of the population and the age/sex of the workforce. 2. Visual Display Terminals The amount of discussion of VDT related issues may seem rather limited. It is a topic which has attracted very much attention over the last 15 years. Authors such as Grandjean (1986) have written whole books about VDT and office ergonomics. However, there are very few issues which are unique to the design of the VDT workplace apart from the particular issue of whether dedicated VDT work is a health hazard. Even here, fundamental principles and design recommendations of a more general nature are applicable and are described

40 INTRODUCTION TO ERGONOMICS

throughout the book. The approach has been to present two figures (4.15 and 4. 16) which summarise the ergonomic requirements for the design for VDT workplaces. Further discussion of these issues can be found in other parts of the chapter and if the book. 3. DEMONSTRATION Using the approach and equipment described in chapter 2, an electromyographic demonstration can be carried out to illustrate the ergonomics of standing and seated work. 3.1. Standing Work Electrode Placement: Place electrodes on the erectores spinae muscles approximately at the level of L3 (third lumbar vertebra). L3 is approximately at the same height above the ground as the iliac crests (tops of the iliac bones) which can be found by palpation. Electrode position can be optimised by carefully palpating the low back to find the body of the muscle. Place another set of electrodes over the neck muscles (i.e. place the two electrodes unilaterally and posteriorly in the neck shoulder region). It may be helpful if the subject first elevates the shoulder to shorten the muscle and disclose its location. Test the placement of the electrodes by asking the subject to elevate the shoulders. A strong signal should be detected almost immediately and for very small elevations of the shoulder. Channel 1: Low back muscles Channel 2: Neck muscles. Subject Position: Subject stands at a workbench carrying out a light task (any light task with clear visual and manual requirements will do such as doing a jigsaw puzzle or typing text into a word processor). The worksurface height should be approximately the same as the subject’s standing elbow height. Monitor low back muscle EMG while the subject is working with the hands close to the body. Next move the working away from the subject so that the hands are at the limit of the preferred reach area (i.e. approximately 40 cm from the front of the bench). Observe any postural changes and also any changes to the EMG signal. Next, demonstrate the effects of foot position and the requirements for foolscap by placing an obstruction by the subject’s feet (a wooden box or board can be placed on the floor in line with the leading edge of the workbench so that the subject cannot get the feet under the workbench and thus has to stand farther away. Observe any postural changes also any changes to the EMG signal. Further “experiments” on foot position in relation to working posture can be carried out by making the subject stand in a shallow wooden box or frame to

INSTRUCTOR’S MANUAL 41

constrain the position of the feet and moving the frame in relation to the work area. This demonstration is intended to impress upon students the need for careful consideration of the worker’s requirements for free space at the feet and the need for good housekeeping in working areas to ensure that floor area is not used inappropriately as storage space, thus constraining the standing worker’s posture. 3.2 Additional Demonstration: Effects of work-surface height Using the same apparatus as described above, select several different worksurface heights to demonstrate their effects on posture and postural load. It is generally recommended that work should be carried out at approximately standing elbow height. Try several heights above and below this in steps of 10 cm. If the bench height is fixed provide the subject with blocks to stand on. Look for elevation of the shoulders and increased neck/shoulder EMG activity when the worksurface is too high. Look for increased trunk and neck flexion when the worksurface is too low. Additional Demonstration: Working with the hands too high and too far away. In many situations, workers have to reach forwards and upwards to manipulate work objects—as, for example, when filling shelves or cupboards above workbenches or when painting or drilling holes in walls. This places a static load on the shoulder muscles and increases the flexion moment about the lumbar spine. With the subject standing in such a position, monitor EMG activity in the low back and shoulder region. If low back EMG activity does not increase, observe whether compensatory extension of the lumbar spine has occurred (this is a common postural strategy for dealing with increased forward loading which obviates the need for increased back muscle EMG at the expense of increased lumbar lordosis. 3.3. Chair Sitting Demonstration Electrode Placement: As above. Also place electrodes on the abdominal muscles and on the inguinal canal to detect iliopsoas muscle activity as described in chapter 2 (it is not always possible to get a good signal since the iliopsoas are a deep muscle, however, with perseverance and by experimenting with electrode position and different subjects, a good signal can be obtained which is indicative of hip flexor activity). Subject Position: Ask subject to sit on the floor or on a long bench in the long sitting position (Figure 4.17). In this position, the shortened hip flexors and lengthened hip extensors cause the pelvis to rotate backwards and the lumbar lordosis to disappear. Ask the subject to attempt to “sit up straight” by tilting the pelvis forwards and arching the back. Monitor the EMG activity of the erectores

42 INTRODUCTION TO ERGONOMICS

spinae and iliopsoas muscles. Increased activity inn these muscles compared with relaxed sitting in this position demonstrates the important point made in the main text that in certain body positions the cost of maintaining an erect trunk is a physiological one—i.e. increased energy expenditure. This demonstrates that it is futile to attempt to train seated workers to sit “correctly” if the seat and the rest of the workplace is not designed according to the principles of ergonomics. 3.4 Function of Backrests and Lumbar Supports. With the subject sitting on a flat seat (i.e. a seat with no backrest) monitor EMG activity from the abdomen and low-back with the subject sitting in an erect, relaxed posture (i.e. with the trunk in an upright but “slumped” position). In this position, the bones of the spine are held in place by passive tension in the posterior ligaments, anterior wedging of the intervertebral discs and pressurisation of the abdominal contents. These prevent further forward flexion of the spine and consequently there is no EMG activity from the relaxed trunk muscles. In the absence of a backrest and lumbar support this is the relaxed posture which is found in the 90-degree sitting position. It is also the posture which people choose to rest in but can cause low back pain if assumed for long periods due to the deformation of soft tissues. A simple mechanical analysis suffices to demonstrate that when a sitting person reclines against a backrest, the action of gravity on the upper body mass can be resolved into 2 force components, a vertical component (i.e. the backrest supports some of the mass of the trunk) and a horizontal component. The latter is the component of forward thrust which tends to eject the sitter from the seat if not overcome by good design—normally the mid-front part of the seat is raised above the rear part which supports the ischial tuberosities. This contouring provides pelvic stabilisation when the sitter reclines (as is illustrated below). As can be seen from the above, the more the sitter reclines (i.e. the greater the angle between the seat and the backrest) the greater is the vertical component of force and the less is the horizontal component. What this means is that the more the sitter reclines, the less compression there is on the spine. It also indicates that in the position of reclining the backrest does not “support the back muscles”. This is because it resists what is in reality an extensor moment about the low back (or what would be an extensor moment if the backrest were to suddenly disappear while the sitter reclined against it). Somewhat paradoxically, it can be said that a backrest relieves the abdominal muscles! This can be demonstrated by having the sitter recline against a “backrest” provided by the hands of the instructor (i.e. the instructor supports the person’s back while the person reclines). If the instructor suddenly removes his hands, an immediate contraction of the abdominal muscles will be observed electromyographically. Activity will also be observed from the hip flexors.

INSTRUCTOR’S MANUAL 43

For the lumbar support demonstration, monitor EMG activity from the erectors spinae muscles and from the iliopsoas muscles. Have the subject move into an upright sitting position from a relaxed position by “arching the back”. It may be possible to observe increased back and hip flexor activity as the subject executes this manoeuvre. The manoeuvre can now be repeated with the instructor acting as the lumbar support. The subject should sit in a relaxed position. The instructor should position himself on the left of the subject and stabilise the subject’s upper body by placing his left arm across the subject’s chest with the left hand on the subject’s right shoulder. The experimenter should then use his right hand to apply steady pressure against the subject’s low back in such a way as to mimic the action of a lumbar support (i.e. the lumbar spine should extend and the pelvis should tilt forwards slightly). The subject should remain relaxed throughout this manoeuvre. If the instructor releases subject, it can be seen that the subject will immediately return to a more flexed (“slumped”) way of sitting. This demonstrates the need for correct lumbar support if an upright sitting posture is to be adopted. The manouvre can be repeated while EMG activity from the low-back and hip flexor muscles is monitored except that the subject is instructed to maintain the erect sitting posture after the instructor releases the subject. In other words, the demonstration begins with the subject sitting in a relaxed flexed posture. Negligible EMG activity will be observed. It is important that the subject continues to relax while the instructor positions him in the upright sitting posture. If the instructor suddenly releases the subject, a sharp increase in EMG activity from the hip flexors and some back muscle activity will be observed indicating the role of these muscles in maintaining this posture in the absence of external support. This demonstration does not always work well. If problems are experienced in eliciting EMG activity under these conditions repeat the exercise but with the subject sitting with the knees fully extended (it may also be instructive to repeat the manoeuvre with the knees flexed and the legs tucked under the seat i.e. it is easier to sit erect when the knees are flexed rather than

44 INTRODUCTION TO ERGONOMICS

extended which explains why it is important to provide clear, unobstructed space underneath the seats of people who have to work in an upright position). 3.5 VDT Work Subject Position: Subject sits at a VDT on a height adjustable chair and desk, entering data into the system. Experiment with different chair and desk heights. Look, in particular for increased EMG activity from the shoulder/neck region when the keyboard is too high. Shows the importance of optimising the chair/ desk heights for every worker in relation to their anthropometry. 4. EXERCISES 1. Hints for exercise number 1. Some of the main areas of anthropometric mismatch in office furniture are as follows: —Seat not adjustable or lowest seat height exceeds popliteal height of small females. —Seat depth exceeds buttock popliteal length of small females —Desks too high and no footrests provided —Desks too low for very tall users —Insufficient legroom underneath desk (or legroom taken up by obstructions, documents etc. due to insufficient storage space) or legroom impeded by poorly designed “modesty boards”. —Split level desk for secretaries not provided —VDT height not adjustable or too high or too low —VDT footprint too large or insufficient desk space —Desks too close together—invasion of privacy —Workstations too close together—impedes access, ingress and egress —Inadequate accessories (e.g. document holder needed, keyboard drawer needed, more shelving or filing space needed). It may also be desirable to encourage students to devise some kind of classification scheme for problems identified. For example, separate surveys can be carried out for different categories of employee: Secretarial/Clerical (non VDT) Data entry Programmer/Systems Analyst Administration Management Other For each group, problems can be divided up into different categories such as: —Workspace/User Mismatches

INSTRUCTOR’S MANUAL 45

—Task/User Mismatches —Task/Workspace Mismatches Specific recommendations for redesign or upgrading can be made under each category, for example: —Workspace/User Mismatches: Height adjustable seats needed for clerical workers Footrests needed for data entry personnel —Task/User Mismatches: Task/User mismatches relate more to job content than physical design but may impact on physical evaluation. For example, workers may complain about physical design deficiencies such as badly designed chairs when the real cause is a too-high work rate or inadequate training. —Task/workspace Mismatches: —Insufficient desk space for programmers —inadequate storage space for managers —Document holders needed for VDT users 2a. How to measure trunk-thigh, knee, trunk flexion and pelvic tilt angles. Trunk-thigh angle: Angle between lines drawn between the knee and hip joint and the cervical spine (C7) and hip joint. Knee angle: First measure the angle between lines drawn between the ankle and knee joint and the hip and knee joint. Then subtract this from 180 degrees to get the angle of knee flexion. Trunk flexion angle: Draw a line from the hip joint through the iliac crest and extend it beyond this. From the iliac crest to the C7 marker, draw another line. The angle between these two lines is the angle of trunk flexion. Pelvic tilt angle: This can only be estimated by drawing a line through the anterior and posterior superior iliac spines and noting its angle with respect to the horizontal. To draw the graphs, use Trunk-thigh angle as the x-axis and note how each of the other angles varies with changes in trunk-thigh angle. This exercise is really just to get students practicing the use of simple techniques to quantify posture. These techniques are useful in motivating for ergonomic redesign of workplaces and for quantifying the benefits by carrying out “before/after” comparisons. 2b.Trunk-thigh angle is about 70 degrees—comparison with ISO 1226 graph indicates that it is not sustainable for more than a few minutes. Postural stress and task workload: Lower back—passive strain on flexed lumbar and thoracic spines Upper back—High trapezius muscle load Neck—lower part of C spine flexed, probably passively Upper part actively extended to peer at book (isometric muscle contraction— note chin is jutting out) C spine unstable

46 INTRODUCTION TO ERGONOMICS

Left hand acting as a vice to stabilise book—increased carpal tunnel pressure, static contraction of finger flexors, increased compressive load on the elbow joint and tension at tendon insertions in the humerus Right elbow resting on knee to close postural chain and stabilise right hand— adds to trunk flexion Assymetry of elbow positions will probably set-up a twist to the right in the spine and cause localised back pain to be worse on the left side Left foot resting on footrest Viewing distance approx 40 cm—suggests task is visually demanding and/or lighting is poor Curved seat tending to eject sitter from the seat Worksurface too low, too far away 3. Hint. If time permits, have students contact facilities management at some large corporations. Ask them about the company’s approach to furniture acquisition and workspace design. Get statistics on sick leave due to back problems. Try to obtain or estimate data on the relative costs of labour (ideally, the cost to the company of employing someone, not what the person receives) and office furniture. Express the cost of the cheapest and most expensive furniture as a percentage of the total salary cost over the lifetime of the furniture. 5. FURTHER READING The book by Zacharow (1988) is the recommended point of departure for anyone seriously interested in design for sitting and standing workers. The classic paper by Keegan (1953) should be read by anyone intending to do research on the sitting. Taylor and Francis are bringing out a new book, in 2003, on “Working Postures and Movements” edited by Dellemann, Chaffin and Haslegrave. This will probably be the best single source text on posture.

Chapter 5

On completing this chapter, the student should understand:

1. The concept of a “work-related disorder” (this is, in fact, a very difficult concept which takes time to acquire) 2. The conceptual model of Armstrong et al. (1993). 3. The main ways in which different body tissues can respond to excessive loading and the names and etiology of the WMSDs described in the chapter. 4. The role of ergonomics in causation and prevention. 5. The basic principles for the design of hand tools and equipment. The student should be able to:

1. Evaluate tools and hand-held equipment. 2. Evaluate tasks which require the use of hand tools. Identify risks and suggest improvements. 1. COMMENTARY This chapter attempts to build on the theme of the preceding chapter and on the foundation laid in chapter 2. In many ways, it is artificial to distinguish between design for seated and standing workers and the upper body at work since it is the case that the worker will be either sitting or standing and many of the design issues will overlap. However, there has been such growth in the literature on the upper body at work, the design of hand tools and the causes and cures of “cumulative trauma disorders”, reflecting a healthy interest in the area, that the topic seems to demand a chapter of its own. The main approach taken in the chapter is to base the discussion on a conceptual model of work-related upper body musculoskeletal disorders (Armstrong et al) and to introduce appropriate anatomical information and epidemiological concepts. Modern approaches to the area have to be multidisciplinary.

48 INTRODUCTION TO ERGONOMICS

2. EXERCISES We will concentrate on exercise number 2 which can also serve as a useful laboratory or demonstration class, lead by the instructor and involving the students as subjects and observers. A simple approach is described below. Screwdriver EvaluationÐSimple Worked Example Method There are several approaches to the evaluation of screwdriver design and efficiency. In the present approach, we will use performance as the evaluative index. A repeated measures design can be used such that each of a number of students attempts to screw in a number of screws in a limited period of time using different screwdrivers. We will take the number of screws fully inserted over a 3 minute period as the index of screwdriver performance and analyse the results using a statistical technique known as Analysis of Variance (ANOVA). In ANOVA terminology, we are applying several treatments in succession to a number of different subjects. We wish to test the hypothesis that differences between treatments (i.e. screwdrivers) and subjects are due to sampling error (i.e. uncontrolled or random factors affecting screwdriver performance. Apparatus Use a standard thickness and hardness of wood so as not to confound the effects of screwdriver design on performance by extraneous variables. Use several commercially available screwdrivers differing in handle design or blade length. Use a standard size and type of screw (i.e. in terms of diameter, thread and type of metal). Procedure Have a number of subjects participate in the experiment. Give them either a standard number of screws for each screwdriver and record the time it takes them to insert the screws into the wood for each one. Alternatively, give them a limited time to insert as many screws as possible and record the number of screws (this is a better method for balancing out fatigue effects over screwdrivers since for many subjects, fatigue will be a function of time). After each treatment, give each subject a 10 minute rest period. It is extremely important to balance out or randomise the order in which subjects use each screwdriver so that practice effects or fatigue do not confound any significant findings.

INSTRUCTOR’S MANUAL 49

Results The procedure should yield a table of data as follows which is amenable to analysis using ANOVA to test the statistical significance of the main factors (subjects and screwdrivers). Screwdriver Number 1

2

3

5 9 3 7 9 3 7 43

6 8 4 5 2 4 3 32

5 7 3 5 9 3 7 39

Subject 1. 2. 3. 4. 5. 6. 7.

Data are the number of screws inserted in a 3 minute period by 7 subjects using 3 different screwdrivers. Analysis Most good statistical analysis programmes contain a suite of ANOVA models to handle the computation required to test the hypotheses. For those unfamiliar with the use of ANOVA, a worked example is given. It can be followed to enable lab results to be tested manually or it can be used to test the data entry procedures required by one of the commercially available statistical packages. The approach is as follows: First, we calculate the total sum of squares (Tss) in the data using the formula:

Then we partition the sum of squares into components due to treatments (Screwdrivers, for convenience “A”) and components due to subjects (“S”) Where n1 etc is the number of observations on which the column total is based Since the Tss=Ass+Sss+Error

50 INTRODUCTION TO ERGONOMICS

we can calculate the error term by subtraction.

Degrees of freedom are given by A degrees of freedom=a−1=2 S degrees of freedom=s−1=6 T degrees of freedom=t−1=20 Error degrees of freedom is given by

Summary of Analysis Source

ss

df

ms

F

significance decision Ho*

A 8.85 2 4.43 1.29 p>0.05 Retain S 51.14 6 8.52 2.48 p>0.05 Retain Error 41.15 12 3.43 Total 101.14 * Ho is the null hypothesis that differences between treatments or subjects are due to sampling error.

Discussion In this experiment, we observed no statistically significant effects of screwdriver design on performance at a screwing task. Differences between subjects were also not statistically significant. Possible reasons for the lack of significant differences are as follows. The lack of a significant difference between screwdrivers may be because the screwdrivers selected for evaluation are really rather similar. A wider range of screwdrivers should be selected next time or more attention paid to selecting screwdrivers on the basis of substantive differences rather than styling or appearance. The lack of a significant difference in performance between subjects suggests either that the subjects were all similar in skill and ability or that their is no simple relationship between screwdriver design and task performance. It may be that there is a significant interaction between subjects and screwdrivers which obscures any simple differences between subjects. That is, whether or not a particular subject performs better than other subjects depends on which

INSTRUCTOR’S MANUAL 51

screwdriver is being used. This could have if both males and females participated in the experiment. We might predict a significant interaction between subjects and screwdrivers in this case because we know that there are differences in hand anthropometry and strength between males and females. Unfortunately, the experimental design used does not permit us to test the significance of the interaction term. Even if the experiment had yielded significant findings, we would only be able to say that performance depended on the type of screwdriver, we would have had to speculate about the reasons why. Recommendations The findings of the experiment suggest that screwdriver design has no statistically significant effect on task performance. Any of the screwdrivers tested are appropriate for the task investigated. A more sophisticated experimental design might be used on a wider range of screwdrivers in order to investigate the problem more thoroughly and determine whether there is a significant interaction between subject and screwdriver characteristics. Screwdriver EvaluationÐAdvanced Example To answer the more detailed questions arising out of the first experiment, we can carry out a technically more sophisticated experiment using more advanced experimental models and more powerful analysis techniques. Apparatus Let us devise a more standardised and flexible set of screwdrivers to use in the evaluation. We will construct experimental screwdrivers with cylindrical knurled handles of 3 cm and 1.5 cm diameters. The handles are designed to hold either of two experimental heads—a conventional blade-type head and a cross (or Phillips) head. Identical screws differing only in head type (slot or cross-head) will be used together with wood of a standard hardness. workbench heads will be adjusted to 15 cm below each subject’s standing elbow height. Procedure The procedure is the same as in the previous experiment. Each subject uses all screwdriver types in random order with a rest between each screwdriver. Subjects are instructed to screw as many screws as possible over a 3 minute period with each handle-head combination. The model used to design the experiment is known as a Split Plot design. We wish to investigate subject characteristics and have decided to partition the sample into males and females. Gender is of obvious interest from a design point

52 INTRODUCTION TO ERGONOMICS

of view since we may wish to make recommendations for the design of “nonsexist” screwdrivers. However, it would be possible to partition the sample in some other way such as between skilled and unskilled users (people whose jobs require them to use screwdrivers on a daily basis versus those who never use screwdrivers). Gender, then, becomes the first factor (which we will label “A”) in our experimental design. The next two factors relate to screwdriver design and are handle diameter (factor “B”) and head type (factor “C”). Each of these have two levels (i.e. two head types and two handle diameters although we could have used more, there is no restriction, per se, on the number of levels we choose for a factor). We will combine them so that each subject works under all levels of each factor (i.e. all factor combinations) in random order. Experiments of this nature are known as “Factorial Designs”. Results The procedure will yield a data set which can be written down as follows: B1 Subjects

C1

B2 C2

C1

C2

13 14 13 9 11 10 70 11.6

9 8 10 4 7 6 44 7.3

A1

A2

Totals mean Where

1. 2. 3. 4. 5. 6.

A1=Males B1=3 cm Handle C1=Slotted head

14 11 13 12 13 12 10 8 10 7 11 8 71 59 11.8 9.8 A2=Females B2=1.5 cm Handle C2=Cross Head

Data are the number of screws inserted in a three minute period. Even for a data set as small as this one, considerable computation is required to generate F-ratios which can be used to test the experimental hypothesis that differences between subject means and tool means are due to sampling error. It is recommended that commercially available software be used to carry out the analysis. For those unfamiliar with the use of such software and the particular data entry procedures required for the analysis, a complete summary of the analysis for the above data is presented below. Also included in the next section is an interpretation of the findings in the context of the actual experiment.

INSTRUCTOR’S MANUAL 53

Summary of Analysis Source

ss

df

ms

F-ratio

significance

Decision Ho

1. A 2. S(A) 3. B 4. AxB 5. S(A)xB 6. C 7. AxC 8. S(A)xC 9. BxC 10. AxBxC 11. S(A)xBxC Total

70.04 2.83 9.37 0.04 4.84 63.37 0.38 1.50 7.04 0.38 2.82 162.61

1 4 1 1 4 1 1 4 1 1 4 23

70.04 0.71 9.37 0.04 1.21 63.37 0.38 0.38 7.04 0.38 0.71

98.65

p<0.001

reject

7.74 0.03

p<0.05 p>0.05

reject retain

166.76 1.00

p<0.001 p>0.05

reject retain

9.91 0.52

p<0.05 p>0.05

reject retain

Discussion The experiment has yielded a number of significant findings which suggest that the differences in task performance are not due to sampling error alone: 1. Sex of Subjects. Subject sex had a highly significant and large effect on task performance (p<0.001). Males completed a mean of 11.9 screws compared to 8.4 for females over all experimental treatments—a difference of approximately 42%. The greater strength of males is likely to be a contributory factor as is a likely greater amount of experience in this type of manual work involving hand tools. 2. Handle Diameter. Handle diameter had a moderate and statistically significant effect on performance (p<0.0). The 3 cm diameter handle produced better performance than the 1.5 cm handle (a mean of 10.8 as opposed to 9.5 screws respectively). The interaction between handle diameter and subject sex was not statistically significant (p>0.05) which can be interpreted to mean that the effect of a larger handle diameter on performance is the same, irrespective of subject sex. 3. Screwdriver Head Design. Head type (standard or cross head) had a statistically significant effect on performance (p<0.001). A mean of 11.75 screws were inserted for the standard screwdriver compared to a mean of 8.6 for the cross head design. This seems to apply both to male and female users because the interaction between subject sex and head design was not statistically significant. 4. Handle-Head Interaction. A statistically significant interaction (p<0.05) was found between handle diameter and head design. The figure below illustrates the nature of this interaction using the column means from the table above.

54 INTRODUCTION TO ERGONOMICS

As can be seen, with the standard design of screwdriver head, the difference between the 3 cm and 1.5 cm handle diameters is small (mean screw numbers of 11.8 as opposed to 11.6). For the cross head screwdriver, handle diameter has a big effect on performance (9.8 screws with the 3 cm handle compared to 7.3 with the 1.5 cm handle). Thus, the standard screwdriver design seems to yield good performance over a range of handle diameters. for the cross head screwdriver, handle diameter can have a much larger effect on performance—small diameter handles are associated with poorer performance. Recommendations The standard design of screwdriver seems best for the experimental task investigated here. A 3 cm handle produced slightly better performance than a 1.5 cm handle. Interestingly, handle diameter seems to have a much bigger influence on the usability of cross head screwdrivers compared to standard screwdrivers. The present data do not permit us to offer a causal explanation for this result although we might speculate that the narrower handle requires greater effort to overcome the resistance of the wood which is incompatible with the need for precision in engaging the tool tip with the cross head of the screw. Further evaluations might also record the number of times users have to engage conventional and cross head screwdrivers when inserting a screw and the number of “misengagements” (e.g. slips, misses, re-engagements, shearing of screw head, etc.) as well as the mean departure from vertical (i.e. perpendicular) at the screw-wood interface. 3. FURTHER READING There are a number of useful references in the mainstream ergonomics literature which can be found in the References section of the main text. The book by PutzAnderson is well-illustrated and deals with many of the practical questions an ergonomist might have about hand tools and upper body problems at work. Pheasant’s (1992) book is also recommended. Individual papers on the design of hand tools can be found in the references section and are quoted in the main text. The model of the development of work-related upper body musculoskeletal disorders by Armstrong et al. is essential reading for anyone considering doing serious research in this area. In addition to the ergonomics journals, The Scandinavian Journal of Work Environment and Health is a good source of research literature. Also worthy of perusal are the American and British Journals of Industrial Medicine and the Journal of Occupational Medicine. Those wishing to carry out surveys of the prevalence of disorders are urged to adopt an epidemiological approach and to consult a basic epidemiological text or the

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NIOSH Occupational Exposure Sampling Strategy Manual (NIOSH 1990) and also Schierhout et al. (1993).

Chapter 6

On completing this chapter, the student should understand:

1. The biomechanics of manual handling. 2. The epidemiological extent of back injury. 3. The main ways the trunk can fail during manual handling operations 4. Other hazards such as slipping, tripping or falling. The student should be able to:

1. Use the NIOSH 1993 equation to specify an RWL for a lifting task. 2. Identify hazardous manual handling tasks and suggest improvements to reduce the hazard 1. COMMENTARY Manual handling is a major area of research in modern ergonomics. Its discussion has been left until chapter 6 for a number of reasons—firstly, it is necessary to have acquired an understanding of anatomy and related postural concepts in order to understand the problems manual handling presents and the solutions ergonomics offers. Secondly, manual handling tasks are dynamic and involve large muscle groups and it is appropriate, in order to ensure a progression of concepts, to follow the discussion of manual handling with a discussion of physiological factors which influence work performance (Chapter 7). 1.1 Anatomy and Biomechanics of Manual Handling A number of basic concepts are needed before students can really appreciate why manual handling of loads is hazardous. In particular, they need to know the role that various anatomical structures play in lifting and carrying of loads and how these structures can fail.

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A knowledge of the anatomical fundamentals is essential before embarking on the study of biomechanical models. From an instructional point of view it is better to begin with a description of the anatomical structures and explain their interaction with handling tasks and then move on to more detailed modelling. 1.2 Design of Manual Handling Tasks As in preceding chapters, a systems approach to the design of manual handling tasks has been taken. This stresses the importance of considering worker characteristics, workspace design and task requirements in designing manual handling tasks. The NIOSH approach is described in some detail and is recommended as a design and evaluation method. 1.3 Carrying A great deal of research has been carried out on the design of lifting tasks and is to be found in the ergonomics literature. Less attention has been paid to what happens to the load and the operator after the load has been lifted. The approach taken in the chapter is to describe the attainment and maintenance of the standing position and the factors which affect postural stability in standing—including the bearing of loads. This is followed by a discussion of walking. Key points in the gait cycle are described to provide a framework for discussing industrial slips, trips, falls and other injuries which can occur when walking and can be magnified when a load is being carried. Personal and workplace factors which increase the risk of injury are described. A considerable amount of work has been done on methods of load carriage— much of it in military applications. Less attention seems to have been paid to this research by industrial ergonomists and it is common to see workers in industry carrying loads in by holding them in front of the body. Apart from the use of trolleys or fork-lift trucks, industry seems to make little use of lifting aids although these are almost always used by people in third world countries (devices such as yokes or head straps are common) who typically carry heavy loads on their heads. For these reasons (the emphasis in the literature on lifting rather than carrying and the under-utilisation of carrying aids) a section on the design of carrying tasks has been added. 2. DEMONSTRATION Using the equipment and procedures described previously, electromyography can be used to demonstrate some of the key concepts about manual handling which were described in the chapter.

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1. Lifting Demonstration: Comparison of Stoop and Squat Lifts

Electrode Placement: Place one set of electrodes over the hamstring muscles at the back of the thigh. Place the other set of electrodes on the low back to detect erectores spinae activity. Subject Position: First ask the subject to stand erect in a relaxed position. From this position, the subject flexes at the hip as if touching his toes while keeping the knees extended. This simulates an extreme version of the use if the “stooplift” to pick something off of the floor. Note how in the fully flexed position there is an absence of activity from the low-back muscles. Ask the subject to straighten-up slowly. It will be observed that the activity in the low-back muscles is only apparent during the last third of the movement. Before this, the spine is flexed and the motion is one of hip extension. During this early phase much EMG activity can be detected from the hip extensors. This demonstrates the involvement of the hip extensors in all lifting activities. It will be observed that the low-back EMG activity occurs during the last part of the movement when the lumbar spine begins to extend and the lumbar lordosis reappears. The exercise can be repeated but with the subject performing a “squat-lift” manoeuvre—one foot is placed in front of the other, the knees are allowed to flex and the trunk is kept more erect. Again, the subject should mimic picking something off of the floor or even be asked to pick up an object of negligible mass such as a piece of paper. It should be possible to detect low-back activity throughout the entire movement. It will also be noted that there is activity from the hip extensors and from the hip flexors for a large part of the lifting cycle. What this demonstration should show is the greater involvement of different muscle groups when the squat lift is used. We can conclude therefore that the squat lift incurs greater metabolic cost which explains why workers do not voluntarily use this method of lifting and why it is rarely observed in developing countries where people have never been trained to lift objects “correctly”. 2. Carrying Demonstration: Load Design and Load Position

Subject Position: Relaxed standing. With the subject in this position, demonstrate the effect on the intensity of the low back EMG activity of holding a 5 kg weight in various positions. Monitor EMG activity from the low back with the subject holding the weight symmetrically at waist height and as close as possible to the body. Ask the subject to slowly move the weight away from the body by reaching forwards. It should be possible to demonstrate an increase in low-back EMG activity as the weight moves away from the body. This is due to the increased load moment caused by the forward displacement of the weight and the arms away from the spine. This indicates the important distinction in the analysis of manual handling tasks between the load and the moment exerted on the lumbar spine by the load. The first depends on the load itself and the second on where it is in relation to the body—both are amenable, in principle, to ergonomic redesign (with some subjects and with a sufficiently sensitive EMG

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monitoring system, it may be possible to detect an increase in low back EMG purely by having the subject stretch his/her arms out in front as in reaching for a distant object. This illustrates the importance, for the spine, of having everything within “easy reach” and can also be used to demonstrate the use of proper armrests or arm support to reduce stress on the low back of standing subjects). It will be observed that when the load is placed on the head, there is a marked reduction in low-back muscle activity compared with holding the weight in front of the body (an increase in activity of the extensor muscles of the neck may however be observed). It is interesting and instructive to repeat the demonstration using a variety of different objects varying in size or shape. 3. Carrying Demonstration: Confined Spaces

Repeat the basic holding task described in (2) above simulate working in a space with restricted headroom (i.e. ceiling height of about 1.5 metres). It should be possible to demonstrate how the subject’s postural adaptations to the restricted headroom increase the load moment about the lumbar spine reflected by an increase in EMG activity. 4. Carrying Demonstration: Carrying Aids

For this demonstration, a variety of carrying aids will be needed and about 20 kg in weights. It is worthwhile to carry out the demonstration using a modern backpack and any other carrying aids available. Subject position: For the backpack demonstration, first have the subject hold the weight in front of and close to the body and monitor low-back EMG activity before and after taking the load. A considerable increase in activity should be readily observed. Next, fit the backpack to the subject and insert the load ensuring it is secure. With the subject in a relaxed position, monitor EMG activity from the low back. Very little activity should be observed because of the reduced flexion moment about the lumbar spine when the load is carried on the back. This demonstrates how load position influences the forces acting on the low back and how well chosen lifting and carrying aids can reduce strain on the low back. It may be worthwhile attempting to detect the presence of a trunk extension moment when the backpack is being used. If sufficiently large, this would cause a noticeable increase in the activity of the abdominal and iliopsoas muscles. With a backpack, under the conditions described here, any extension moment would be small and possibly difficult to detect using the relatively crude instrumentation for which these demonstrations have been designed. With the subject in a standing position and wearing the loaded backpack, monitor EMG activity from the iliopsoas muscles (see previous chapter commentaries for details of the electrode placements) and then the abdominal muscles. Repeat the exercise with the 20 kg weight removed from the backpack.

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A reduction in signal amplitude would be expected. If successful, this demonstration can show convincingly that a well-designed backpack actually takes the load off of the back. 5. Load Carriage: Trolleys

Electrode Position: Monitor activity from the erectores spinae and abdominal muscles. Subject Position: Use either a real trolley or simulate pushing and pulling a trolley by having the subject push or pull against a fixed surface—a railing or fixed bar is ideal. Demonstrate the involvement of the low back musculature in pulling by monitoring low-back EMG activity while the sub pulls a trolley or pulls against a fixed bar. Note an increase in activity when the subject pulls and a further increase in activity when the subject is instructed to extend the lumbar spine during the pull. Repeat the exercise and monitor activity from the abdominal muscles. Some activity may be observable in the form of a reflex Cocontraction of these muscles. A quite different pattern of muscular involvement will be observed when the subject pushes instead of pulls. It will be seen that there is less activity from the low back muscles and more from the abdominal muscles. As in all of these demonstrations, the instructor may wish to experiment with different electrode positions and different muscle groups. In the case of pushing, it is recommended that electrodes be placed over the inguinal canal to detect any iliopsoas/hip flexor activity which is hypothesised to be high during this activity. It may also be of interest to experiment with different bar heights to determine whether these have any effect on the amplitude of the EMG activity in the muscles. As with all of the other demonstrations of electromyography, it is hoped that these exercises enable students to "see under the skin" as it were, and develop a working knowledge of functional anatomy applicable to the analysis of work tasks. 3. ESSAYS AND EXERCISES 1. A worker unloads 20 kg sacks of apples from a conveyor and loads them onto a shute, from whence, they are despatched. He loads for 2 hours per day at a rate of 5 sacks per minute. The height of the conveyor is 60 cms and the height of the shute is 100 cm. there is an angle of asymmetry of 45 degrees and the load is held 30 cm from the body. • Use the equations below to calculate the RWL and the lifting index (LI) • Comment on the safety of the task and identify the risk factors

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RWL=LC×HM×VM×DM×AM×FM×CM Where

The task is not safe. Although the lifting index is less than 3 it is well above 1 and indicates that efforts are needed to improve the task. Listing the multipliers in ascending order we get: FM HM AM DM VM CM

0.6 0.83 0.86 0.93 0.955 1.0

Clearly, the first thing to do is alter the lifting frequency. However, this will result in an increase in the unit cost of handling the sacks (assuming that the worker does other work for the remaining part of his shift). We may have to look at mechanising this part of the operation or changing from lifting to sliding by redesigning the conveyor and the shute. 2. In this essay question, the intention is to get the students to write about the problems of manual handling and heavy work and think about the causes of trunk

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failure. The question is really the verbal counterpart to question 1 and we are trying to get the students to organise for themselves, in a qualitative way, the risk factors for injury. A good answer will exhibit some sort of classification scheme for the various risk factors. There are personal factors, load design factors, work organisation factors (shift work and fatigue), workspace factors (e.g. slippery floors, bad lighting) and other factors such as the corporate “culture” and its attitude to manual handling). 3. This exercise has been included to give students practice in evaluating a wide variety of manual handling tasks, to put the NIOSH work in a wider context and to build awareness in students’ minds of how pervasive the problem is. 4. FURTHER READING The book “Occupational Biomechanics” by Chaffin and Anderson is a key reference for students requiring further information about manual handling. It can be updated by referring to the paper by Waters et al. (1993) on the new NIOSH lifting equation. There are several important papers in recent issues of the journals Ergonomics and Spine. Several of these are referenced in the main text but readers should monitor these journals as new findings are continuously being added to the literature. The NIOSH applications standard for the new lifting equation DHHS (NIOSH) Pub. #94–110 should also be consulted. A good, modern book on manual handling is the one by Ayoub and Mital in the “Further Reading” section.

Chapter 7

On completing this chapter, the student should understand:

1. The meaning of the terms “Stress” and “Fatigue” 2. The basic biochemistry and mechanism of muscle contraction 2. The difference between oxygen dependent and oxygen independent processes and the implications of this for work design. 3. The concept of work capacity, particularly aerobic capacity and maximum oxygen uptake. 4. The physiological constraints on human work capacity and the causes of fatigue. The student should be able to:

1. Recognise physiologically demanding jobs. 2. Suggest ways of characterising the physiological work load. 1. COMMENTARY It is not straightforward to determine an appropriate level of detail in which to present physiological concepts to students on an introductory ergonomics course. The upsurge in interest in sports and exercise physiology has, on the one hand, produced a very large and interesting literature which can be drawn upon to illustrate the practical application of the fundamental knowledge. On the other hand, the industrially developed countries have introduced mechanisation and automation which has reduced the amount of manual work in production. The respiratory and cardiovascular limitations of workers are rarely exceeded on a day to day basis except in specialised occupations. However, it is essential for the ergonomics student to develop a basic understanding of the fundamental physiological concepts used in ergonomics. The provision of energy and muscle physiology are two key concepts which have to be acquired early on by the ergonomics student. The role of oxygen uptake

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and heart rate as both measures of workload and limiting factors on performance should also be stressed. 2. DEMONSTRATION A demonstration of the treadmill or bicycle ergometer test of maximal aerobic power is a useful addition to a series of lectures on physiology in ergonomics. It requires specialised equipment not normally found outside physiology or sports and exercise departments. Instructors based in engineering or other faculties should make suitable arrangements with their colleagues to enable the demonstration to be made. The selection of a subject to engage in the test of maximum aerobic power is best left to experienced teachers or researchers. However, those with a history of high blood pressure, heart problems, chest pains should be excluded as should anyone, no matter how well-trained, suffering from any acute infection such as a cold or flu or anyone on a course of medication. For the treadmill test, the subject is required to run on a treadmill at several intensities while measurements of heart rate and oxygen consumption are made. The subject begins at a comfortable pace for 2–3 minutes before any measurements are made. The slope is then increased by about 1.5 degrees every three minutes until exhaustion. The oxygen uptake data may be plotted against the heart rate data or against the subject’s power output in watts. 3. ESSAYS AND EXERCISES 1. Fatigue. Key Concepts: (a) Recognition of need to document and describe nature of fatigue —Distinguish between localised fatigue of a muscular nature and fatigue due to cardiovascular or respiratory factors (i.e. accumulation of waste products or lack of fuel) —Distinguish between physical and non-physical causes of fatigue (e.g. lack of sleep, boredom, lack of motivation) (b) Description of Investigation Strategy —Describe system of work: Shifts Hours of work Machine-paced or self-paced

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Pace of work Frequency and duration of rest periods System of supervision/management —Characterise workforce/people complaining: Age Sex Qualifications Experience Years on job Years with company State of health —Describe and analyse task: Task description Task Analysis Task requirements: Physical requirements Mental requirements Other (e.g. level of responsibility) (c) Evaluations —If physical workload suspect: Biomechanical analysis Physiological analysis: Oxygen uptake during performance Heart rate -

steady state peak resting recovery

Control measures are, for example: a. Document problem - characterise work demands - characterise work capacity of employees b. Reduce workload of heavy tasks - use more workers - provide better job aids - mechanise

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c. Optimise work-rest schedule d. Optimise shift system 2. First calculate the mean and standard deviation push force: mean=(25+17+30+23+23+24+19+35+34+25)/10=25.5 kg. sd=6 Now, the 5th and 95th percentile push forces: 5th percentile push force=mean−1.64 (6)=15.7 kg 95th percentile push force=mean+1.64 (6)=35.54 kg Since T=−90+126/P−36/P2+6/P3 Where P=Required exertion/Individual’s maximum exertion P (5th percentile)=15/15.7=0.95 5th P (95th percentile)=15/35.4=0.42 T (5th percentile)=−90+126 (0.95)−36/(0.95)2+6/0.953 T (95th percentile)=−90+126 (0.42)−36/(0.42)2+6/0.423 T (5th percentile)=9 seconds T (95th percentile)=87 seconds Clearly, even the 95th percentile endurance time is too low for sustained work. Perhaps the engineers should consider changing the resistance of the valve or providing longer handles. 3. It is quite possible for them to do this, but there will always be a certain arbitrariness about the level that is chosen (because there are difficulties in specifying a particular generic task that will require a certain level of aerobic fitness). More seriously, whatever level is chosen will be confounded by other factors. Most military tasks that require people to work aerobically also require them to carry loads. Thus, absolute body size and absolute weight of the load will confound the VO measure (because it is normalised for body weight). The key issue really is spare aerobic capacity to carry the load. Larger people will have more spare capacity when working at a given level compared to smaller people with the same (relative) aerobic capacity. There are many other issues as well, including equal opportunities legislation, being gender free in selection (most women have lower VO2 max expressed per kg of body weight because of body fat. 4. Selection tests are the FMJ approach and do represent the failure of ergonomics. However, there are many situations where it is not possible to change the environment or provide job aids and the question is really asking the student to think about this issue and to give examples. 5. Use of these terms is quite acceptable as long as their use is preceeded by a scientific definition. 4.

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FURTHER READING The main reference is the book by Astrand and Rodahl (1986) “Textbook of Work Physiology”. Recent volumes of the journals “Ergonomics” and “Applied Ergonomics” may also be perused. The forestry industry remains one of the few which relies very heavily on manual labour and there are several recent physiological papers of interest such as: Hagen et al. 1993. Ergonomics, 36(5):479–488. Trites DG et al. 1993. Ergonomics, 36(8):935–940 Robinson DG et al. 1993. Ergonomics, 36(8):951–962.

Chapter 8

On completing this chapter, the student should understand:

1. The use of direct and indirect methods for estimating oxygen consumption. 2. The physiological basis of effort and its subjective correlates. 3. The use of physiological methods to evaluate tasks. 4. Physiological correlates of mental workload. 5. The difference between health and fitness for work. The student should be able to:

1. Calculate the linear relationship between heart rate and oxygen consumption. 2. Estimate a person’s maximum aerobic capacity from these data. 3. Estimate oxygen consumption from heart rate data. 1. COMMENTARY This chapter aims to provide students with an understanding of human physiology applicable to the evaluation of workload in industrial tasks. Also, it introduces some of the wider physiological issues which affect work performance. The student is encouraged to think of work as entailing a physiological cost which has to be met by rest and recuperation. The measurement of the physiological cost of work is usually done using heart rate and oxygen uptake. The Douglas bag method is briefly described but has been surpassed by less intrusive methods made possible by devices such as the oxylog. If possible, the use of one or other of these methods to evaluate workload should be demonstrated.

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1.1 Indirect Methods of Energy Expenditure Measurement In practical situations it is not always possible to measure oxygen consumption directly because of the intrusive nature of the equipment (face masks, tubing etc.) and so subjects are calibrated in the lab to determine their heart rate/oxygen consumption relationship. There are two major constraints which affect the validity of this approach: 1. The testing situation should be similar to the work situation in terms of climate and the clothing worn by the subject. Variables such as temperature can also increase heart rate independently of the energy expenditure due to the work itself. If a person was tested in a cool laboratory and then observed working in a hot environment, the observed heart rate would be higher in the heat than in the lab and so the oxygen consumption would be overestimated. 2. The calibration method should resemble the work to be evaluated in terms of the type of muscles used. For some tasks it may be more appropriate to use a bicycle ergometer than a treadmill or to use a method which requires the use of the upper as well as the lower body. The data yielded by the calibration procedure are in the form of paired heart rate VO2 measurements made at a number of submaximal levels on each subject, i.e. HR X1 X2 Xn

VO2 Y1 Y2 Yn

Heart rate in taken to be the independent variable and oxygen consumption, the dependent variable (later on, we want to predict oxygen consumption from heart rate and not the other way around). Linear regression analysis can be used to calculate the calibration curve for the data. We want the regression of y (oxygen consumption) on x (heart rate) of the form:

where and

y=oxygen consumption x=heart rate a and b are constants which give the intercept and slope of the line.

The formula for calculating the slope of the line is given by: The formula for calculating the intercept is given by:

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The analyses are done separately for each subject and the equations so derived are personal statements about each subject’s cardiovascular/respiratory functioning. They are unique to each subject. The calibration procedure is completed by determining the upper an lower limits for heart rate and oxygen consumption. The upper limit is the individual’s predicted maximum volume of oxygen uptake in litres/min. It is calculated from the calibration procedure by means of a predicted maximum heart rate given by:

Young persons from 20–30 years of age are assumed to have a maximum heart rate of 200 beats/minute. Maximum oxygen uptake is estimated from the calibration curve by extrapolation, under the assumption that the relationship remains linear at higher levels of heart rate. Suppose an 25 year old individual’s heart rate/oxygen curve was as follows:

given that the maximum heart rate is taken to be 200 beats/min, then maximum oxygen uptake is given by:

Resting heart rate (the lower limit) is defined according to pre-determined criteria in which a basal metabolic level is assumed to be reflected by an oxygen uptake of 0.25 litres/minute. A second calibration curve must be calculated from the data. This is again of the form: However, y is now heart rate and x is now oxygen consumption. That is, heart rate is the dependent variable and oxygen consumption the independent variable. This has to be done because we want to predict a theoretical resting heart rate from a criterion level of oxygen consumption. Suppose the above subject has a calibration curve of For an oxygen consumption of 0.25 litres/min, the resting heart rate will be:

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Having obtained calibration curves for each of the subjects and established physiological limits for them, it is possible to evaluate the physiological cost of performing work using heart rate data taken in the work situation. When moving from the laboratory situation to the work situation this estimate of resting heart rate can be compared with a measurement taken when the worker is at rest. If, in the work situation, the resting heart rate is higher, it may not be valid to use the procedure. 1.2 Practical Applications Physiological criteria for assessing the severity of work sometimes state that an oxygen consumption of 1litre/min can be maintained throughout the workday. However, this takes no account of human variability. Suppose two workers are working at this level and one has a maximum capacity of 2 litres of oxygen per minute and the other a capacity of 4 litres per minute. The first worker will be working at 50% of maximum and the second at 25%. Physiologically, the latter will be working twice as hard as the former, despite the fact that their outputs are the same (this is the major crux of using physiological methods—they tell you how hard someone is working with respect to his/her personal capacity i.e., the effect of work on the worker). It is more meaningful to define workload in terms of the workers’ aerobic capacity. Example

We wish to determine whether a worker can carry out a task for 8 hours (with the usual rest periods). Suppose the worker has a maximum aerobic capacity of 3.99 litres/min. 40% of this (the maximum load for 8 hours) is 1.6 litres/min. The worker’s heart rate is recorded while he is carrying out the task—let us assume a level of 147 beats/ min is recorded. From the calibration curve, the oxygen consumption is then calculated, say 2.64 litres/min. This is: of maximum. Another way of expressing this is as a workload index with respect to 8 hours, in other words:

The worker is working at 165% of the recommended 8-hour work-rate or standard physiological cost. In this case, either the rate of work would have to be lowered or the task redesigned to give more rest periods. The equation of Murrell (see main text) can be used to calculate rest periods.

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2. ESSAYS AND EXERCISES 1. Step test. This can be done as a lab class involving all students. If equipment for measuring oxygen consumption and heart rate is available, it is preferable to do the calibration procedure as described in the chapter. 2. This exercise has been included specifically for instructors who do not have access to a physiology laboratory. Its main purpose is to give students some direct experience of simple physiological measurements that can be used to evaluate workload. A well-written answer will comment on the main trends in the data and on the size of the differences between different activities. 3. This essay gives the student more freedom to explore the concepts of health and fitness and incorporate their own ideas. For example, in developing countries, organisations can provide cheap or free nutritious meals and some degree of primary health care. In developed countries, organisations can counter substance abuse, limit tobacco smoking at work and provide stress management and exercise facilities. 3. FURTHER READING See chapter 7.

Chapter 9

On completing this chapter the student should understand:

1. The basic terminology used in environmental measurement: Dry Bulb Temperature Wet Bulb Temperature Globe Temperature The WBGT Relative Humidity 2. The fundamentals of human thermoregulation: 3. Common thermal problems in the work environment and their amelioration 4. Approaches to the control of the working environment The student should be able to:

1. Use a heat stress monitor to determine wet bulb, dry bulb and globe temperatures. 2. Carry out an evaluation of a working area and evaluate the findings 3. Suggest simple methods of rectifying any problems identified. 1. COMMENTARY The design of the physical environment is an issue which has long attracted the interest of physicists, architects, engineers and medical doctors. In many ways, it is an area of research and development that predates ergonomics itself. For this reason, it is more difficult to specify the knowledge and skills required of an ergonomist in this domain than in others such as the design of workplaces, user interfaces or manual handling tasks. The ergonomist is not an air conditioning engineer or an industrial hygienist but should be able to detect and rectify simple problems and determine when specialist intervention is needed. In keeping with other chapters, the approach to the discussion of the thermal environment proceeds from the person outwards with a discussion of human thermoregulation and the concept of thermal balance. It is unlikely that most

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instructors will have access to a lab where direct calorimetry can be carried out so a less theoretical approach to the concept is taken. Essentially, there are three components to the concept of thermal balance, energy consumed by work, energy converted to heat and heat gained or lost to the environment. 1.1. Measuring the Thermal Environment Students entering ergonomics from disciplines such as psychology or engineering may be unfamiliar with several of the key concepts needed to understand the chapter material. The following notes are included to assist the instructor in conveying these ideas at an introductory level. Modern heat stress monitors facilitate the measurement of thermal environment. For demonstration purposes it is worthwhile to show students the older generation of equipment—the “whirling hygrometer” and globe thermometers using “mercury in glass thermometers”, as well as newer instrumentation. The key points to be learnt in this section are the relationships and differences between dry bulb, wet bulb and globe temperature. The distinction between dry bulb and wet bulb temperature requires some understanding of the concept of “water vapour pressure”. The water vapour pressure of the air at a given temperature can be described loosely, for purposes of explanation, as the “wetness” of the air at that temperature. The first point to bring across is that the hotter the air is, the “wetter” it can be. The water vapour pressure at a given temperature corresponds to the “wetness” of the air. When air is as “wet” as it can be at a particular temperature, it is said to be saturated. Thus, the saturated water vapour pressure of the air rises with temperature (see figure 5.1 in the main text). The dry bulb temperature measures the temperature of all the constituent gases of the air at a particular temperature. Strictly speaking, the energy of the Brownian motion of the molecules in the air is transferred to the thermometer until equilibrium (in the absence of draughts, the thermometer heats-up by conduction) takes place. Thus, dry bulb temperature is not affected by the water vapour pressure. Wet bulb temperature does depend on the water vapour pressure of the air at a given temperature. If the bulb of the wet bulb thermometer is wetted and the thermometer is placed in saturated air, no evaporation from the bulb will take place and the wet bulb and dry bulb temperatures will be the same (because in the absence of evaporation from the bulb of the wet bulb thermometer, the two thermometers are measuring the same thing). If the air is not saturated at a given temperature, evaporation from the wet bulb will occur and the bulb will be cooled down. Thus the wet bulb temperature will be lower than the dry bulb temperature. The lower the water vapour pressure of the air at a given temperature, the faster will evaporation from the wet bulb take place and the lower will be the wet bulb temperature.

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The term relative humidity is used because saturated water vapour depends on air temperature. The relative humidity of the air is the water vapour pressure of the air at a given temperature divided by the saturated water vapour pressure (see figure 9.1). In other words, in order to say how “wet” the air is at a particular temperature we need to know its “maximum wetness” at that temperature and express the measured “wetness” in relation to the maximum. In ergonomics, it is important to know about the relative humidity of the air. If the dry bulb and wet bulb temperatures are known, relative humidity can be found using a psychometric chart such as the one below. Modern instruments are commercially available which give direct measures of relative humidity (this is why no psychometric charts have not been included in the main text). Relative humidity affects the rate of evaporation of sweat and hence the efficiency of evaporative cooling of workers. It is a major determinant of heat stress and of indoor and outdoor comfort. A final term that be usefully be introduced to round-off the conceptual discussion is “Dew Point”. The dew point is the temperature at which the ambient water vapour pressure would be the saturated water vapour pressure. Below the dew point, water will condense. 1.2 Thermoregulatory Mechanisms Discussion of the physiological mechanisms controlling thermoregulation in humans and heat acclimatisation is essential to the understanding of the effects of hot and cold climates on work efficiency. It is important to emphasise the distinction between core and peripheral temperatures since the former can only fluctuate in a narrow range whereas the latter may fluctuate over a much wider range. Examples are given in text of practical ways of protecting workers from extreme climates. 1.3 Effects of Hot and Cold Climates on Performance, Maintaining Performance in Extreme Climates There is a large and complex literature on this subject and in the present text it has only been possible to summarise some of the main findings. First it should be emphasised that hot and cold climates seem to effect performance indirectly— via their effects on the worker’s thermoregulatory system. Hot climates only effect performance if they cause a rise in core temperature. Cold climates effect performance if they lower skin temperature. It may be also that less severe climates effect comfort and this in turn effects performance. However, we can equally well argue that discomfort is a subjective phenomenon which occurs as a result of some or other perturbation to the worker’s thermoregulatory system. The way to prevent performance decrements occurring is to prevent the climate

76 INTRODUCTION TO ERGONOMICS

from affecting the worker (ways of doing this are described). It also seems to be the case that since aerobic fitness and heat tolerance both depend on cardiovascular and respiratory capacity, physical fitness prevents hot climates from affecting performance. Since cold effects performance mainly through its effects on peripheral tissues (nerve conduction, muscle function etc.) increasing worker’s level of skill will also prevent performance decrements. 1.4 Comfort and the Indoor Climate In the industrially developed countries, exposure to climatic extremes at work is localised to particular industries. A more widespread problem is that of providing a comfortable indoor climate. Much work has been done on this over the years but advances in building design and building technology coupled with the requirements for energy savings, mean that this is still an important issue. Several sections have been added to the text to draw the students’ attention to the importance different building design technologies have on indoor climates through their influence on the building’s thermal properties. Ergonomists involved in the planning of new facilities should develop a sensitivity to these issues with regard to there possible consequences for satisfaction and performance at work. 2. DEMONSTRATIONS An optimal demonstration of human thermoregulatory responses can be a valuable addition to the more theoretical material presented in the chapter. However, it requires access to a laboratory fitted with a “hotbox” and instrumentation for remote monitoring of heart rate, body temperature etc. Deep body temperature can be monitored in several ways. For research purposes, rectal temperature is often used. For demonstration purposes, it is adequate to use a probe placed in the external auditory canal. If this equipment is available, then many demonstrations are possible which illustrate important aspects of ergonomics. 2.1 Example Demonstrations 1. Effects of Protective Clothing on Endurance of High Temperatures.

Two subjects sit in the hotbox at a WBGT of 33 degrees Celsius. One wears light shorts only, the other wears protective clothing. Heart rates and body temperatures are monitored over time. If possible, obtain a clothing ensemble worn by a worker such as a fireman or military pilot consisting of heavy boots, thick

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socks, underwear, waterproof trousers, 1 or more jerkins or pairs of overalls, a heavy jacket and a helmet. If a real protective clothing ensemble cannot be obtained one can be simulated using any suitable clothing available (motorcyclists’boots, jacket, helmet etc.). Record heart rate and body temperature over time (about every minute) and note the trends. The semi-naked subject should be able to tolerate the conditions quite well, with little increase in heart rate and skin temperature. The clothed subject should exhibit a steady increase in both heart rate and body temperature as the peripheral blood flow is increased to remove heat from the core tissues. This illustrates some of the problems in designing protective clothing for people who work in hot environments and how performance is reduced. If the resting heart rate of either subject exceeds 120 beats per minute or the body temperature exceeds 38.5 degrees Celsius, remove the subject from the hotbox. Weigh subjects naked before and after the demonstration to obtain an estimate of the amount of sweat lost. Express the sweat loss as a percentage of the subject’s initial body weight and note any differences between the two conditions. It may be of interest to supplement the above demonstration with other conditions. For example, a third subject can be included, also wearing shorts only, but carrying out a task (for example, stepping up and down onto a box every 2 seconds). This can be compared with same task carried out at room temperature (approximately 20–23 degrees Celsius) The differences and similarities between all of these conditions illustrate some of the ways which heat, clothing and workload can impose physiological strain on workers and why the ergonomist must aware of the possibly severe constraints hot environments can have on productivity. 2. Effects of Task/Tool Design on Work Efficiency in the Heat.

This demonstration is intended mainly for instructors who do not have access to a hot box or sophisticated measuring equipment. In hot conditions, the physiological cost of work can be reflected by the amount of sweat lost over the work period. The cost of work depends not only upon the output but also upon the ergonomics of task and tool design. This can be demonstrated in hot weather by carrying out outdoor trials of a variety of tasks. Weigh subjects before and after carrying out specified tasks in hot weather and express the weight loss as a percentage of the body weight. By dividing the work done by the percentage loss, we get an index of work efficiency (i.e. work done per % weight loss). If a running track is available, carrying tasks can be evaluated using a variety of carrying aids. A load of 20kg will be suitable for demonstration purposes, carried, for example, in a backpack , in a suitcase, using a suitcase with wheels or trolley, in two bags, one in either hand, in a sack on the shoulder or on the head. Have subjects walk for an hour in hot weather (or add extra clothing to simulate these conditions) and measure the distance the load is carried (the work

78 INTRODUCTION TO ERGONOMICS

done). This project can be done as a class exercise. An alternative task, if suitable land is available, might be digging with different designs of spade to see if some designs are more efficient than others. 3. ESSAYS AND EXERCISES 1. Measurements in different workplaces. The main purpose of this exercise is experiential. The intention is to give students practice at using the equipment to take measurements in different environments and also to experience different temperatures in these environments. From this, it is hoped that students will develop an intuitive feel for the measurements as well as a theoretical understanding. Measurements should be about 1.2 metres from above the floor and the thermometers should be given 5 minutes to stabilise (or more if manufacturer’ instructions say so). The findings should be presented graphically or in the form of a “league table” for all temperatures. They should comment on the suitability of the environments for carrying out different types of work and the implications for the comfort of people working inside. Reference to published standards and guidelines should be made to assist in the interpretation of the findings. If protective clothing or other measures are needed, appropriate suggestions should be made. The environments will differ not only in the overall WBGT but also in relative humidity and radiant heat and extreme environments should be commented upon. The students should also note any differences between the indoor and outdoor temperatures and comment upon them. As an extension to this exercise, measurements can be made at different times of the day and related to the external temperature and the working activities indoors. 2. This exercise involves measurement of the thermal environment in a modern office. It is intended to provide students with practice at applying their knowledge to the assessment of a working environment. Air conditioned offices do not always have the uniform temperature distribution that they are sometimes thought to have. There may be local differences due to the position of the sun at different times of the day or cold or draughty regions due to the interaction of air inlets and the placement of furniture/screens etc. Clearly, measurement of the temperature in the office will have to be carried out at several different locations possibly using a plan map of the office so that measurements can be related to the position of the sun and air inlets and outlets. The findings of the survey should be evaluated in conjunction with published standards and guidelines. The development of the questionnaire to assess satisfaction is an important part of the survey. When dealing with problems of environmental design it is almost never sufficient to only take physical measurements. The “open-closed” approach to questionnaire design may be taken in which the interviewer begins with open questions before moving on to more detailed, closed questions (open questions imply no particular answer, closed questions permit only a small

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number of answers such as “yes, no, don’t know”, “too hot, too cold, about right” etc. The questionnaire should also characterise the occupants of the room in terms such as age, sex, job title, years in job, years in office. Possible items to be included in the questionnaire are : —An initial rating of occupant’s level of satisfaction with the environment (e.g. a rating scale where 1=totally satisfied and 7=totally dissatisfied). —Likes and dislikes about the environment —Similar, more specific ratings of satisfaction with temperature —Ratings of thermal comfort (possibly on a body diagram) —More specific questions about time of day, time of year, are any positive/ negative aspects associated with the weather outside. Information about the occupant’s working area, closeness to windows, air inlets, outlets etc. 3. Essay on thermal comfort. The topic involves many of the key concepts covered in the chapter. The purpose of this exercise is to provide the student with practice in writing about the main physical and physiological concepts presented in the chapter but with an emphasis on people’s response to their working environment. 4. FURTHER READING For more detailed information about the thermal environment, the reader is referred to the publications of ASHRAE and ISO/DIS7933. The following paper can be used as a point of departure for more advanced reading: Haslam RA Parsons KC. 1994. Using computer-based models for predicting human thermal responses to hot and cold environments. Ergonomics, 3:399–416.

Chapter 10

On completing this chapter the student should understand:

1. The structure and function of the human eye 2. The SI units of lighting 3. How lighting design can influence visual function 4. The causes and cures of “eyestrain” and “visual fatigue”. 5. Other ways in which the ambient light can affect human performance The student should be able to:

1. Carry out a simple lighting survey in an office or factory and interpret the findings 2. Identify common visual problems in the workplace and suggest ways of alleviating them. 3. Comment on the “quality” of lighting in a work place. 1. COMMENTARY This section on lighting is a good example of the classical concept of ergonomics in which concepts from physics and from biology are bought together to analyse and solve practical problems in the workplace. After a brief description of the nature of visible light, we move straight on to a discussion of the structure and function of the human visual system. The discussion is rather more detailed than is usually found in ergonomics textbooks. This is intentional for several reasons. Firstly, vision is dealt with in some depth in keeping with the previous chapters where quite detailed discussions of anatomical and physiological topics took place. It is regarded as essential that students develop a clear understanding of vision before proceeding to a discussion of the applications. Secondly, many jobs nowadays place very severe demands on the visual systems of employees and the requirements for a good visual environment are almost certainly higher today than in previous years. Thirdly, their is a current trend in many organisations to use lower levels of illuminance in order to save energy. This may sometimes

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present a conflict between energy efficiency and ergonomic design. Finally, although the current view is that occupational VDT use neither causes permanent changes in visual function nor exacerbates existing conditions, this may change in future years and the ergonomist will require a fundamental understanding of the structure and function of the visual system in order to follow developments in research and, perhaps, legislation. The discussion of photometric terminology has been kept to a minimum in the main text. Detailed discussion of these terms takes the reader into the field of illumination engineering rather than ergonomics. Instructors may wish to include more detail on this topic and for this reason a number of worked examples are given below which can be used to supplement the main text. An important area of concern for the ergonomist is that of lighting standards. These vary from country to country but are almost always indispensable when carrying out lighting surveys and interpreting the results. Table 10.3 gives a range of recommended illuminances for a variety of tasks. Instructors should consult the regulatory organisations of their own country to obtain more detailed information. 1.1 The VDT Workplace A section on the visual aspects of VDT work has been included since this is perhaps one of the most pervasive visually demanding tasks in modern organisations. However, there does not appear to be anything unique about the visual demands of VDT work, other jobs (such as microscopist) also impose large visual demands and if the student understands the general principles of vision, he or she will be able to apply this knowledge to the analysis of all types of visually demanding work. 2. WORKED EXAMPLES Summary of the main photometric terms and their interrelationships. Luminous Intensity (I) refers to the power of a source to emit light. The light emitted is Luminous Flux which is measured in Lumens Consider a sphere of unit radius. A non-directional source with luminous intensity of 1 candela, place in middle of the sphere will, by definition, emit 4 lumens. These will be evenly distributed to give 1 lumen/steradian. Note, the luminous flux/steradian does not vary with distance from source. i.e. Flux (lumens)=Intensity×solid angle (cd.w) (w=solid angle) F=I.w. The illuminance on the inside of the sphere (ignoring first and inter-reflections) will be 1 lumen/unit area. If the radius of the sphere is 1 metre, then the surface area of the sphere is 4 m^2. Hence, the illuminance, E will be:

82 INTRODUCTION TO ERGONOMICS

Assuming that the surface of the sphere is a perfect diffuser its luminance will be the same as its illuminance i.e. 1 lumen/m^2 or 1 Apostilb Imagine that the sphere is a lamp of 1 metre radius. Seen from below, the lamp appears as a disc of area Its luminance is given by, Therefore, 1 Apostilb=1/ cd/m^2

Apostilbs. Illuminance (E) is given by:

* wd^2=surface area of the curved wavefront 2.1 The Cosine Law This states that the illuminance on any surface varies as the cosine of the angle of incidence between the direction of light and the normal to the surface. Consider the illuminance, En on a plane normal to the direction of the incident light.

The illuminance on a horizontal plane is given by:

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The illuminance on a vertical plane is given by:

2.2 Example 1. A spherical lamp of radius 0.1 m is suspended 2 m above a table whose reflection coefficient is 50%. The sphere emits 1256 1 m.

What is the luminance of the sphere? Its intensity will be the same in all directions Since 1 candela=1 lumen/steradian, the luminous intensity (I) of the sphere is given by: Looking at the lamp, we will observe a disc of radius 0.1m. It surface area is Hence, luminance towards p, the projected area is given by

84 INTRODUCTION TO ERGONOMICS

b. The illuminance at P is given by

If the table were tilted by 60 degrees, the resulting illuminance would be:

c. The luminance of the sphere is given by

Since the area of a sphere is 4 r2 The lumen output of the lamp=10,00×4 ×0.1×0.1=1267 lumens The luminance of the table at p is given by: Where R=50% Since

3. DEMONSTRATION There are many interesting and useful demonstrations which can be made to accompany the lecture(s) on vision, light and lighting. To complement, the more technical material described above, a demonstration of color rendering and quality of light may be useful. A series of lightboxes may be constructed as shown below.

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Different lamps may be installed in each of the boxes. The main purpose of the lightboxes is to demonstrate the different color rendering properties of commercially available lamps. If resources are limited, choose cool and warm white fluorescent lamps, an incandescent lamp and one or more low color rendering lamps used for general industrial lighting. Test materials of varying types can be placed in the boxes and illuminated using the different bulbs. Try, for example, parts of a wiring loom, color photographs of faces, color charts used by paint manufacturers to advertise their products etc. The differences in color rendering ability of the lamps will be made readily apparent. 4. ESSAYS AND EXERCISES 1. This is essentially an experiential exercise to provide students with practice in taking measurements and to give them an intuitive feel for the units of measurement (and also to demonstrate differences between one’s subjective impression and objective measurements). The first measurement will demonstrate how bright it is out of doors even on a dull compared to an indoor working environment. The other environments provide useful readings, which can be compared to published standards. Measuring the illuminances in an office with the lights off should demonstrate how little daylight contributes to the illumination in buildings. 2. The ability to carry out a simple lighting survey is an essential skill requirement for an ergonomist. In addition to measurement of desktop illuminances and comparison with standards, students should be look for sources of glare and shadow and sudden discontinuities between light and dark areas (due to uneven lighting for example). It is often worthwhile to find out the orientation of the building so as to determine the possible effects of sunlight on the occupants’ visual environment and to make several visits at different times of day. Students should take not of the power rating and type of lamps used as well as the glare resistant properties of luminaires. A short questionnaire, similar to that described for the previous chapter, may be of use in determining employee’s subjective responses to the visual environment. Once again, it is suggested that the “open-closed” approach be used and the interviewer first ask employees to rate their level of satisfaction with the visual environment using the 7-point scale. For workers who are dissatisfied with the lighting or other visual aspects, more detailed questions will be needed. Try not to ask leading questions or questions which imply that there is a “right” answer (e.g. “How often do you get spots in front of your eyes?”) rather, try to get the employee to volunteer sources of dissatisfaction and to describe any visual symptoms. Find out if the employees use spectacles for any activities and when they last had their eyes tested (even people who wear spectacles may have poor eyesight because the original prescription is out of date).

86 INTRODUCTION TO ERGONOMICS

The aim should be to combine the objective measurements of illuminance and the layout of the office and lighting system with subjective data to arrive at an overall evaluation of the office. 3. Structure and function of the eye. This is a straightforward essay question which requires students to reproduce the fundamental knowledge described in the chapter. Since the discussion takes place in the context of ergonomics, a wellwritten answer should always be suggesting practical implications of the anatomical and physiological facts. 4. “Lighting should be right not bright”. Essentially, what we are looking for here is the concept of the “U”—shaped cost benefit function. The student should appreciate that lighting costs money and that excess lighting not only wastes electricity and incurs higher maintenance costs but can also degrade performance due to glare. A useful additional exercise would be for the student to carry out some mini lighting surveys in a variety of workplaces (offices, clothing factories, light engineering workshops etc.) and find out from the management how much they spend on lighting per year. Additional data on the performance of visual tasks, such as production, number of rejects, quality etc. should be sought. From this, and a comparison with standards, the student should be able to decide whether the money spent on lighting is being well-spent and whether any costsaving improvements are possible. 5. FURTHER READING The IES lighting handbook is recommended for more advanced study.

Chapter 11

On completing this chapter the student should understand:

1. The terminology used to describe sound 2. The structure and function of the human ear 3. How noise can affect auditory functioning 4. The causes and prevention of noise-induced ear damage 5. Other ways in which noise can affect the human component The student should be able to:

1. Carry out a simple noise survey in a factory or other noisy workplace 2. Identify “safe” areas and areas where ear protection must be worn 3. Suggest simple ways of reducing hazardous noise 4. Comment on the design of and need for auditory alarms and warnings. 1. COMMENTARY The chapter on noise follows on from that on lighting, taking a similar approach to the ordering and description of concepts. The description of sound is somewhat more technical than that of light in the previous chapter. It is regarded as essential that the ergonomics student develop a clear appreciation of the nature of sound and its measurement. It is important to emphasise the logarithmic nature of sound measurement using the decibel scale (particularly when teaching students from non-engineering backgrounds). These concepts are necessary for the analysis of workstations both from the point of view of health and from the point of view of the design of auditory displays (the design of visual displays is returned to in chapter 13, whereas chapter 11 is the only chapter where acoustic stimuli are discussed). In keeping with the previous chapters, a fairly detailed discussion of the structure and function of the ear has been included. This is essential if the student is to grasp firstly the way the ear responds to sound and how excessively loud sounds can damage the ear and secondly concepts such as auditory masking

88 INTRODUCTION TO ERGONOMICS

which cannot be properly understood without a knowledge of how the ear processes acoustic data. 1.1. Measurement of Sound The ergonomics student should learn the basic ways in which sound is measured. Of fundamental importance is a knowledge of: —Sound Pressure Levels —Sound Levels (A-weighted scale) —Frequency Analysis —Measuring Noise Exposure A knowledge of common, commercially available sound level meter is also needed. Photographs of suitable equipment have been provided. 1.2 Design of the Acoustic Environment Some key concepts in building and room design are included to provide students with a wider perspective on some of the options for creating suitable work environments. This leads on to a discussion of industrial noise control and of the design of alarms. The section finishes with a discussion of the effects of noise on task performance and on satisfaction. Some key aspects of vibration and its effects on people are also discussed but not in detail as this is regarded as being a specialised area beyond the scope of the present text. 1.3 Vibration The section on vibration has been expanded considerably. One of the key issues here is vibration white finger. All ergonomists should be aware of the whole body and localised effects of vibration and the existence of standards for exposure. However, measurement of vibration requires specialised equipment and is normally carried out by engineers or by occupational hygienists. It is unlikely that an ergonomist would be asked to carry out this service. It is recommended that instructors cover the basic principles pointing out that noisy equipment often vibrates as well (which is why it is noisy) and that when ergonomists find problems with noisy machines they should recommend that a vibration specialist be bought in to assess the problem.

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2. WORKED EXAMPLES 1. Manual calculation of the LAeq from first principles. A worker is exposed to noise levels of 80 dB(A), 90 dB(A) and 100 dB(A) for 2, 4 and 2 hours respectively over the length of an 8 hour shift. What is the LAeq? The LAeq is given by: LAeq=10 Log (f1 Antilog 80/10+f2 Antilog 90/10+f3 Antilog 100/10)* where f=proportion of time spent at a given noise level with respect to 8 hours. LAeq=10 Log (0.25Antilog 8+0.5 Antilog 8+0.25 Antilog 10)

* All logs to base 10 2. Evaluation of ear protectors. In this example, we wish to illustrate some of the ways in which the different sound measurements can be used in practice. Suppose we have a noisy machine and wish to determine whether it is a threat to hearing and whether ear protectors will provide the operator with the necessary protection. We carry out an octave analysis which yields the following data: Center Frequency (Hz)

SPL

“A-weighted” SL*

31.5 63 125 250 500 1000 2000 4000 8000

80 90 100 95 110 105 101 90 70

40.6 63.8 84.9 86.4 106.8 105.0 102.2 91.0 68.9

Using Table 11.2 (main text) we can calculate the total noise from the machine from these A-weighted SL’s. We take the noisiest and next noisiest frequencies, L1 and L2 and from Table 11.2, look up the amount to be added to L1 when the two are summated: L1=106.8 dB(A) L2=105 dB(A) difference between them=1.8 and from Table, amount to be added to L1=2.3. Combined

90 INTRODUCTION TO ERGONOMICS

Again New

difference between them=6.9 and from Table, amount to be added to L1=0.8 Combined

Again New L1=109.9 dB(A) next L2=91.0 dB(A) difference between them=18.9 and from Table, it can be seen that this has a negligible effect on the total SL of 109.9 dB(A). This exercise demonstrates very clearly that it is only the noise centred at 500, 1000 and 2000 Hz that has much effect on the human operator. In selecting ear protectors we must ensure that the protectors attenuate noise at these frequencies by at least 17 dB, otherwise the operator will still be exposed to noise levels over 90 dB(A). Suppose we have two sample ear protectors, A and B, and data on their noise reduction characteristics. We can compare the suitability of the protectors for use when operating the machine: Center Frequency

SL.

Reduction A

SL at Ear

Reduction B

SL at Ear

31.5 63 125 250 500 1000 2000 4000 8000

40.6 63.8 84.9 86.4 106.8 105.0 102.2 91.0 68.9

−15 −15 −15 −15 −15 −15 −10 −5 −5

25.6 48.8 69.9 71.4 91.2 90.0 92.2 86.0 63.9

−5 −5 −5 −5 −22 −23 −20 −10 −10

35.6 43.8 64.9 65.4 84.8 82.8 82.2 76.0 58.9

We can calculate the total noise at the operator’s ear when the plugs are being used by means of the method described above and Table 11.2. For plug A, the total noise is 96 dB(A) For plug B, the total noise is 88.2 dB(A) So, for noise of this nature, where the most intense components are centred around 500 Hz and above, earplug B seems best. Although this data are fictitious, they illustrate some of the ways in which frequency analysis, combined with A-weighting, can be used to evaluate the

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effects of noise on operators and to predict the effectiveness of protective measures. 3. DEMONSTRATION There are many possible demonstrations, which can be used to accompany the lectures depending on the facilities available to the instructor. If there is an acoustics lab at the instructor’s institution, it is recommended that a visit be arranged so that students can see testing facilities such as the anechoic chamber. In the absence of an acoustics lab, the instructor should demonstrate the equipment available for carrying out noise measurements. The ability to use a sound level meter in ergonomic assessments should be regarded as a minimum skill for an ergonomist. However, noise surveys for assessing compliance with health and safety regulations normally requires the assessor to have some kind of certification. Ergonomist wishing to carry out such assessments will require further training. 4. ESSAYS AND EXERCISES 1. Noise surveys are described in the main text. Ideally, area sampling and personal sampling should be carried out if a noise dosimeter is available. The data in Table 11.5 should also be obtained as well as data on work organisation (for example, do employees have a fixed workplace, does the work involve moving from one place to another, is job rotation practised etc.?). The main objectives of the exercise are: —to characterise the work environment in terms of ambient Sound Levels —to identify the main sources of noise and their locations —to locate, in space, safe and unsafe working areas (with respect to noise exposure) —to suggest suitable forms of ear protection for different categories of worker —to suggest suitable locations for the placement of warning notices (e.g.“you are now entering a noisy area where the wearing of ear protection is compulsory”) —to suggest ways of reducing the noise —to suggest other ways of controlling worker exposure to noise 2. 1 hr 20 mins, approximately, for 3 dB(A) rate. For 5 dB(A) rate the answer is 2 hrs 24 mins. 3. No, it won’t work. All workers will have an 8-hour dose of 97dB(A) assuming a doubling rate of 3 dB (95 dB(A) for a rate of 5 dB). 4. If the workers didn’t take the muffs off, their daily exposure would be 80 dB (A). Plug wearers are exposed to 90 dB(A). Because muff wearers do take their muffs off for 1 hour per day in the noisy environment, they are exposed to 110 dB

92 INTRODUCTION TO ERGONOMICS

(A) for 1 hour. Assuming a doubling rate of 3dB(A) this is equivalent to 101 dB (A) for 8 hours. So, the ear plugs will offer by far the best protection in practice. 5. FURTHER READING The Industrial Noise Manual published by the American Industrial Hygiene Association is an excellent point of departure for further reading in the area. Equipment manufacturers such as Bruel and Kjaer have also produced many useful documents on practical aspects of noise measurement. *The “A-Frequency” weightings are as follows (from Industrial Noise Manual, 3rd Edition, American Industrial Hygiene Association).

Frequency (Hz)

Correction

25 32 40 50 63 80 100 125 160 200 250 315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000 5000 6300 8000

−44.7 −39.4 −34.6 −30.2 −26.2 −22.5 −19.1 −16.1 −13.4 −10.9 −8.6 −6.6 −4.8 −3.2 −1.9 −0.8 0.0 +0.6 +1.0 +1.2 +1.3 +1.2 +1.0 +0.5 −0.1 −1.1

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Frequency (Hz)

Correction

10000 12500 16000 20000

−2.5 −4.3 −6.6 −9.3

Chapter 12

On completing this chapter, the student should understand:

1. The concept of the human as a processor of information 2. The main stages in the human information processing system 3. The control processes which govern information flow, storage and retrieval 4. The differing needs of skilled and unskilled users 5. Key concepts of cognitive ergonomics—mental models and the human as a problem solver The student should be able to:

1. Characterise the main information processing component of simple tasks using Wicken’s model (e.g. psychomotor, short term memory etc.) 2. Describe simple tasks using Rasmussen’s typology (e.g. say whether a task is primarily skill-based, rule-based or knowledge-based). 3. Compare and contrast the information processing demands of a variety of tasks 1. COMMENTARY The material in this chapter is likely to be unfamiliar to most students except those with a background in experimental psychology. For this reason, care must be taken to ensure that the material is conveyed in a way that students can understand. Chapter 12 is the “psychological counterpart” of chapters 2 and 7. It reviews the basic psychological theory that is used by many ergonomics researchers and system designers. Like these other chapters, it is important that the basic concepts be mastered if the student is to understand the following chapters and more advanced literature including research papers. It may be noted that the growth of ergonomics as a discipline has bought many new people into the subject, some with highly specialised backgrounds in professions such as physical therapy or occupational health. Although this is a positive development

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in many ways, there is a disturbing tendency in some countries for ergonomics to be divided into physical and psychological camps, the former being mainly interested in employee health and the latter in product usability and system performance issues. The present book attempts to present a context independent and truly inclusive approach to ergonomics in which physical, physiological and psychological aspects of ergonomics are given equal weight. The student should acquire key concepts in congitive ergonomics and appreciate that these issues are very much alive in the design of the world of electronic information that surrounds us. Instructors should strive to illustrate the psychological ideas using examples from everyday life. 1.1 Comments on the Information Processing Model In presenting the information processing model to students, the aim should be to emphasise the flow of information through various processing stages. We may first distinguish between information that is obtained externally, via the senses, and is processed to suucessively higher (i.e. more abstract, symbolic) levels until such time as a response is initiatied and information that is obtained internally. The response will provide some form of internal (proprioceptive) and/or external feedback which may lead to further information processing or signify the completion of the task. This type of information processing can be described as “psychomotor” when it takes place in tasks such as car driving or games such as tennis. Information can also enter the system from internal sources i.e. from long term memory. Remembering that today you have an appointment at the dentist is an internal stimulus which sets-off a train of behaviour. In cognitive tasks, most of the information processing may be internal as facts and rules are retrieved from long term memory and operated on in working memory until the results can be stored again for future use. 1.2 Coding All information that comes into the human information processing system is coded in some way. All this means is that it is converted into a form appropriate to the level of processing at that particular stage in the system. To use an analogy, in computer systems information is coded in the sense of being represented in different ways depending on the level of the system we are operating on at the time. The letter “d” displayed on a VDT screen bears a close resemblance to its printed (paper) form but is represented in quite a different way in the system’s ROM. The point about coding in ergonomics is that we have the ability, through design, to pre-code information in such a way that it will be most compatible with the level of information processing which occurs while the operator is

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carrying out the task (assuming that we know, or can find out, what level of processing is occurring). In this sense, it can be seen that the concept of coding is fundamental to any discussion of display design. 1.3 Memory The short term/long term memory distinction is an important one. STM can be likened, crudely, to the RAM on a PC and LTM to the hard drive. In ergonomics, STM is of interest in that it presents a major capacity limitation in many tasks where information has to be held and operated on. It is also fragile and currently held items can easily be displaced by new ones. STM limitations underly many problems in the control of complex systems. Bad display design can exacerbate these problems whereas building in external memory aids into the hardware or software interface can ameliorate them. LTM issues are important when we look at the issues involved in learning to use new systems. Thoeries such as Anderson’s provide us with a framework and vocabulary for discussing the design issues involved in acquiring new information. 1.4 Attention and Skill This section is of paramount importance and should not be glossed over. Originally attention was thought of in terms of the capacity limitations of the system for information flow (channel capacity). Nowadays, the multi-channel view emphasises many, modality specific capacity limited channels feeding into an executive processor (working memory or consciousness) which is coupled to modality -specific subroutines (which are built up with practice and can off-load the processor). We can think of the subroutines as being like “plug-in” dedicated modules or “hard-wired” interface cards take “off-load” the CPU on a computer. Many of the ergonomic issues to do with voice control and other novel interface modalities cannot be easily understood without these concepts. Similarly, the concept and definition of skill is an important one. This is because the design issues and requirements for skiled and unskilled operators are completely different. 1.5 Cognition A section on cognition and ergonomics has been included since this is an expanding application. The term “cognitive ergonomics” is gaining popularity despite its many drawbacks. A major thrust of this area is to characterise the tactics and strategies used by operators when controlling a complex system. From

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this characterisation, better decisions about interface design requirements can be made and a more formal approach to interface specification is made possible. 1.6 Problem Solving Most instructors will acknowledge that the topic of problem solving is not an easy one for an introductory course. On the one hand, there is the danger of presenting too advanced material which will not be understood. On the other hand, there is a danger of presenting the material in too superficial a way such that no general principles or concepts emerge. The approach which has been taken is to discuss the factors relevant to ergonomics, which can influence a person’s ability to solve problems and illustrate these using example. These are the factors, which are under the designer’s control. Firstly, working memory limitations can influence the solving of problems which have a high information content that has to be held in memory and acted upon by mental operations. We can therefore improve a person’s problem solving ability in this situation by either reducing the information load (possibly by better integration of system data) or by providing an external memory aid—some sort of external memory support that keeps track of key aspects of the problem solving process. Predictor displays are an example of this type of memory enhancing display (that is, they increase the human-machine system’s working memory and thus its problem solving capacity). In the missionaries and cannibals problem, we can see how memory limitations affect problem solving ability—it is impossible to think more than a one or two steps ahead and we have to keep track of things using a pencil and paper. The solution to this problem is given below. The concept of problem representation is discussed and illustrated using two versions of Wason’s 4 card problem. This is a famous problem in the literature, although it was not intended for use in ergonomics it illustrates the concept of problem representation. One of the best ways to enhance problem solving is by designing the information so that it best discloses the problem’s true nature. The two liquids problem continues this theme and presents several different representations of the same problem, discussing the different mental operations needed to solve the problem depending on which representation is used. It can readily be seen that the way an interface is designed determines not only the information made available to the operator but also the representation of the system and the way problems of system operation are represented. 2. DEMONSTRATION Many demonstrations are possible to illustrate the characteristics of human information processing at various stages. Instructors employed in psychology

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departments will have no problems in arranging these. Instructors employed in other department may wish for their students to visit an experimental psychology laboratory where basic demonstrations can be carried out. Some suggested demonstrations are as follows: 1. Illustration of Short Term Sensory Store (“iconic” memory). Replicate the experiment of Sperling (1960) (see references section, main text). This requires the use of a tachistoscope to present 3×3 letter matrices for short durations (10– 500 milliseconds). A cue is presented to the subject AFTER the brief visual presentation which instructs the subject to identify 3 of the letters in the matrix. Reproduction performance deteriorates as the lag between matrix presentation

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and cue increases, thus demonstrating the existence of a short-term “iconic” store for visual information which decays rapidly. 2. Short Term Memory. Replicate either of the experiments of Murdoch or Petersen and Petersen mentioned in the main text. Present subjects with 3 consonants and have them recall the consonants after counting backwards in 3’s from a particular number. Vary the length of time between consonant presentation and recall. Repeat the exercise and plot the probability of correct recall against the retention time to illustrate decay in short term memory. Generate list of random words of different lengths (say 7–20 words per list). Present them to subjects and have the subject recall the lists immediately and after a 2 minute delay. Plot the probability of correct recall of a word against its position in the list to demonstrate the primary and recency effects. A further elaboration of this experiment is to present lists of words from the same and different categories (e.g., plants, animals, names of countries etc). Are single category lists recalled better, overall, than mixed category lists (i.e. 2 or 3 categories per list). Does this have any implications for the way long codes are designed? 3. Factors Influencing Psychomotor Performance. This demonstration requires the use of a “pursuit rotor” to simulate a tracking task. The subject has to keep a stylus in position on a rotating surface. Performance is scored on the ability to maintain the stylus in place. Many factors can be investigated which influence tracking performance, rotor speed, time spent tracking, amount of practice, intake of alcohol etc. Illustrate some aspects of the multichannel attention model by combining the tracking task with other types of task carried out simultaneously. Can subjects count backwards in 3’s equally well when tracking compared with when doing nothing else? Does it depend on how much practice they have had on the tracking task? 4. Encoding Factors and Memory. Replicate the Craik and Lockhart experiment in simplified form. Divide subjects into 4 groups and give them a list of words each (approximately 30 words). The groups are asked to do different things with the lists: 1. Tick every word that has an “E” 2. Count the number of letters in a word 3. Rehearse the words (repeat them, internally, to yourself over and over again) 4. Find a rhyme for each word 5. Find a synonym for each word 6. Think of a visual image that “goes with” each word Ask the subjects to recall the words immediately, the next day and one week later. Does recall differ between the groups? Which encoding method is best? 5. Problem Solving. Have students carry out the problem solving exercises in their own time but with answers prepared for presentation to the group. Discuss

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the reasons underlying correct and incorrect solutions and the problem solving processes which gave rise to them. 3. ESSAYS AND EXERCISES 1. The purpose of this essay is to get students to think about the human information processing model, to understand its various components and to identify the information processing components of familiar tasks and relate them to the model. For example, short term memory is often involved in following an unfamiliar recipe, following directions as to how to get from one place to another or remembering the details of a phone number. Long term memory is involved in remembering the steps required to replace the dust bag on a vaccum cleaner or setting-up a VCR to record a to-be-broadcasted TV program. Car driving is a common psychomotor task, but there are many others such as sawing wood, laying bricks, making an omlette all of which require a certain degree of manual skill. Attention is required when driving to, for example, navigate through busy traffic (avoiding collisions while remaining aware of traffic signs and directions). Higher level cognitive processes permeate many aspects of daily life, language comprehension is involved in interpreting instructions and manuals, decision making and problem solving are involved in diagnosing and fixing faults with domestic appliances or automobiles. Elements of more complex cognitive control and optimisation can be found in activities such as cooking a dinner consisting of several dishes each with different ingredients. These can be thought of a different subsystems with different dynamic proporties (cooking times and temperatures) which all have reach an end state in a particular future window of time. 2. Paella Recipe Recall Task. This has been chosen as an exercise for students in order to bring some of the ideas out into the world. Subjects should be given approximately 5–10 minutes to memorise the recipe (taking notes or using other forms of external memory is not permitted). They should then be asked to recall the recipe immediately, the next day, after 1 week and after 1 month. The student should record the subject’s remembered version for later analysis. The purpose of the exercise is to give students practice in analysing errors and dealing with “soft” data and also in applying error analysis typologies and methods. There is no fixed way of analysing the data but the students are expected to cast the data into some structured format based on their readings of the material in the book. For example, in remembering the recipe, there are three main types of information: —The ingredients to be used in the dish —The preparation of these ingredients themselves —The sequence of combining them to make the dish Immediately, it can be seen that there are three basic types of error—forgetting ingredients, forgetting how to prepare an ingredient or including an ingredient in

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the wrong sequence. The student may also look for additions to the recipe. These occur when the subject includes other knowledge in an “effort after meaning”. The exercise should be written-up as a report detailing the process of forgetting in terms of the loss of ingredients over time, the procedural errors which occur over time and the inclusion of ingredients or procedures not in the original material. The student might also suggest ways of representing the material to make it easier to remember. 3. The difference between skilled and unskilled performance should be discussed using the model described in chapter 12. The ergonomic implications center around the different needs of skilled and unskilled performers in interface design, the types of error they are susceptible to, the ability of skilled performers to recover from error more easily and the ability of skilled performers to paln ahead. 4. The main task here is for the student to develop some kind of classification scheme for the subject’s verbal responses and then to put these categories of response together in some way, so as to arrive at a representation of the person’s problem solving behaviour. A key component of the exercise is for the student to show how the subject determines the structure of the problem and analyses this representation of structure to arrive at the solution. The student must suggest reasons for the subject’s progress in arriving at a correct or incorrect answer. The student must, of course, thoroughly understand the problem. 5. The algorithm must generate a search tree of the following form: The system must only make legal moves according to the rules of the problem (only 1 or 2 men at a time) and be able to recognise legal and illegal states. There are many possible ways of writing an algorithm to solve the problem and only the main points will be discussed here. One approach is to use a “depth first”

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strategy in which the system always moves men in the order c, mm, cc, mc, m. That is, at the starting point, the system tries to move one cannibal across. It will then have to move the cannibal back to return the boat and will be back at the starting point. Therefore, the algorithm needs some means of detecting loops. The system needs to store a given state, then compare the next state but one with that state. If a loop is detected, the system returns to the initial state and tries moving mm across. Immediately, this leaves the remaining missionary outnumbered and an illegal state is detected so the system returns to the initial state. Now, cc move across, the state is legal and c moves back with the boat. Having completed a legal move, the algorithm repeats the process of building the search tree from the new position. C is moved across, a loop is detected and

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the system backs up 1, mm is moved across, which is illegal and we continue as before. The search tree grows as follows: As can be seen, the depth first strategy gets there in the end but is totally mechanical in its operation. There are however, variations on this theme. The system can be made to back-up by more than step. Unlike with some human subjects, however, the system does eventually solve the problem because it can generate the search space and at any point, it tries all moves at that point until it has reached a dead end. An algorithm for the depth first strategy might look something like this: An alternative approach is the depth first strategy where the system generates all the states that can be reached in one crossing. It then generates all the states that can be reached from those states. Illegal states are abandoned when found.

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The breadth first strategy will find the shortest solution to the problem but for complex problems will require a lot of computation. 4. FURTHER READING Wickens (1992) book is a recommended text for more advanced study. The psychology textbook by Howard is recommended for those who wish to pursue psychological theory more deeply (particularly the information processing

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paradigm). There are many other interesting and easy to read Introductory Textbooks on Psychology including Eysenck and Kean’s Introductory text which was published in 2000. Instructors should seek texts with a bias towards experimental and cognitive psychology since these are of the most relevance to ergonomics.

Chapter 13

On completing this chapter the student should understand:

1. Fundamental considerations for the design of visual displays 2. Principles for control/display integration 3. Advantages and disadvantages of some of the new user-system interaction modes 4. Ergonomic issues in the design and use of virtual environments The student should be able to:

1. Use the gestalt and other principles in conjunction with task analysis data to evaluate control panels. 2. Use task and system analysis to characterise the operator’s information needs 3. Suggest ways of improving the design of a user interface 4. Explain why good interface design is cost-effective 1. COMMENTARY In this chapter an attempt is made to condense information from a large and growing field into a limited space. The approach has been to begin with a description of the traditional “knobs and dials” ergonomics which was developed in the 1950’s and 1960’s and then elaborate on this theme by describing some of the new interface modalities and the issues which have arisen out of technological advance. One of the fundamental ideas that has to be communicated is the concept of figure-ground differentiation. This has visual and perceptual aspects. In Chapter 10 we saw how the retina functions as an edge detector rather than a lightmeter. This is the first step in coding a visual stimulus into a figure and a background. One of the best ways to facilitate the perception of a display as a figure against an unobtrusive background is to enhance its contour.

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The grouping principles are a good way to continue the discussion of display design since they are relatively easy to understand and because they shift the focus of the debate away from purely visual considerations (can the operator see the displays?) to perceptual ones (given that the displays can be seen, can the information that is displayed be correctly perceived?). These principles illustrate the concept of coding which was introduced in the previous chapter—i.e. the perceptual system uses proximity to code an array of dials as a series of rows in figure 13.4. Therefore the information presented on the dial must be compatible with this coding principle (e.g. the information presented on dials within a row is more closely related than the information presented between the rows). There has been a considerable amount of research into the field of visual search because of its importance in military applications and fault detection in complex systems. The literature on this topic is rather difficult and potentially unsuitable for an introductory course. An attempt has been made in the chapter to simplify some of the main findings whilst avoiding making the area seem trivial. The main point to be emphasised is that in the design of complex display panels there are several options for facilitating visual search. These include cueing, which may be specific or general (telling the operator to look for something) and using spatially oriented sound to direct visual attention to a particular part of the display. This discussion can be supplemented with the information in chapter 11 on the design of auditory warnings and cues. 2.1 Computer Displays The CRT is rapidly becoming the standard display device in many industries. The main difference between CRT displays and electromechanical displays is the flexibility available in the former for representing system variables and states in different ways—data can be represented as histograms, pie charts, time series etc. Related to this are purely computer related display issues such as how to represent computer system objects on the confined space of a CRT. This leads into the discussion of maps, icons, navigation aids etc. and onto more complex ideas such as the representation of system structure in hypertext and other systems. 2.2 Other New Developments. Stereoscopy was known to the ancient Greeks and was very popular in the 19th century for viewing photographs. It has long been used by surveyors as a measurement tool and has also been used to make 3-D movies. Split-screen CRT and LCD technology now make 3-D displays on computer screens possible. Synthetic speech can be used as a display device (see also chapter 11).

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Controls

Many of the detailed issues concerning control design are really only relevant to vehicle designers. For these reasons, only the main points have been included. Voice recognition technology makes possible the use of a voice as a control modality. It is already possible to activate and interrogate some systems using voice as an input medium. Voice is a potentially pervasive control modality that can potentially be included into many systems and is actively being researched at present. For this reason, a rather extensive discussion of the problems and prospects for VR technology and voice control has been included. Combining Displays and Controls

This area is of fundamental importance in ergonomics and must be given a high profile in any set of lectures on displays. We are dealing here with some of the oldest and most basic aspects of interface design and introducing concepts such as display control compatibility, population stereotypes etc. Virtual Environments

Virtual environments can be used for training, as in the examples given, or as prototyping aids. There are many ergonomic issues involved, however, most of them are really just extensions of the ergonomic principles used in display design. 3. DEMONSTRATIONS Many demonstrations are possible depending on the time available and the facilities. Some examples are given below: 1. Copy the two dials in figure 13.6. Remove the pointer from the dials and make 10 copies each of the dial faces. Onto the copies, replace the pointers to give 10 different versions of each dial with different readings (make sure that the readings cover the entire range of possible readings on the dial faces. Stick the dials onto cardboard. You should now have two sets of 10 cards one of dial A and one of dial B with a different reading on each pair of cards (i.e. there are 10 different readings in total, each reading appears once on dial A and once on dial B). a. For instructors with access to a tachistoscope. Present the cards to each subject in random order, one at a time. The subject’s task is to say out loud the reading on the dial. Use a voice switch to record the reaction time i.e. the time taken for the subject to read the dial. If a voice switch is not available, have the subject stop the timer manually. Tell the subject beforehand that his task is to read the dials as accurately and a quickly as possible (present one or two test cards beforehand, to orientate the subject to the task). For each card, record the subject’s reaction time and the magnitude of any error i.e. the difference between the actual reading on the dial and the subject’s response.

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Repeat the exercise with 5 subjects. Calculate the mean reading time, mean number of errors and mean errors × time for each subject/dial combination. Cast these data into sets of 3 columns as follows:

sub 1 sub 2 sub 3 sub 4 sub5

Dial A

Dial B

xa1 xa2 xa3 xa4 xa5

xb1 xb2 xb3 xb4 xb5

Analyse the data using the matched-pairs t-test where*: and df=n−1=4 (n=number of subjects) Test the hypothesis that reading time, errors and errors xtime are greater with dial A than with dial B (note, the null hypothesis is that=0, i.e. there is no difference between reading time and errors for the two dials). Calculate a 95% confidence for the true size of the difference as follows: Express the limits of the confidence interval as the percentage improvement in reading performance when dial B is used. *Instructors familiar with the technique may prefer to use ANOVA on the raw scores to estimate the within groups variability b. For instructors with no access to a tachistoscope. Make a third set of test cards to enable subjects to practice the reading task. Give the subject a deck of 10 cards (either of dial A or of dial B). Instruct the subject to sort through the deck as quickly as possible saying out loud the dial reading before moving on to the next card. Record the time it takes the subject to read the entire deck. Record the subject’s responses and compare the subject’s response with the actual reading on the card. Record any discrepancy. Give a score of 0 if the reading is correct. Next give the subject the other deck (dial A or B) and repeat the exercise. Use 6 subjects in total, 3 do deck A first and then deck B and the other 3 do B then A. Shuffle the cards in each deck beforehand. Give each subject 3 practice trials using a mixed set of cards. Analyse the data as above using total time/deck, total errors/deck and time × errors as the dependent variables. Estimate the performance improvement to be obtained using the redesigned dial (if any). 2. Use of Secondary Task to Measure Mental Workload. Carry out a simplified replication of the famous driving studies of Brown and Brown and Poulton that were carried out in the 1960’s. The objective of the exercise is to demonstrate two things: (a) The use of the secondary task technique to estimate mental workload

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(b) How a knowledge of mental workload can have design implications Select a route of about 3 miles in a city near you. The route should pass through all the major (i.e. heavily used) areas. Have subjects drive the route at a busy time of day, carrying out a secondary task such as mental arithmetic (test their arithmetical ability beforehand with no primary task). The experimenter sits next to the subject calling out mental arithmetic problems and recording the time taken for the subject to respond and the number of errors. Repeat the exercise with a number of different subjects. Express secondary task performance as a percentage of the each subject’s solo performance on the secondary task and superimpose these data on a map of the route which was driven. We now have a “mental workload map” of the route which was driven. Try to obtain accident data about the route. Are areas of high mental workload associated with more accidents or fewer accidents? What are the characteristics of high and low workload areas? 3. Simulation of a “Speechwriter”. This demonstration is purely to give students some indication of the possible benefits of speech as an input medium and the problems of designing a speech-driven interface. The speechwriter can be simulated by using a commercial wordprocessing package with one or two modifications to the hardware set-up. Essentially, we will use an experienced user of the package with good typing skills, as the voice recogniser. This person will be situated in a separate room from the VDT and will have only a keyboard. That is, the typist will not be able to see the screen. The user will be situated in another room with the VDT. The user enters voice commands and text into a microphone which are then relayed to the typist/voice recogniser. There are many interesting problems involved in making such as system work. Firstly, we have to construct a spoken command language and chose meaningful words for editing and file management functions which do not conflict (and are different from the spoken text). With graphic user interfaces we have to find a way of combining pointing with a mouse and issuing verbal commands. At the level of text creation, there are differences between spoken and written English which have to be overcome when creating written text by voice. This demonstration can be done as a problem solving exercise, interactively, with the students. 4. ESSAYS AND EXERCISES 1. This exercise can be graded in two parts. Firstly, there is the analysis of the control display relationships and the design of the controls and displays themselves. The following are examples of the criteria students might use:

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Control Design

—Size, shape, color, texture, resistance to operation, resistance to inadvertent operation, location, visibility Displays

—Size, shape color, texture, modality, loudness, pitch, visibility, Control/Display Relationships

—Layout of controls and displays —Mapping of controls onto displays (spatial transformation) —Conforms with stereotype or stereotype not applicable Secondly, there is the question of response adequacy. When a control is activated, what feedback does the system provide to enable the user to judge that the control action has had the desired effect? Feedback may be intrinsic to the process itself (e.g. the sound, vibration and visual feedback of a foodmixer in operation) or it may be extrinsic to the process (a light or buzzer or other such display device deliberately incorporated to indicate response adequacy). A well written answer to this question will show some analysis of the task sequences involved in using the product and will suggest design improvements. 2. Design options for human-machine interfaces. Essentially, this question requires the student to summarise much of the chapter material in the form of an essay. There is no “best” way of answering the question but a well-written answer should be based upon some type of framework for discussing humanmachine interaction. The framework might be based on the task analysis model in Chapter 15 or it could be based around the human-machine model of Chapter 1 looking at the various ways input, processing and output can be supported in hardware. An alternative approach might be operator-centred i.e. looking at the various display and control modalities. A well-written answer should show a good grasp of underlying theory and fundamental principles and the ability to relate this to practical design problems and suggest solutions. Some mention should be made of the use of virtual environments as user-interfaces. A literature search to find examples of V.E.s used in this way would be appropriate. 3. Checklist for Evaluating Displays. This question aims to encourage the student to process the chapter information more actively and to put it in a form more directly applicable to solving practical problems of workspace design. The following items are offered as hints for the preparation of the checklist. 1. Characterise the display requirements of the system. 1.1. 1.2. 1.3. 1.4.

Describe the controlled process Describe the tasks to be carried out using the display Describe the information needed to carry out the tasks Describe the information needed to know when to carry out a task

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1.5. Describe the information needed to know when a task has been successfully carried out. 2. Characterise the variables currently displayed. 2.1. List the names of the variables to be displayed 2.2. Categorise the variables: Physical Variables: Quantitative Variables: Scale of Measurement Nominal Ordinal Interval Ratio Qualitative Variables

3. Evaluate the need for the displayed variables. 3.1 Relate the variables currently displayed to the information requirements analysis in step 1. 3.2 Identify any areas of redundancy or duplication in the current display. 4. Evaluate the layout of the physical interface 4.1 4.2 4.3 4.4 4.5

Grouping of displays Stereotypes Compatibility with controls Compatibility with task sequences Displayed level of detail appropriate for display requirements (e.g. excessive or insufficient detail)

5. Consult operators. 5.1 Likes and dislikes about current display 5.2 Errors and difficulties in relation to common tasks 5.3 Preferred or suggested improvements 4. Essentially, this question is seeking the student’s insights into the issues about spatial transformations, control display relationships and population stereotypes. The question is testing the student’s ability to apply these principles to a practical problem. There are many ways of approaching the problem and the following is recommended as a structured, analytical method. The key point is to identify where the mappings involved in driving differ between the two options. The best option for Mr. Jones is the one that leads to the smallest number of

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changes in his existing perceptual-motor skills—where the demands of the driving task in France map best onto his existing skill repertoire. So, to answer the question, we might list the different subtasks of driving and see whether or not they differ, as follows: Driving in France Own Car (Right hand drive) Operating Car Steering Foot Pedals Indicators Windscreen wipers Radio Other controls Driving on Road Looking Ahead Looking in Mirror Overtaking Pulling Out Traffic lights Spotting targets Avoiding obstacles Navigating Mental Map Real Map Reading Signs

Hire Car (Left hand drive) Same Same Same Same Same Same

Different Different Different Different Different Different

Same Same Different Different Different Different Different

Different Different Different Different Different Different Different

Different Different Different

Different Different Different

As can be seen, there are clear advantages for Mr. Jones in taking his own car since the skills of operating the vehicle still apply. Thus, this part of the task remains automated, at the skill-based level, leaving plenty of spare mental capacity for the other tasks of driving and navigating. We predict, therefore, that Mr. Jones will find it easier to take his own car and recommend this course of action. The exercise should be repeated at a finer level by considering whether each of the above tasks will be easier or more difficult in the two cars. What will happen is that some of the driving manouvres, such as overtaking, and avoiding obstacles (by looking in the rear-view and wing mirrors, for example) will be easier in the hire car (because the fit between the car and the rest of the road system is better). However, overall, more tasks are easier in the right hand drive car than in the left hand drive car.

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5. The circles represent buttons, pressed to steer the wheels. The square is the forward/reverse switch—it has 3 indented modes “off” centre position “move forward”, slide switch to right, “reverse” slide switch to left. The switch always moves back to the off position when the hand is removed The operator stands facing the side of the vehicle so when it moves backwards it moves to his left and vice versa. The following key presses are used to achieve the following manouvres: Move forward and: Steer left Steer right Crab left Crab right Reverse and: Steer left Steer right Crab left Crab right

Flip switch to right Push top right button Push bottom right button Push top right and bottom left buttons Push bottom right and top left buttons Flip switch to left Push top left button Push bottom left button Push bottom left and top right buttons Push top left and bottom right buttons

The buttons cause each wheel to steer independently. There are lock outs to prevent illegal steering configurations (e.g. toe-ing in or out). 5. FURTHER READING There is a very large literature on displays and controls and human-machine interaction issues. Probably the best journal for research into this area is Human Factors. For computer applications, Human Computer Interaction and Behaviour and Information Technology are both good sources of recent research. The book by Wickens on Engineering Psychology can be recommended as the next step for more advanced reading in this area. For a general discussion of human

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interaction with systems the reader is referred to the articles and book of Norman. Details of these publications can be found in the “Further Reading” section of the main text.

Chapter 14

On completing this section, the student should understand:

1. The modern approach to design for usability as exemplified by ISO and other standards 2. The distinction between analogue and symbolic modes of representation 3. The use of memory theory in code and database design 4. The language comprehension model and its application in ergonomics 5. The advantages and disadvantages of various human-computer interaction styles and the kinds of research questions that are investigated by HCI specialists The student should be able to:

1. Evaluate a symbol set used to build a coding system 2. Critically evaluate forms, instructions, warnings and manuals using the language comprehension model and other information in the chapter 3. Suggest improvements to the design of the verbal material described in 2. 4. Together with chapter 13, critically evaluate the design of the user interface of an an interactive system. 1. COMMENTARY This chapter has been included in response to the growing literature on humancomputer interface/interaction issues and the increasingly cognitive nature of the design issues which ergonomics is addressing. In contrast to earlier work on human machine interaction, where most of the emphasis is on perceptual-motor issues and the interaction is skill-based, much of human-computer interaction takes place at a symbolic level. The user and system exchange information, which is represented symbolically, rather than being in analogue format like the movement of a steering wheel and the reading on a speedometer. As is emphasised in the introduction to the chapter, there are fundamental differences between symbolic and analogue reasoning. The design issues in human computer interaction are different from more conventional areas and, as

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has been argued by researchers in the field (such as Long, for example), the principles and methods used in conventional ergonomics to study human-machine interaction are inadequate to tackle many of the most important human computer interaction issues. An approach, based on a level of the cognitive processes involved in the interaction, is necessary. For this reason, an attempt has been made to introduce the student to appropriate concepts and theories of cognitive psychology and to center the discussion of design issues within a cognitive framework. The chapter begins with a review of the ISO approach to the design of interactive systems and describes the various stages that have to be worked through from concept formation to final testing. Next some of the research issues in HCI are reviewed to illustrate the links between design (which is essentially a creative exercise) and science. HCI ergonomics provides the scientific evidence for decision making about user-interface design options. This is followed by some fairly straightforward information about the design of codes. Much of the code information is derived from the work of Bailey and it serves as a useful introduction to some of the practical implications of memory theory. In this way, we begin by showing some of the ways memory theory (and the information processing approach, in general) can be used to understand the problems of interface design and suggest solutions. The discussion then develops to include recent research on the database retrieval. A database is nothing more than an external memory which has to be interfaced with the user’s memory in some way or another. Instead of talking about interface design issues such as the color of the screen or the grouping of items in menus, the discussion is initiated at the cognitive/symbolic level. The work of Lansdale is an excellent example of a cognitive analysis of database retrieval. Central to this is the notion that some knowledge domains are intrinsically easier to decompose into mutually exclusive categories than others. Where ambiguity exists, ergonomics has a role to play in ensuring that the system uses a representation of the domain that is compatible with the user’s representation. To achieve this, ergonomists must find ways of discovering what the users’ representations really are. We have now introduced the notion of cognitive compatibility in system design. 1.1 Human Computer Dialogues There is now a large body of literature on the design of human computer dialogues. In keeping with the rest of the chapter, the approach has been to select a fairly simple framework in which to couch the debate. In this case, the framework of Shneiderman has been used. Within this framework, reference to classic and recent research has been made. Instructors may wish to expand this section using research articles of their choice.

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1.2 Language A fairly detailed discussion of language has been presented in keeping with the philosophy of the book that ergonomics should be based on sound theories and that the teaching of ergonomics should proceed from the theoretical level to the practical level. Given the growing importance of this area, it has been decided to spend considerable effort explaining some of the fundamentals of language, more so than would be found in similar, introductory ergonomics texts. The language comprehension model of Greene and Cromer (see references section, main text, for more details) has been chosen as the framework for the discussion of language design in ergonomics. This model has been developed for the teaching of introductory psycholinguistics and has the advantage of presenting the problem of language comprehension in a hierarchical way, which is compatible with the discussion of ergonomic design issues. It is emphasised that language comprehension is a problem solving process which requires the use of several layers of knowledge. It directs our attention to what the listener or reader needs to know to understand a phrase or utterance. All we have done in presenting the model of language comprehension is to exemplify the various points using examples of ergonomic relevance. There is considerable, some would say excessive, discussion of grammar and syntax. The theory of grammar which was proposed by Chomsky has been briefly described. The purpose of presenting this, now rather old, theory is firstly to establish a formal definition of grammar in which to couch the ergonomics debate and secondly to make students aware that such theories exist, thereby introducing a new avenue for possible further study. Since Chomsky’s theory was the first of its kind, students will find that most discussions of grammar in psycholinguistics texts begin with Chomsky. If students undertake further theoretical reading of grammar they will start where they left off in the present text. A more practical reason for discussing grammar in this depth is that the parsing of sentences is a requirement for natural language interfaces. There is a fundamental difference between understanding an isolated word (as with a “voice button” system) and understanding a number of words put together to communicate an idea more complex than any of the isolated words themselves. It is a non-trivial problem that has to be overcome if natural language interfaces are to be usable. Therefore students should understand, rather more deeply than would otherwise be the case, what a grammar is and why a knowledge of grammar is necessary to understand language. Parsing programs have been written in a variety of programming languages, which illustrate the practical application of grammatical rules to the linguistic analysis of phrases. More generally, students should be beginning to acquire concepts about language design. Language and communication are crucial in all systems, not just those based on information technology. We can talk about

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“ergonomicallydesigned” language in the same way we talk about ergonomicallydesigned seats. The example given in the text (“The Sugar Debate” has been included to illustrate why communication problems occur when the parties have a mutually inconsistent understanding of the words that they use. In fact, the example illustrates the link, first introduced in Chapter 12, between the semantic network and the lexicon (or mental disctionary). Words in memory are linked to nodes in a semantic network. It is quite possible for people to share the same words but use them differently resulting in a failure of communication. If the mapping between the semantic networks of the different parties is poor, communication problems are bound to occur. The theoretical discussion leads directly onto a discussion of applications. The design of visible language has been discussed at length by Wright (see references section, main text) and this work, together with that of Broadbent is described together with the main recommendations of these authors. The design of warnings is discussed here with the emphasis on linguistic issues in design (chapter 11 described other issues in warning design). The chapter finishes with some practical research by Wogalter on the design of warnings and a discussion of the evidence for the cost effectiveness of work in this area of ergonomics. 2. DEMONSTRATIONS Many demonstrations are possible depending on the facilities available. 1. Demonstrations of Code Design. Obtain examples of codes from your own institution or organisations you interface with. Use these as discussion and illustrative material to enhance the contextual relevance of the lecture. Some common examples are: —Bank account numbers —Driver’s licence number —Employee payroll or staff numbers —Medical insurance number Find out on what basis the codes where designed, how many objects the coding system has to discriminate between and how the code is communicated around the system (e.g. computer keyboard, handwriting, verbal relay, telephone, fax etc.). See if you can find examples where the transaction is particularly fragile and error prone. Are numeric codes more common than alphanumeric codes? 2. Communication of verbal material. Obtain a set of instructions for settingup a machine or consumer product or devise a set of instructions of your own. The total number of words should exceed 250 i.e. be enough to impose a considerable memory load on the subjects. Alternatively, use the Paella recipe in chapter 12. The purpose of this demonstration is to show how verbal material gets altered when communicated from one person to another. Write the instructions down

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onto a sheet and read them to a student. Have this student verbally transmit this information to a second student (no other people must be able to hear what is said). The second student then transmits the information to a third, non-cognisant student and so on. Repeat the exercise until the information has passed through 10 students. Have the last student recall the material and present it to the class. Compare with the original material. Identify the omissions, additions and distortions to the original information. This demonstration is based on a famous memory experiment originally carried out by Sir Frederick Bartlett in the 1930’s. It demonstrates how information can be distorted when human memory is relied upon to as the storage medium. 3. Demonstration of human computer interaction styles. Engineering students will probably be sufficiently computer literate that they will not need to have different interaction styles demonstrated to them. Students from other disciplines may not have had such exposure to different types of systems and will benefit from a demonstration, particularly if they do not have programming skills. Obtain sample software based around each of the interaction styles described by Shneiderman. Demonstrate how the interface supports tasks of different types and analyse the skills and knowledge required of users. There may be difficulty in locating examples of a natural language interface and a “form-fill-in” interface. Some commercial programming packages contain demonstrations of limited natural language interfaces to illustrate the power and versatility of their systems (Borland’s Turbo Prolog package for example). The Computer Science Department in your institution may have sample software appropriate for demonstration purposes. “Form-fill-in” interfaces are often found in large institutions such as banks and hospitals for capturing the details of new customers or patients. Obtain an old wordprocessing package that runs on MSDOS rather than Windows or the Apple Mac (such as “Wordstar”). Demonstrate to the students the use of the command language to create, save and edit text. Compare the tasks and knowledge needed to use this system compared to MSWord, for example. Is MSWord really easier to use and easier to learn to use or does it just look nicer? 3. ESSAYS AND EXERCISES 1. Operating instructions of consumer products. Examples of products are, a VCR, a CD player, a “foodprocessor”, a washing machine or a microwave oven. The answer can be divided into two parts. Firstly, there is a straightforward analysis of the language used in the instructions, which is analysed using the language comprehension model: Spelling/location of words: size of letters, color of letters and background, location of letters on product or on manual, typeface style and size. Essentially these are to do with the legibility and visibility of the instructions.

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Normal meanings of words: can an average user be expected to understand the vocabulary—avoidance of jargon, avoidance of technical words, appropriate US or “English” usage. Grammatical rules: sentence length, embedded clauses, avoidance of excessive use of conditional statements etc. Metaphorical uses:

Contextual knowledge: do the instructions depend on the context in which the product is used? Is an understanding of the product’s possible states necessary before an instruction can be understood? Is this information made available to the user and, if not, is it reasonable to expect the user to already know about the product’s possible states (see below)? General Knowledge: Are assumptions made about the general knowledge of the reader (e.g. knowing about the electricity supply system in general, the voltage and frequency of the supply, how to wire a plug etc.? Are these assumptions valid? Secondly, the user may require a knowledge of the system’s possible states before being able to understand the instructions. This is like contextual knowledge and the author of the instructions will either have provided this information or assumed that users will already know about the product’s possible states. In evaluating the instructions, we must also decide whether they can be understood correctly if a knowledge of the system’s possible states is not available for any reason. Detailed discussion of system state analysis is beyond the scope of the present work, but can be illustrated as follows (see the work of Stanton for further information). All systems (in this case products) have a number of finite states which they can be in. A knowledge of a system’s states and the transitions from one state to another are sometimes essential if the instructions are to be understood. For example, a domestic kettle has a number of states: Some kettles automatically switch-off when the water boils, which is an automatic transition from one state to another. There is a correct procedure for moving from one state to another and the user interaction be based around this procedure should be reflected in the instructions (e.g. “don’t switch on an empty kettle”, “wait until the kettle has boiled before emptying”). 2. Evaluation of a menu-based system. There are many ways to carry out this exercise. It can begin with an analysis of the screen layout and use of color using material from the previous chapter. Here we will concentrate on cognitive issues. We can begin by making explicit the menu structure, writing it down on paper in its actual (probably hierarchical) format. This discloses the number of levels in the menu (its depth) and the number of alternatives at each level. Presumably, we would like to see a good balance between depth and breadth—not too deep so

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that the user has to go through too many screens to find out something and not so wide that user is presented with complex screens full of data at each point. Next, we can evaluate the static and dynamic aspects of the menu design at a cognitive level. For the static analysis we need to know something about the knowledge domain which is accessed by the menu. We also need to know something about the users of the menu and how they categorise knowledge in the

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domain and then look for cognitive incompatibility between users’ representations of the domain and the system’s. It is also necessary to characterise any ambiguity of menu alternatives e.g. are mutually exclusive categories used to formulate menu items? Based on these two sources of data, would an alternative menu structure be better (i.e. more compatible, less ambiguous)? For the dynamic analysis, we need to evaluate user interaction with the system, the provision of navigation aids, jump-ahead, jump-back techniques such as command language override, and any other graphical or other types of orientation aid. 3. The icons in Figure 14.2 are (from top left to bottom right): Effectiveness

Identifiability

Open file Save Print Check spelling Cut Paste Undo Inset address Insert Drawing Show/Hide Characters Tip Wizard Bold Align Left Justify Bullets Borders

4. Carry out a literature search in the area. Look for recent research by Wogalter. Do the warning labels comply with ergonomic best practice? What percentage of the warning labels use are composite, such as Figure 14.4? 4. FURTHER READING Interesting discussions of language comprehension and applied psycholinguistics research can be found in Greene and Cromer and in the book by Howard. If these are not available, any modern psychology text should have a section on language. The papers by Wright (see main text, references section) can be recommended for a discussion of the design of visible language and of forms. The journals Behaviour and Information Technology, Human Computer

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Interaction, Human Factors, Ergonomics and Applied Ergonomics are all good sources of research articles on human computer interaction.

Chapter 15

On completing this chapter, the student should understand:

1. The ergonomic approach to Human Error analysis based on the description and analysis if human-machine interaction 1. System design factors influencing operator ability to solve problems and detect errors. 2. Some key characteristics of human decision making 3. The difference between the designers model of the user and the user’s model of the system 4. The elements of Risk Homeostasis Theory and why, when we change one system component, we should not expect the rest of the system to remain as it was The student should be able to: 1. Carry out simple task analyses and comment upon the information requirements and design of task materials and interfaces 2. Carry out analyses of the errors and difficulties experienced by people in problem solving situations 3. Collect and interpret verbal reports of problem solvers and decision makers 4. Suggest ways of improving the design of a system or product to reduce errors and difficulties experienced by users. 1. COMMENTARY As was described in the commentary for chapter 14, cognitive issues are becoming prominent in ergonomics. Somewhat surprisingly, there are few introductory books or articles on cognitive ergonomics. Most of what is available seems to be aimed at graduate students, specialists and researchers. Presumably because the field is a young one, it tends to attract “leading edge” researchers

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who are more interested in pushing forward the boundaries of knowledge than writing for studets on an introductory course. Certainly, the research on human problem solving is both specialised and advanced. It is dominated by cognitive psychologists and by researchers in artificial intelligence. Similarly, human decision making has received much attention by mathematical psychologists and statisticians. Task analysis, however, is a well-established tool in ergonomics and, beyond the simple descriptions given here, is best learnt by doing rather than by reading. 2.1 Human Machine Interaction and Human Error The chapter begins at the user interface as this is where many errors have their cause. Examples from medicine are given to illustrate, in concrete terms, how bad design can increase the likelihood of error. The key concept here is that some designs “invite” people to make errors whereas others don’t. It is through an understanding of the task and the principles of interface design that error prevention can be implemented. Some systems design factors that can increase error likelihood include modes, which are described in some detail. The key point to understand is that modes are less of a problem if they are explicit (e.g. the system or the interface changes colour when the mode changes) than when they are not. Modal structure relates to system complexity and “nested” modes can be a real problem requiring a wellmapped mental model to run with the interaction and keep track of the mode. Mental models are discussed from various points of view and contrasted with system models. It is not essential for users to have the same model as designers, but it is essential for the models to give the same predictions about system behaviour down to and including the physical device level. 2.2 Modelling of Human Operators Although the book is not designed to convert novice students into practitioners, it has been felt necessary to briefly review methods for building a model of an operator’s control strategy. This involves “getting inside the operator’s head” and includes methods such as verbal protocol analysis, structured interviews and questionnaires. The later technique is ubiquitous in ergonomics, we must always consult the users of systems for their comments on any type of ergonomic problem. Verbal protocol analysis is however, a more formal technique and requires some discussion. Students should be given some experience in the use of this technique.

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2.3 User Models of Interactive Systems The paper by Young is a seminal work on user models and should be presented in detail. Students from non-engineering backgrounds may be unfamiliar with RPN calculators and required additional demonstrations or coaching. 2.4 Decision Making The classic paper by Tversky and Kahnemann is the point of departure for the discussion of decision making. It should be pointed out that human decision making is not “wrong” because heuristics are used. If we know the heuristics and how they are used in a domain we can predict when they will be effective and when biases will occur. Designing decision aids to overcome the limitations of heuristic problem solving and decision making is the basis of the notion of “man machine symbiosis”. 2.5 Improving Human Decision Making Some common methods for improving human decision making are described. These are by no means all new or involve complex technology. Biologists have long used pencil and paper techniques to help them classify animals and plants. Rural health care workers in developing countries also use them to make preliminary diagnoses and treatment decisions. A brief introduction to expert systems and artificial intelligence in aiding decision making is given. 2.6 Human Error Reduction Some design options for making systems less likely to invite people to make errors are summarised. These include methods for increasing the likelihood of selfdetection of error. 2.7 Risk Homeostasis Theory Still controversial, instructors should be careful to distinguish between RHT itself and the mechanism of risk compensation. Many experts might reject RHT but allow that there are indeed circumstances when risk compensatio does occur. For students, it is important to know the difference between a theory and one of its constiuent mechanisms.

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3. DEMONSTRATION 1. Decision Aids. Demonstration expert systems are available in many programming packages available for major suppliers. The Department of Computer Science at your institution may have real systems which can be demonstrated to students. 4. ESSAYS AND EXERCISES 1. Vending machines differ in the way they present themselves to users. Some of the key questions are: What states can the machine have (e.g. empty, nothing to sell, goods to sell, but not switched-on in the case of powered machines, full of goods but coin stuck in coin slot or delivery mechanism jammed). These might be called “error states”. Is there anything in the design of the machine that indicates these staes? Examples are; glass front to show available products, machine lights-up when power is on, LED display reads “empty” when the machine is empty etc. If the machine sells a variety of products, how does the user know how to choose a particular one and how to pay the correct amount? Does the machine indicate whether it gives change? Regarding user behaviour, the kinds of errors and difficulties experienced might include; struggling to find out what is available and how much it costs, difficulties deciding how to pay (what coins or notes to use), difficulties knowing how to insert coins or notes (with notes there are interesting problems with spatial transformations in machines that only accept notes in a particular orientation—how is the user advised about this?). Finally, there are difficulties/anxieties waiting for the coin to be accepted by the machine and for something to happen—what re assurance does the machine give the user? Time how long the transactions last and decide whether this is acceptable. Essentially, this is a warm-up question for the next question which involves carrying out a task analysis and identifying legal and illegal operations that might cause errors. 2. Use the format specified in the chapter. First draw the flow diagram from: • • • • • •

Insert ticket Read amount Insert coins/notes Take change Take ticket End

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From this list, for each operation, and describe the: • • • •

Indications when to do the task Control objects and operation Indications of response adequacy Error correction

For each step in the diagram, record average times, note the times for transitions between steps and the time lost due to errors/difficulties inserting notes etc. Use the task analysis to relate the errors and difficulties to the design of the interface and the sequence of steps. Do any machines issue the ticket before they issue change? 3. Examples of tasks that might be usefully studied include: • Driving a tour bus and giving a running commentary to tourists • Operating an automatic teller machine in a foreign country • Driving a car using an in-car navigation system 5. FURTHER READING Salvendy’s Handbook of Human Factors (see Bibliography) contains some useful summary papers on various aspects of cognitive ergonomics. For more on Human Reliability analysis, see Kirwan’s book in “Further Reading” in the main text. Stanton and his colleagues have done a great deal of work on human error and task analysis. Stanton’s edited volume on Human Factors in Consumer Products (is a good palce to start (Taylor and Francis, 1998).

Chapter 16

On completing this chapter, the student should understand:

1. The need for a systems approach to design and the main components of the process 2. The main theories of motivation and job satisfaction and their limitations 3. The sociotechnical and other approaches to job design 4. The concept of macroergonomics and the difference between macroergonomic and microergonomic interventions 5. Why standards are essential if proper quality control of systems design and management is to be achieved 6. The key components and main players in an ergonomics intervention program in industry The student should be able to: 1. Describe the role of the ergonomist in systems design and development 2. Refer to international standards for guidance in the management of the systems design process 3. Relate ergonomics to wider issues of system reliability and efficiency and the well-being of people at work. 4. Explain why psychosocial factors are important mediating variables that can affect the outcome of ergonomic interventions 1. COMMENTARY This chapter is intended to provide an overview of many of the issues discussed in the previous chapter by placing them in a wider context. The focus of attention is shifted away from the individual worker at his/her machine to the role of ergonomics within the context of system design, development and management. In this respect, this final chapter brings to an end our introductory excursion into

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the basic theories, principles and applications of ergonomics and points the way to the study of systems ergonomics and systems theory. The need for a systems approach is discussed with reference to ISO 6385. Most of the early chapters of the book are dominated by analytic thinking about human-machine interaction and have a compartmentalised approach to the analysis of problems. This has been intentional, one must be able to recognise the pieces of a jigsaw before being able to put them together. One of the main differences between the present text and texts on ergonomics is that this section appears at the end of the book rather than at the beginning. This has been done intentionally because it is felt that students should first acquire an understanding of basic principles in their simpler manifestations, before tackling more complex manifestations of problems. The section on work organisation and satisfaction is derived from the literature on occupational psychology. This is the final discipline whose theories have a bearing on the practice of ergonomics and its inclusion is essential in order to complete the description of the components of the Human-Machine Model. No treatment of ergonomics can be complete without some discussion of the factors which compel people to work and which mediate the effects of ergonomic stressors on performance and health. Job enlargement and job enrichment are described together with their advantages and disadvantages. In task analysis terminology, these approaches take place at the level of assignments rather than basic tasks. 2.1 Psychosocial Factors Psychosocial factors are becoming increasingly important and are recognised as important mediators of the effects of ergonomics, or indeed of a lack of it. The attitude of people towards their jobs and towards their colleagues mediates the effects of work stressors. It can go either way—well-motovated employees can cope well with bad working conditions whereas those whose level of motivation and sense of control is poor may find something to complain about in even the most well-designed facility. At the practitioner level, an awareness of these factors is vital if the practitioner is to provide good advice and find a workable way to implement his ideas. At the resrch level, tools such as the “Psychological Aspects of Work” questionnaire may be to be included in the study. 3. DEMONSTRATION One way of summing-up the material presented in the book is to have students generate an ergonomic checklist for the evaluation of jobs. Alternatively the checklist below (which is adapted from the IEA checklist and others) may be presented and each point discussed with the class or in tutorials. The checklist

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covers many of the main points, but can certainly be added to in class, or broken down into a number of more specific checklists covering each of the areas of ergonomics. Ergonomic Checklist 1. Job Analysis 1.1 1.2 1.3 1.4 1.5

What are the main assignments and segments of the job? Is there high physical workload? Is there high mental workload Does the operator have a high level of responsibility? What are the skill/knowledge requirements of the job?

2. Work Organisation 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8

Does the operator work alone or with others? Is the work machine-paced or self-paced? What is the system of supervision and accountability? What shift system is in operation, if any? What are the hours of work and rest periods? Is overtime worked? Is there time pressure due to deadlines or meeting production targets? Is the work carried out on a piece rate basis?

3. Workspace Design 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 3.13 3.14 3.15

Is there sufficient space for the operator to work? Can the worker sit while working at all? Does the position of the body demand static muscular work? Does the workspace permit a stable, neutral posture? Is the worksurface appropriate for the visual and manual requirements? Are foot controls necessary? Do they permit a suitable posture? Are hand controls correctly placed and designed to allow a good upper limb posture? Is the seat height adjustable? does the chair have a backrest? Can seated workers use footrests, armrests, lumbar pads if needed? Are hand tools correctly designed? Are any body parts exposed to constant pressure? Is adequate personal protective clothing provided where needed? Is there any vibration? Are any surfaces hot enough to cause burns?

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4. Physical Demands 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11

Do large forces have to be exerted? Does the work involve lifting or twisting, bending, stooping or reaching? Is muscular work mainly static or dynamic? Are movements centred around the mid-point of the joint range? Can static work be eliminated by providing clamps or vices? Can loads be lifted and carried safely? Is the workload greater than 40% of maximum aerobic capacity? Are large or small muscle groups involved? Can the operator vary the workrate or take rest periods at will? Are lifting aids or powered tools available? Are cycle times less than 30 seconds?

5. Mental Demands 5.1 Does the task carry a high mental workload? 5.2 Is the task carried out at a predominantly skill-based, rule-based or knowledge-based level? 5.3 Does the task place high demands on the perceptual or attentional systems or on short or long term memory? 5.4 How must information be processed before a response can be made? 5.5 Can mental workload be reduced using external memory aids, predictor displays, decision support systems, navigation aids etc.? 5.6 Does the operator have to carry out more than one task at a time and are the task modalities compatible? 5.7 Are the sequences of mental operations compatible with the physical layout? 5.8 Is the representation of the system compatible with the operator’s representation? 5.9 Does information from different channels/modalities have to be integrated? 5.10 Are great demands made on visual search, can cueing be used to reduce these? 6. Human-Machine Interaction 6.1 6.2 6.3 6.4 6.5 6.6 6.7

Does the information presented satisfy the operator’s requirements? Is the rate of information flow too high or too low? How many sources of information does the operator use to work? Are data readily available, in the right form and unambiguous? Are data embedded in noise? Are there many distractions due to noise, speech or other disturbances? Is the layout of controls and displays compatible with prevailing stereotypes?

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6.8 Are controls close to their corresponding displays? 6.9 Are control/display ratios and control dynamics compatible with system order? 6.10 Is the “grain size” of feedback appropriate for the control actions and decisions that have to be made? 6.11 Does the operator have to monitor several channels simultaneously? 6.12 Are warnings, instructions and other displays suitably designed and accessible? 6.13 Does the system provide timely feedback or other indications of response adequacy? 6.14 Are verbal instructions/displays in the correct language and easily comprehensible? 6.15 Are human-computer dialogues user-friendly? 6.16 Is the human-computer interaction style appropriate given the expertise of users? 6.17 Are colors used in an appropriate way? 6.18 Are coding systems compatible with human memory limitations? 7. Work Environment? 7.1 7.2 7.3 7.4 7.5 7.6 7.7

Are temperature, noise, lighting and vibration within recommended limits? Is there excessive brightness or glare in the workspace? Are there sudden loud noises? Does temperature vary throughout the day and are there hot or cool spots? Are there reflective surfaces or hot or cold surfaces? Does the room, have an appropriate reverberation time? Are the colors and reflectances of objects in the environment appropriate for the work? 7.8 Is the relative humidity and ventilation satisfactory? 7.9 Are protective clothing and devices available for workers in extreme environments? 7.10 Can exposure be reduced by taking rest periods in suitable areas or by rotating workers? 7.11 Are there toxic or radioactive chemicals or other hazards in the work environment? 7.12 Are warning signs or other notices placed in appropriate places? 8. Workforce Characteristics 8.1 8.2 8.3 8.4 8.5

Is the anthropometry of the workforce known? Are the workers mainly male or female? What language(s) are spoken proficiently by the workers? What is the average age of the workers? What is the educational level of workers?

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8.6 8.7 8.8 8.9

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Are all workers literate? What is the level of health and firness in the workforce? Are the workers suitably nourished? Are the workers mainly full-time, part-time or seasonal? 4. ESSAYS AND EXERCISES

1. This is an “open-ended” essay which has been included to provide students the opportunity of writing about ergonomics in a general way using information from the book freely. This essay will be of value professionally, providing useful practice in explaining ergonomics to colleagues from other disciplines. Some examples of possible factors include: • • • • • • • • • • • • • • • • •

Demographic trends (e.g. availability and cost of labour) Legislation The Trade Unions The “culture of litigation” Psychosocial factors The profitability of the business The need to add value to products ISO 9000 and its successors International competition and “free trade” Consumer demand Attitudes and values of top management Macroeconomic climate Real or perceived accountability of organisation for employee health Skills and knowledge of practitioners Ability of professional societies and consultancies to market ergonomics Real or perceived consequences of not implementing ergonomics Sensational press reports of disasters or huge compensation payments for “ergonomic” injuries

2. An open-ended question to give students free reign to their imagination and also to further stimulate their interest in the subject and maybe enter a more advanced program of study. 3. Some examples of ergonomic issues are: • Global warming: Change in working hours to include a “siesta” from 12: 00 to 16:00. More night work work and shift work, widespread use of cooling garments, redesign of offices to improve efficiency of heating and cooling systems, more attention to the planting of trees to provide shade

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around buildings and in urban areas, move of businesses away from congested urban areas. New designs of “floodproof” buildings. Thermal physiology makes a comeback in the training of ergonomists • Ozone layer: Work outdoors banned between 11:00 and 15:00hrs, new designs of work hats with wider brims, protective goggles needed at certain times of day to prevent cataracts in the eyes. Ergonomists taught to diagnose skin cancer as part of their training • Demographic ageing: Increase in lean-manning” and an increase in mechanisation and automation, more part-time work to keep retired people in the workforce, more use of computer assisted training and support to make-up for shortages of new skills, decentralisation as more people work at home, the concept of an “employee” becomes fuzzy and everyone is a part-time consultant to everyone else. • Solar/hydrogen power economy. Demise of process industries such as nuclear power, more use of solar panels and wind farms to generate electricity to electrolyse water and provide power. Rise in the percentage of people who work out of doors inspecting and maintaining these facilities, older areas of ergonomics make a comeback. Centres of large cities become increasingly humid as water vapour is the byproduct of the hydrogen fuel cell. Big implications for heat stress in manual work durng the day. Power system becomes increasingly decentralised and new concepts for the representation of the system are needed. Sunny oilprducing countries become hydrogen producing countries exporting the gas by global pipelines. Upsurge in interest in safety and integrity of the system. New ergonomic approaches to system safety are needed with a big emphasis on wearable computers for computerassisted fault diagnosis and maintenance in the field. 5. FURTHER READING For macroergonomics, see the two volumes by Brown and Hendrick (1986) Human Factors in Organisational Design and Management, North Holland Publishers. More recently, there is: Meister D. 1991. Psychology of System Design. Elsevier Science, Amsterdam. For an overview of everything, see The Encyclopaedia of Ergonomics and Human Factors by Karwowski.