Physical Strength Assessment 

in Ergonomics

Sean Gallagher, MS, CPE

Research Physiologist

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

P.O. Box 18070

Pittsburgh, PA 15236-0070

J. Steven Moore, MD, MPH, CIH

Professor

Department of Nuclear Engineering

School of Rural Public Health

Texas A&M University

130B Zachry Engineering

College Station, TX 77843-3133

Terrence J. Stobbe, Ph.D., CPE

Professor

Industrial Engineering Department

West Virginia University

P.O. Box 6101

Morgantown, WV 26505-6101

American Industrial Hygiene Association

Fairfax, Virginia
Copyright 1998 by the American Industrial Hygiene Association. All rights

reserved. No part of this publication may be reproduced in any form or by any

other means — graphic, electronic, or mechanical, including photocopying,

taping, or information storage or retrieval systems — without prior written consent of the publisher.

ISBN 0-932627-88-9

AIHA Press

American Industrial Hygiene Association

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Stock No. 335-ER-98

This book was developed by experts with background, training, and experience in ergonomics and physical strength assessment, working with information and conditions existing at the time of publication. The American

Industrial Hygiene Association (AIHA), as publisher, and the authors have

been diligent in ensuring that the material and methods addressed in this

book reflect prevailing occupational health and safety and industrial

hygiene practices. It is possible, however, that certain procedures discussed

will require modification because of changing federal, state, and local regulations, or heretofore unknown developments in research.

AIHA and the authors disclaim any liability, loss, or risk resulting directly

or indirectly from use of the practices and/or theories discussed in this

book. Moreover, it is the reader’s responsibility to stay informed of any

changing federal, state, or local regulations that might affect the material

contained herein, and the policies adopted specifically in the reader’s workplace.

Specific mention of manufacturers and products in this book does not represent an endorsement by AIHA.
3

Contents

Chapter 1:

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Human Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

Purposes of Strength Measurement in Ergonomics . . . . . . . . . . . . .7

Purpose of this Publication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

Chapter 2:

Isometric Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Definition of Isometric Strength . . . . . . . . . . . . . . . . . . . . . . . . . .11

Workplace Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

Isometric Testing Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

Evaluation According to Physical Assessment Criteria . . . . . . . . . .18

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19

Chapter 3:

Maximal Isoinertial Strength Testing . . . . . . . . . . . . . . . . . . . . . . . . . 21

Definition of Isoinertial Strength . . . . . . . . . . . . . . . . . . . . . . . . . .21

Published Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

Evaluation According to Physical Assessment Criteria . . . . . . . . . .29

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31

Chapter 4:

Psychophysical Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Theory and Description of the Psychophysical Methodology . . . . .33

Why Use Psychophysical Methods? . . . . . . . . . . . . . . . . . . . . . . .33

Published Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

Experimental Procedures and Methods . . . . . . . . . . . . . . . . . . . . .34

Important Caveats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

Related Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

Recommended Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37

Validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .39

Evaluation According to Physical Assessment Criteria . . . . . . . . . .42

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43

Chapter 5:

Isokinetic Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Theory and Description of Isokinetic Strength Measurement . . . . .47

Workplace Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .47

Published Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48

Evaluation According to Physical Assessment Criteria . . . . . . . . . .51

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54

Chapter 6:

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Appendix

Typical Written Instructions for Isometric Strength Testing. . . . . . . . . 59
Acknowledgments

This publication benefitted greatly as a result of the reviews and comments by

a number of preeminent researchers in the field of human strength assessment.

The authors would like to take this opportunity to thank the following authorities for graciously imparting their insight and expertise in reviews of all or part

of the present document: Dr. Don B. Chaffin (University of Michigan), Dr.

K.H.E. (Eb) Kroemer (Virginia Tech), Dr. Joe McDaniel (U.S. Air Force), Dr.

Vince Ciriello (Liberty Mutual Insurance Co.), and Dr. Stover Snook (Harvard

School of Public Health).

4
5

1. Introduction

Humankind’s interest in measurement of human physical strength probably

dates to the first humans. At that time, life was truly a struggle in which the

fittest survived. To a great extent, fittest meant strongest. Interestingly, current

interest in human physical strength in the workplace stems from 1970–1980s

vintage research demonstrating that persons with adequate physical strength are

less likely to be injured on physically demanding jobs. Survival in many modern workplaces may still be a case of survival of the strongest.

There is, however, a flip side to this issue — that persons with limited

strength are more likely to be injured on “hard” jobs. To address this problem,

we can apply what we know about physical strength to job design. “Hard” jobs

can be redesigned to be within the physical strength capability of most people.

Since physical strength is important to these jobs, we must find ways to quantify it through testing.

This publication concerns human physical strength testing. Its purpose is not

to recommend any particular type of testing, but rather to describe the types of

testing available and their uses. It is up to each individual user of strength testing to decide which testing technique is most appropriate for his or her particular application. This booklet discusses four types of strength testing: isometric,

isoinertial, psychophysical, and isokinetic.

Human Strength

Before describing the different types of strength measurement, we must define

the term “strength” and explain the concept of strength measurement. Strength

is defined as the capacity to produce force or torque with voluntary muscle

contraction. Maximum strength is defined as the capacity to produce force or

torque with a maximum voluntary muscle contraction.(1,2) These definitions

include some key words that must be explained.

A voluntary muscle contraction is “voluntary.” When a person’s physical

strength is measured, only the effort the person willingly puts forth at the time

is measured. Thus, when we test a person’s “maximum strength,” we are not

measuring his or her actual maximum, but some lesser value representing what

he or she is comfortable expressing at the time with the existing equipment and

environmental conditions. Interestingly, when researchers startled persons being

tested (e.g., by setting off a starter’s pistol behind them), they have found significant increases in measured strength.(3) It has been hypothesized that the

lower strength displayed during normal testing provides a margin of safety

against overloading and damaging muscle tissue. The test equipment and the

tested person’s familiarity with the process also influence the “voluntary”

strength output. The interface between the tested person and the test equipment

is particularly important. A poorly designed interface induces localized tissue

pressures that vary from uncomfortable to painful. In this situation, testers are

measuring voluntary discomfort tolerance — not strength. It is important for

strength researchers to keep the “voluntary” nature of their data in mind when

they are designing their equipment and protocols.
The definition of strength also involves force or torque. Strength researchers

and users of strength data must understand this distinction. We commonly use

the terms “muscle force” and “muscle strength” to describe the strength phenomenon. Technically, this is incorrect. In most human movements and force

exertions, a group of individual muscles (a functional muscle group) actually

works together to produce the observable output. In complicated exertions, a

number of functional muscle groups work together to produce the measured

output. Elbow flexion strength, for example, is the result of the combined

efforts of the biceps brachii, brachialis, and brachioradialis; and a squat lift is

the result of the combined efforts of the legs, back, and arms. In elbow flexion,

each individual muscle’s contribution to the functional muscle group’s output

depends on the posture of the arm when being tested. Thus, when we measure

elbow flexion strength, we are measuring the strength of the elbow flexor muscle group, not the strength of any individual muscle. 

Furthermore, we are measuring (recording) the force created by the functional

muscle group(s) against the interface between the person and the equipment (a

set of handles, for example). Consider the elbow flexion measurement depicted

in Figure 1. The force generated by the elbow flexor muscle group is shown by

Fm. This force acts through lever arm “a.” In so doing, it creates a torque about

the elbow joint equal to Fm x a. The measured force (Q, R, or S) depends on how

far (b, c, or d) the interface (force cuff) is from the elbow. Assuming that the

exertion is static (nothing moves) in this example, the measured force (on the

6

Figure 1—

Given a constant

muscle force (Fm),

forces measured at

various distances

from the elbow will

result in different

force readings (FQ,

FR, or FS).
7

gauge) will equal the elbow flexor torque divided by the distance that the

gauge’s associated force cuff is from the elbow joint. That is,

Q = (Fm x a)/b (1)

or R = (Fm x a)/c (2)

or S = (Fm x a)/d (3)

As we move the interface (force cuff) from the elbow to the hand, the measured force will decrease. This example highlights four points. First, as

Kroemer et al. wrote in the International Journal of Industrial Ergonomics,

“muscular strength is what is measured by an instrument.”(4) Second, people

publishing or using strength data must report or understand in detail how the

measurements were done. Third, the differences in published strengths of the

various body parts may be due to differences in the measurement methods and

locations. Fourth, interface locations selected using anthropometric criteria will

result in more consistent results across the population measured.(5)

In summary, a record of a person’s strength describes what the instrumentation

measured when the person voluntarily produced a muscle contraction under a

specific set of circumstances with a specific interface and instrumentation.

Purposes of Strength Measurement in Ergonomics

People may want to collect human strength data for a number of reasons. One

common reason is to build an anthropometric database of population strength

data that can be used to create design data for products, tasks, equipment, and

so forth, as well as for basic research into the strength phenomenon. This publication focuses on two common uses of physical strength assessment in

ergonomics: worker selection and placement and job design.

Worker Selection and Placement

Worker selection and placement programs ensure that jobs involving heavy

physical demands are not performed by those who lack the necessary strength

capabilities.(6) It should be noted that this method is not the preferred strategy

of the ergonomist; it is a provisional measure for controlling work-related musculoskeletal disorders (WMSDs) when job design cannot be used to alleviate

task demands. Nonetheless, this method can be effective in reducing the harmful physical effects caused by the mismatch of worker and job, given adherence

to two fundamental principles: ensuring that (1) the strength measures closely

simulate the actual high-strength elements in a job and (2) strength assessment

is performed only under circumstances where those who may be at risk of

WMSD can be predicted. The following paragraphs describe these issues in

more detail.

It has become quite clear over the past several years that strength, in and of

itself, is a poor predictor of the risk of future injury to a worker.(7-9) A worker’s

strength capacity predicts risk of injury only when it is carefully equated with

job demands.(10) All too often, collecting data on individual workers’ strength is

emphasized, while evaluation of actual job demands receives little or no
attention. Recent evidence shows that job demands cannot be generalized as

“light” versus “heavy”;(11) a careful biomechanical evaluation of strenuous tasks

performed by the worker needs to be done.

The following scenario illustrates the need to analyze strength in relation to

specific job demands: An employer has an opening for a worker in a physically

demanding job and wishes to hire an individual with strength sufficient for the

task. This employer decides to base his employment decision on a strength test

given to a group of applicants. Naturally, he selects the applicant with the highest strength score to perform the job. The employer may have hired the

strongest job applicant; however, he may not have decreased the risk of injury

to his employee if the demands of the job still exceed this individual’s maximum voluntary strength capacity. This example should make it clear that only

through knowing both the person’s capabilities and the job demands can worker selection protect workers from WMSDs.

The second issue to be considered when implementing worker selection is the

test’s predictive value. The predictive value is a measure of the test’s ability to

determine who is at risk of future WMSD.(6) In the case of job-related strength

testing, the predictive value appears to hold only when individuals are tested for

jobs where high risk is known (i.e., for jobs known to possess high strength

demands). Strength testing does not appear to predict the risk of injury or disease to an individual when job demands are low or moderate.

It should be clear from the preceding arguments that worker selection procedures are not the preferred method of reducing the risk of WMSDs, and they

should not be applied indiscriminantly in the workplace. Instead, care must be

exercised to ensure that these strength testing procedures are applied only in

select circumstances. This procedure appears to be effective only when jobs

are: known to entail high strength demands, and only when the worker’s

strength is evaluated in the context of those demands. However, if attention is

paid to these limitations, worker selection can be an effective tool to decrease

the risk of WMSDs.

Job Design

Physical strength assessment in ergonomics can also be used in job design. Job

design has been a primary focus of the psychophysical method of determining

acceptable weights and forces. Rather than determining individual worker

strength capabilities and comparing these to job demands, the psychophysical

method attempts to determine workloads that are “acceptable” (a submaximal

strength assessment) for populations of workers. Once the acceptable workloads for a population are determined, the job or task is designed to accommodate the vast majority of that population. For example, a lifting task might be

designed by selecting a weight that is acceptable to 75% of females and 90% of

males. Strength assessment in job design has been shown to be an effective

method of controlling WMSDs. Proper design of manual tasks using psychophysical strength assessment has been estimated to reduce the risk of back

injuries by up to 33%.(12)

8
9

Purpose of this Publication

Muscular strength is a complicated function that varies greatly depending on

the assessment. As a result there is often a great deal of confusion and misunderstanding of the appropriate uses of strength testing in ergonomics. Not

uncommonly, these techniques are misapplied by persons who are not thoroughly familiar with the inherent caveats and limitations of various strength

assessment procedures. The purposes of this publication are (1) to familiarize

the reader with the four most common strength assessment techniques used in

ergonomics (isometric, isoinertial, psychophysical, and isokinetic); and (2) to

describe the proper applications of these techniques in controlling WMSDs in

the workplace.

Four chapters cover these four strength measurement techniques. Each chapter describes the strength measurement technique and reviews the relevant published data. Equipment considerations and testing protocols are described, and

the utility of the tests in the context of ergonomics is also evaluated. Finally,

each chapter concludes with a discussion of the measurement technique with

regard to the Criteria for Physical Assessment in Worker Selection.(6) Each

measurement technique is subjected to the following set of questions:

1. Is it safe to administer?

2. Does it give reliable, quantitative values?

3. Is it related to specific job requirements?

4. Is it practical?

5. Does it predict risk of future injury or illness?

This publication is intended as a resource for better understanding and proper

application of these strength assessment techniques in the effort to reduce the

risk of WMSDs.

References

1. Caldwell, L.S., D.B. Chaffin, F.N. Dukes-Dobos, K.H.E. Kroemer, L.L.

Laubach, S.H. Snook, et al.: A Proposed Standard Procedure for Static

Muscle Strength Testing. Am. Ind. Hyg. Assoc. J. 35:201–206 (1974).

2. Chaffin, D.B.: Ergonomics Guide for the Assessment of Human Static

Strength, Am. Ind. Hyg. Assoc. J. 36:505–511 (1975).

3. Ikai, M., and A.H. Steinhaus: Some Factors Modifying the Expression of

Strength, J. Appl. Physiol. 16:157–163 (1961).

4. Kroemer, K.H.E., W.S. Marras, J.D. McGlothlin, D.R. McIntyre, and

M. Nordin: On the Measurement of Human Strength. Int. J. Ind.

Ergonomics 6:199–210 (1990).

5. Stobbe, T.J.: “The Development of a Practical Strength Testing Program

in Industry.” Ph.D. diss., University of Michigan, Ann Arbor, Mich., 1982.

6. Chaffin, D.B., and G.B.J. Andersson: Occupational Biomechanics, 2nd

Ed. New York: John Wiley & Sons, 1991. pp. 464–466.

7. Troup, J.D.G., J.W. Martin, and D.C.E.F. Lloyd: Back Pain in Industry.

A Prospective Study. Spine 6:61–69 (1981).
8. Battie, M.C., S.J. Bigos, L.D. Fisher, T.H. Hansson, M.E. Jones, and

M.D. Wortley: Isometric Lifting Strength as a Predictor of Industrial Back

Pain. Spine 14:851–856 (1989).

9. Mostardi, R.A., D.A. Noe, M.W. Kovacik, and J.A. Porterfield:

Isokinetic Lifting Strength and Occupational Injury: A Prospective Study.

Spine 17(2):189–193 (1992).

10. Chaffin, D.B.: Ergonomic Basis for Job-Related Strength Testing. In

Disability Evaluation, 1st Ed. (S.L. Demeter, G.B.J. Andersson, and G.M.

Smith, eds.). Mosby, St. Louis, Mo., 1996. pp. 159–167.

11. Mooney, V., K. Kenney, S. Leggett, and B. Holmes: Relationship of

Lumbar Strength in Shipyard Workers to Workplace Injury Claims. Spine

21:2001–2005 (1996).

12. Snook, S.H.: The Design of Manual Handling Tasks. Ergonomics

21(12):963–985 (1978).

10
11

2. Isometric Strength

Definition of Isometric Strength

Isometric strength is defined as the capacity to produce force or torque with a

voluntary isometric (muscle[s] maintain[s] a constant length) contraction. The

key thing to understand about this type of contraction and strength measurement is that no body movement occurs during the measurement period. The

tested person’s body angles and posture remain the same throughout the test.

Isometric strength has historically been the type most studied and measured.

It is probably the easiest to measure and to understand. Some strength

researchers feel that isometric strength data may be difficult to apply to some

“real life” situations because in most real circumstances people are moving —

they are not static. Other researchers counter that it is equally difficult to determine the speed of movement of a person or group of persons doing a job (each

moves in his or her unique manner and speed across the links and joints of the

body). Thus, dynamic strength test data collected on persons moving at a different speed and/or in a different posture from the “real world” condition will

be just as hard to apply. In truth, neither is better — they are different measurements. Both researchers and users should collect and use data that they understand and that fits their application.

Workplace Assessment

When a worker is called on to perform a physically demanding lifting task, the

external load produces moments — tendencies to produce motion, also called

torques — about various joints of the body.(1) Often these moments are augmented by the force of gravity acting on the mass of various body segments.

For example, in a biceps curl exercise, the moment produced by the forearm

flexors must counteract the moment of the weight held in the hands as well as

the moment caused by gravity acting on the forearm’s center of mass. To perform the task successfully, the muscles responsible for moving the joint must

develop a greater moment than the combined moments of the external load and

body segment. It should be clear that, at each joint of the body, there is a limit

to the strength that the muscle can produce to move ever-increasing external

loads. This concept forms the basis of isometric muscle strength prediction

modeling.(1)

The following procedures are generally used with this biomechanical analysis

technique. First, workers are observed (and usually photographed or videotaped) as they perform physically demanding tasks. For each task the posture of

the torso and the extremities are documented at the time of peak exertion. The

postures are then re-created using a computerized software package, which calculates the load moments produced at various joints of the body as the task is

performed. The values obtained during this analysis are then compared to population norms for isometric strength obtained from a population of industrial

workers. In this manner, the model estimates the proportion of the population
capable of performing the exertion, as well as the predicted compression forces

acting on the lumbar discs as a result of the task. 

Figure 2 shows an example of the workplace analysis necessary for this

approach. Direct observations of the worker performing the task provide the

necessary data. For example, one must know the load magnitude and direction

(in this case, a 200 N load acting downward), the size of the worker, the postural angles of the body (obtained from photographs or videotape), and whether

the task requires one or two hands. Furthermore, the analysis requires accurate

measurement of the load center relative to the ankles and the low back. A computer analysis program can be used to calculate the strength requirements for

the task and the percentage of workers who would be likely to have sufficient

strength to perform it. Results of this particular analysis indicate that the muscles at the hip are most stressed; 83% of men but only slightly more than 50%

of women would have the necessary strength in this region. These results can

then be used as the basis for determining which workers have adequate strength

for the job. However, such results can also be used as ammunition for recommending changes in job design.(1)

12

Figure 2—Postural data required for analysis of joint moment strengths using

the isometric technique. (From Occupational Biomechanics, Chaffin, D.B., and

G.B.J. Andersson, ©1991 by John Wiley & Sons, Inc. Reprinted by permission

of John Wiley & Sons, Inc.)
13

Isometric Testing Protocol

The basic protocol for isometric strength testing was developed by Caldwell et

al.(2) and published in an AIHA ergonomics guide by Chaffin.(3) The protocol

outlined herein includes additional information determined by researchers since

that time. When conducting isometric testing, a number of factors must be considered and controlled (if possible) to avoid biased results. These factors

include:

● Equipment used to make the measurements;

● Instructions given to the person tested;

● Duration of the measurement period;

● Person’s posture during the test;

● Length of the rest plod between trials;

● Number of trials a person is given for each test;

● Tested person’s physical state at the time of testing;

● Type of postural control used during the tests; and 

● Environmental conditions during the test.

Test Duration

The length of an isometric strength test impacts the result in two ways. If it is

too long, the subject will fatigue and the strength score will decline. If it is too

short, the subject will not reach his or her maximum force level before the test

is terminated. Chaffin suggests a 4-second test, with the score being the average strength displayed during the second through fourth seconds.(3) The appropriate 3-second period is determined as follows:

If the measuring equipment has the capability, collect strength data by having

the person begin contraction with the equipment, monitor the force until some

preselected threshold is reached (usually 20%–30% below the expected maximum force for the person and postures, have equipment wait 1 second, and

then have the equipment average the displayed force for the next 3 seconds.

This is easily done with computerized systems.

If the equipment does not have this capability, have the person tested begin

the test and gradually increase his or her force over a 1-second period. The

force should be measured and averaged over the next 3 seconds. In complex

whole body tests involving multiple functional muscle groups, persons may

take a few seconds to reach their maximum. Under these conditions, the data

collector must adjust the premeasurement time interval accordingly and carefully monitor the progress of the testing to ensure that the maximal force during the 3-second period is, in fact, being measured.
Instructions

The instructions to the person tested should be factual, include no emotional

appeals, and be the same for all persons in a given test group. This is most reliably accomplished with standardized written instructions since the test administrator may reveal feelings about the testee or the desired outcome during verbal

instruction.

The following additional factors should also be considered. The purpose of

the test, the use of the test results, the test procedures, and the test equipment

should be thoroughly explained to the persons tested. Generally, the anonymity

of the persons tested is maintained, but if names may be released, the tested

person’s written permission must be obtained. Any risks inherent to the testing

procedure should be explained to the persons tested, and an informed consent

document should be provided to and signed by, all participating persons. All

test participants should be volunteers.

Rewards, performance goals, encouragement during the test (e.g., “pull, pull,

pull, you can do it”), spectators, between-person competition, and unusual noises all affect the outcome of the tests and must be avoided. Feedback to the tested person should be positive and qualitative. Feedback should not be provided

during the test exertion but may be provided after a trial or when the test is

complete. Quantitative results provided during the testing period may change

the person’s incentive and thus the test result.

To the tested person, a 4-second maximal exertion seems to take a long time.

During the test, tester–testee agreed-on feedback, such as a slow 4 count,

should be provided so the tested person knows how much longer a test will last.

Rest Period Length

Persons undergoing isometric strength testing generally perform a series of

tests, with a number of trials for each test. Under these conditions, localized

muscle fatigue must be avoided since it will result in underestimating strength.

Studies by Schanne(4) and Stobbe(5) have shown that a minimum rest period of 2

minutes between trials of a given test or between tests is adequate to prevent

localized muscle fatigue. The data collector must be alert for signs of fatigue

such as a drop in strength scores as a test progresses. The person tested must be

encouraged to report any symptoms of fatigue and the rest periods should be

adjusted accordingly. Whenever possible, successive tests should not stress the

same muscle groups.

Number of Trials for Each Test

The test–retest variability for this type of testing is about 10%. It is higher for

people with limited experience with either isometric testing or forceful physical

exertion in general. In addition, these people often require a series of test trials

14
15

to reach their maximum. The use of a single trial of a test generally underestimates a person’s maximum strength, possibly by more than 50%. A 2-trial protocol results in less of an underestimate, but it may still exceed 30%.(6)

For this reason, it is preferable to determine the number of trials for each test

based on performance. Begin by having the subject perform two trials of the

test. Compare the two scores, and if they are within 10% of each other, use the

highest of the two values as the estimate of the person’s maximal strength, then

proceed to the next test. If the two values differ by more than 10%, perform

additional trials of the same test until the two largest values are within 10% of

each other. Using this approach, Stobbe and Plummer averaged 2.43 trials per

test across 67 subjects performing an average of 30 different strength tests.(6) In

any case, a minimum of two trials is needed for each test.

When to Give Tests

A person’s measured strength is, for a variety of reasons, somewhat variable. It

will not be constant over time or over a workday. In the absence of specific

muscle strength training, however, it should remain within a relatively narrow

range. It is generally higher at the beginning of a workday than at the end. The

fatigue-induced strength decrement varies from person to person and depends

on the nature of the work done during the day. A person who performs repetitive lifting tasks all day can be expected to have a large lifting strength decrement over a workday, whereas a sedentary worker should have little or no

decrement. Based on these results, the fairest evaluation of a person’s maximum strength can be done at the beginning of, or at least early in, a workday.

Test Posture

Measured strength is highly posture dependent. Even small changes in the body

angles of persons being tested or changes in the direction of force application

can result in large changes in measured strength. When collecting strength data,

a researcher should first determine what type of data are sought, and then

design one or more strength tests to provide that specific type of data. If, for

example, the test is to determine whether people are physically fit for a job, the

test posture should emulate, to the extent possible, the posture required on the

job.

Once the test posture has been determined, the researcher must ensure that

the same posture is used on each trial of the test, monitoring the test to ensure

that the posture does not change during the test. If these things are not done,

the test results will be erratic and may seriously overestimate or underestimate

the person’s actual maximal strength.
Restraint Systems

Restraint systems are generally used either to confine a person to the desired

test posture or to isolate some part of the tested person’s body so that a specific

muscle group (or groups) can be tested (Figure 3). In addition, restraint systems

help ensure that all persons participating in a given study are performing the

same test. The type and location of restraint system used can have a major

impact on test results. Similarly, the lack of a restraint system can allow the

posture to vary or allow the use of the wrong or additional muscle groups,

either of which will impact test results.

Any restraint system used should be comfortable and padded in a manner that

prevents local tissue stress concentrations during the test; it should be positioned so that the correct muscle group(s) and posture(s) are used and maintained. Achieving the latter condition often requires some experimentation.

For many strength tests, restraint systems are necessary to achieve consistent

and meaningful results. Researchers reporting strength testing results should

describe in detail the restraints used and their location so that other researchers

and persons applying their data can interpret it correctly. The nonuse of

restraints should also be reported.

16

Figure 3—Example of a test fixture designed to restrain various body segments

during isometric strength testing. (From Occupational Biomechanics, Chaffin,

D.B, and G.B.J. Andersson, ©1991 by John Wiley & Sons, Inc. Reprinted by

permission of John Wiley & Sons, Inc.)
17

Environmental Conditions

The environmental conditions selected for the testing periods should be appropriate to the purpose of the test. For most testing, the environmental conditions

found in a typical office building or medical department are acceptable. In

cases where the effects of the environment on measured strength or physical

performance must be determined, appropriate conditions can be established

(e.g., work sites requiring exposure to hot or cold temperature extremes).

Equipment

Isometric strength testing equipment has not been standardized. Any equipment

capable of performing the necessary timing and averaging described previously

under Test Duration is probably acceptable. Today, this varies from dedicated

force measurement devices, such as the force monitor developed in the 1970s at

University of Michigan, to a force transducer coupled to a PC via an A-D converter and managed by appropriate software or complex multiple-mode strength

measuring devices manufactured by companies such as Cybex, Chattex,

Loredan, and Isotechnologies. Prices vary from $1000 plus to as high as

$50,000 or $100,000.

The issue is not equipment price, rather, it is equipment function. Researchers

should select or build equipment suited to their needs. Researchers must also

understand what is happening inside the device (and its associated software) in

order to properly interpret the data they collect.

The human–equipment interface is another matter that affects the test results.

The interface must be appropriate to the task measured, it should be comfortable (unless discomfort effects are being studied), and it should give the person

tested a sense of security about the test. Persons generally provide a maximal

exertion in a situation in which there is no movement. If they fear that the testing system may fail or move unexpectedly, they will not give a maximal performance. Similarly, the equipment must be strong enough to remain intact under

the maximum load placed on it. If it fails unexpectedly, someone is going to be

injured — perhaps severely.

Subjects

The subjects selected for strength testing will determine the results obtained.

When collecting strength data, the subjects selected must therefore appropriately represent the population the test claims to describe (e.g., design data for

retired persons should be collected on retired persons, and design data for

entry-level construction workers should be collected on young, healthy adults). 

For general research purposes, persons participating in a strength testing project should not have a history of musculoskeletal injuries. Other medical conditions, including hypertension, may pose a threat of harm to a participant.

Whenever possible, prospective participants should be medically evaluated and

approved before participating in a strength testing project.
The following data should be provided about the subject population when

reporting strength testing results:

● Gender;

● Age distribution;

● Relevant anthropometry (height, weight, etc.);

● Sample size;

● Method by which sample was selected and who it is intended to represent;

● Extent of strength training done by participants, and their experience with

isometric testing; and

● Health status of participants (medical exam and/or health questionnaire

recommended).

Strength Data Reporting

Following are the minimum data that should be reported for strength testing

projects:

● Mean, median, and mode of data set;

● Standard deviation of data set; 

● Skewness of data set (or histogram describing data set); and

● Minimum and maximum values.

Evaluation According to 

Physical Assessment Criteria

A set of five criteria has been proposed to evaluate the utility of all forms of

strength testing. isometric strength testing is evaluated with respect to these criteria in the following sections.

Is Isometric Strength Testing Safe to Administer?

Any form of physical exertion carries some risk. The directions for the person

undergoing an isometric test specifically state that the person is to slowly

increase the force until he or she reaches what feels like a maximum, and to

stop any time during the exertion if discomfort or pain is experienced. The

directions also expressly forbid jerking on the equipment. Isometric testing performed in this manner is quite safe to administer because the tested person

decides how much force to apply, over what time interval, and how long to

apply it. The only known complaints relating to participation in isometric testing are rare reports of some residual soreness in the muscles that were active in

the test(s).

Does Isometric Strength Testing Provide Reliable

Quantitative Values?

The test–retest variability for isometric testing is 5%–10%. In the absence of a

specific strength training program individual isometric strength remains relatively stable over time. When the number of trials is based on the 10% criterion

discussed earlier, the recorded strength is near or at the tested person’s maximum voluntary strength. Assuming these factors, and that test postures are

18
19

properly controlled, isometric strength testing is highly reliable and quantitative.

Is Isometric Strength Testing Method Practical?

Isometric strength testing has already been used successfully in industry for

employee placement, in laboratories for collecting design data, and in rehabilitation facilities for patient progress assessment.

Is Isometric Strength Testing Related to Specific Job

Requirements (Content Validity)?

Isometric strength testing can be performed in any posture. When it is conducted for employee placement purposes the test postures should be as similar as

possible to the postures that will be used on the job. The force vector applied

by the tested person should also be similar to the force vector that will be

applied on the job. When these two criteria are met, isometric strength testing

is closely related to job requirements. However, it should be noted that results

obtained using isometric strength testing lose both content- and criterion-related validity as job demands become more dynamic.

Does Isometric Strength Testing Predict the Risk 

of Future Injury or Illness?

A number of researchers have demonstrated that isometric strength testing does

predict risk of future injury or illness for people on physically stressful jobs.(7,8)

The accuracy of this prediction depends on the quality of the job evaluation on

which the strength tests are based and the care with which the tests are administered.

Summary

Isometric strength is defined as the capacity to produce force or torque with a

voluntary isometric (muscles maintain a constant length) contraction. A characteristic of this type of strength measurement is the absence of body movement

during the measurement period. Isometric strength testing has a long history,

and it may be the easiest to measure and understand. The basic procedures for

testing isometric strength are well-established. Risk of injury appears to be

small, and of relatively minor nature. Residual soreness of muscle groups tested

is occasionally reported. Tests of isometric strength appear reliable, with

test–retest variability on the order of 5%–10%. The approach appears quite

practical and has been applied in many industrial situations. The major limitation of isometric strength testing is in its inability to accurately model materials

handling tasks that have a significant dynamic component. It is therefore recommended that tests of isometric strength be applied when there is little or no

dynamic movement involved. In spite of this limitation, it should be duly noted

that of all the procedures reviewed in this chapter, tests of isometric strength

are the only strength tests that have shown the ability to predict individuals

with a high risk of future injury or illness on physically stressful jobs.(1) The
accuracy of this prediction appears to depend on careful biomechanical evaluations of the jobs on which strength tests are based, and proper administration of

the isometric strength testing procedures.

References

1. Chaffin, D.B., and G.B.J. Andersson: Occupational Biomechanics, 2nd

Ed. New York: John Wiley & Sons, 1991. pp. 105–106.

2. Caldwell, L.S., D.B. Chaffin, F.N. Dukes-Dobos, K.H.E. Kroemer, L.L.

Laubach, S.H. Snook, et al.: A Proposed Standard Procedure for Static

Muscle Strength Testing. Am. Ind. Hyg. Assoc. J. 35:201–206 (1974).

3. Chaffin, D.B.: Ergonomics Guide for the Assessment of Human Static

Strength. Am. Ind. Hyg. Assoc. J. 36:505–511 (1975).

4. Schanne, F.T.: “Three Dimensional Hand Force Capability Model for a

Seated Person.” Ph.D. diss., University of Michigan, Ann Arbor, Mich.,

1972.

5. Stobbe, T.J.: “The Development of a Practical Strength Testing Program

in Industry.” Ph.D. diss., University of Michigan, Ann Arbor, Mich., 1982.

6. Stobbe, T.J., and R.W. Plummer: “A Test-Retest Criterion for Isometric

Strength Testing.” Proceedings of the Human Factors Society 28th Annual

Meeting: San Antonio, Tex., Oct 22–26, 1984. pp. 455–459.

7. Chaffin, D.B., G.D. Herrin, and W.M. Keyserling: Pre-employment

Strength Testing: An Updated Position. J. Occup. Med. 20(6):403–408

(1978).

8. Keyserling, W.M., G.D. Herrin, and D.B. Chaffin: Isometric Strength

Testing as a Means of Controlling Medical Incidents on Strenuous Jobs. J.

Occup. Med. 22(5):332–336 (1980).

20
21

3. Maximal Isoinertial Strength Testing

Definition of Isoinertial Strength

Kroemer(1-3) and Kroemer et al.(4) define the isoinertial technique of strength

assessment as one in which mass properties of an object are held constant, as

in lifting a given weight over a predetermined distance. Several strength assessment procedures possess this characteristic. Most commonly associated with

the term is a specific test developed to provide a relatively quick assessment of

a subject’s maximal lifting capacity using a modified weight-lifting device.(1,5)

The classic psychophysical methodology of assessing maximum acceptable

weights of lift is also considered an isoinertial technique under this definition.(6)

While the definition provided by Kroemer(1) and Kroemer et al.(4) has been

most widely accepted in the literature, some have applied the term “isoinertial” to techniques that differ somewhat from the preceding definition, such as

in a description of the Isotechnologies B-200 strength testing device.(7) Rather

than lifting a constant mass the B-200 applies a constant force against which

the subject performs an exertion. The isoinertial tests described here apply to

situations in which the mass to be moved by a musculoskeletal effort is set to

a constant.

Is Isoinertial Testing Psychophysical or 

Is Psychophysical Testing Isoinertial? 

As various types of strength tests have evolved over the past few decades, some

unfortunate developments in terminology have arisen to describe and classify

different strength assessment procedures. This is particularly evident in sorting

out various tests that have been labeled “isoinertial.” One example was cited

earlier. Another problem that has evolved is that the term “isoinertial strength”

has two connotations. The first is the conceptual definition: isoinertial strength

tests include any strength test in which a constant mass is handled. In practice,

however, the term is often used to denote a specific strength test in which subjects’ maximal lifting capacity is determined using a machine and a constant

mass is lifted.(1,5) Partially as a result of this dual connotation, the literature contains references to both “isoinertial strength test” as a psychophysical variant(8)

and the psychophysical method as an “isoinertial strength test.”(4,9) To lay the

framework for the next two chapters, the authors will briefly discuss some

operational definitions of tests of isoinertial and psychophysical strength.

In stating that the isoinertial strength test is a variant of the psychophysical

method, Ayoub and Mital(8) refer to the specific strength test developed by

Kroemer(1) and McDaniel et al.(5) Clearly, this isoinertial protocol has many

similarities to the psychophysical method: both are dynamic; weight is adjusted

in both; and both measure the load a subject is willing to endure under specified circumstances. However, while both deal with lifting and adjusting loads,

there are significant differences between the psychophysical (isoinertial) technique and the Kroemer–McDaniel (isoinertial) protocol in their procedures and

the use of the data collected in these tests. For our purposes, we designate the
Kroemer–McDaniel protocol Maximal Isoinertial Strength Tests (MIST). This

chapter deals with the latter isoinertial technique, which differs from the psychophysical technique on the following counts:

1. In maximal isoinertial strength tests, the amount of weight lifted by the

subject is systematically adjusted by the experimenter, primarily by

increasing the load to the subject’s maximum. In contrast, in psychophysical tests, weight adjustment is freely controlled by the subject, and may be

upwards or downwards.

2. The maximal isoinertial strength tests discussed in this chapter are

designed to quickly establish an individual’s maximal strength using a

limited number of lifting repetitions, whereas psychophysical strength

assessments are typically performed over a longer duration of time (usually at least 20 minutes), and the subject is instructed to select an acceptable (submaximal) weight of lift, not a maximal one. Because of the typically longer duration of psychophysical assessments, greater

aerobic and cardiovascular components are usually involved in

the acceptable workload chosen.

3. Isoinertial strength tests have traditionally been used as a worker

selection tool (a method of

matching physically capable individuals to demanding tasks). A

primary focus of psychophysical

methods has been to establish

data that can be used for the purpose of ergonomic job design.(6)

22

Figure 4—Incremental Weight Lift

Machine. The barrier has been

removed to expose the stack of weights.

(Reprinted from McDaniel, J.W., R.J.

Shandis, and S.W. Madole: Weight

Lifting Capabilities of Air Force Basic

Trainees (AFAMRL–TR–83–0001).

Dayton, Ohio: Wright–Patterson

AFBDH, Air Force Aerospace Medical

Research Laboratory, 1983.)

Published Data

We describe two primary maximal

isoinertial strength test procedures in

this section. One involves the use of a

modified weight-lifting machine with

which the subject lifts a rack of hidden weights to prescribed heights, as

depicted in Figure 4.(5) Kroemer(1)

refers to his technique as LIFTEST,

and the Air Force protocol has been

named the Strength Aptitude Test

(SAT). The other test uses a lifting

box, into which weights are placed

incrementally at specified times until

the lifting limit is reached.(10) The bulk

of the isoinertial testing literature

deals with the former procedure. 
23

The LIFTEST/Strength Aptitude Test (SAT) Techniques 

The LIFTEST and SAT procedures are isoinertial techniques of strength testing

that attempt to establish the maximal amount of weight a person can safely

lift.(1) In this technique, a preselected mass, constant in each test, is lifted by the

subject (typically from knee height to knuckle height, elbow height, or overhead reach height). The amount of weight to be lifted is at first relatively light,

but the mass is continually increased in succeeding tests until it reaches the

maximal amount that the subject voluntarily indicates he or she can handle.

This technique has been used extensively by the U.S. Air Force(5) and is applicable to dynamic lifting tasks in industry as well.(1,11)

Since a constant mass is lifted in LIFTEST, the acceleration of the load during a test depends on the force applied to the load during the test (in accordance with Newton’s second law: F = ma). The dynamic nature of this procedure, the fact that a constant mass is being lifted, and the subject’s freedom to

choose the preferred lifting technique, all make the LIFTEST generally similar

to certain types of industrial lifting tasks. A unique aspect of the LIFTEST

technique is that it is the only strength measurement procedure discussed in this

document in which results are based on the success or failure to perform a prescribed criterion task. The criterion tasks studied have typically included lifting

to shoulder height,(3,5,11,12) elbow height,(5,11) or knuckle height.(3,11) The USAF

also developed a muscular endurance test using an incremental lift machine

(ILM).(5)

The LIFTEST shoulder height maximal strength test has demonstrated the

highest correlation with manual materials-handling activities.(11) It has been

subjected to a biomechanical analysis by Stevenson et al.,(13) who demonstrated

that this criterion task could be divided into three distinct phases: (1) a powerful upward pulling phase, during which maximal acceleration, velocity, and

power values are observed; (2) a wrist changeover maneuver (at approximately

elbow height), which requires momentum to compensate for low force and

acceleration; and (3) a pushing phase (at or above chest height), characterized

by a secondary (lower) maximal force and acceleration profile.

The analysis by Stevenson(13) suggests that successful performance of the criterion shoulder height lift requires a technique quite different from the slow,

smooth lifting usually recommended for submaximal lifting tasks. On the contrary, lifting a maximal load requires a rapid and powerful lifting motion. This

is largely because of the need to develop sufficient momentum to complete the

wrist changeover portion of the lift successfully. Most lift failures occur during

the wrist changeover procedure, probably because of poor mechanical advantage of the upper limb to apply force to the load at this point in the lift.(13)
Stevenson et al.(13) found that certain anatomical landmarks were associated

with maximal force, velocity, and power readings (Figure 5). Maximal force

readings were found to occur at mid-thigh and maximal velocity at chest

height, minimum force was recorded at head height, and the second maximal

acceleration (pushing phase) was observed at 113% of the subject’s stature. 

The Strength Aptitude Test(5)

The Strength Aptitude Test (SAT) is a classification tool for matching the physical strength abilities of individuals with the physical strength requirements of

jobs in the Air Force.(14) The SAT is given to all Air Force recruits as part of

their preinduction examinations. Results of the SAT are used to determine

whether the individual tested possesses the minimum strength criterion for

admission to various Air Force Specialties (AFSs). The physical demands of

each AFS are objectively computed from an average physical demand weighted

by the frequency of performance and the percentage of the AFS members performing the task. Objects weighing less than 10 pounds are not considered

physically demanding and are not considered in the job analysis. Before the

24

Figure 5—Displacement and timing parameters for a 1.83 m maximal isoinertial lift. Figure illustrates anatomical landmarks for the location of key events,

found to be consistent for both genders. (From Stevenson, J.M., et al.:

Dynamic Analysis of Isoinertial Lifting Technique, Ergonomics 33(2):161–172

(1990). Reprinted with permission of Taylor and Francis Ltd.)
25

physical demands of the AFS are averaged, the actual weights of objects handled are converted into equivalent performance measures on the incremental

weight lift test using regression equations developed over years of testing.

These relationships consider the type of task (lifting, carrying, pushing, etc.),

the size and weight of the object handled, as well as the type and height of the

lift. Thus, the physical job demands are related to, but are not identical to, the

ability to lift an object to a certain height. Job demands for various AFSs are

reanalyzed periodically to update the SAT.

The first major report describing this classification tool was a study of 1671

basic trainees (1066 males and 605 females).(5) The incremental weight lift tests

started with an 18.1 kg weight, which was to be raised to 1.83 m or more above

the floor. This initial weight was increased in 4.5-kg increments until subjects

were unable to raise the weight to 1.83 m. Maximal weight lift to elbow height

was then tested as a continuation of the incremental weight lift test. In the test

of lifting the weight to 1.83 m, males averaged 51.8 kg (±10.5 SD), while

females averaged 25.8 kg (±5.3). The respective weights lifted to elbow height

were 58.6 kg (±11.2) and 30.7 kg (±6.3). Figure 6 shows the distributions of

weight-lifting capabilities for both male and female basic trainees in lifts to 6

feet. Results of the elbow height lift are presented in Table I. 

Figure 6—Distribution of weight lifted in a 1.83 m maximal isoinertial lift for

male and female United States Air Force recruits. (Reprinted from McDaniel,

J.W., R.J. Shandis, and S.W. Madole: Weight Lifting Capabilities of Air Force

Basic Trainees (AFAMRL–TR–83–0001). Dayton, Ohio: Wright–Patterson

AFBDH, Air Force Aerospace Medical Research Laboratory, 1983.)

NOTE: THE TOTAL

RESOURCE REPRESENTS

1980 MIX OF 82% MALE AND

18% FEMALE

TOTAL RESOURCE—

PERCENT OF TOTAL RESOURCE

WEIGHT LIFTED TO 6 FEET (POUNDS)
26

Table I

Weight Lifted by Male and Female U.S. Air Force Recruits Using Maximal

Isoinertial Lift to Elbow Height.(5)

Males Females

Percentile Pounds Kilograms Pounds Kilograms

1 80 36.3 40 18.1

5 93 42.2 48 21.8

10 100 45.4 52 23.6

20 109 49.5 58 26.3

30 116 52.6 61 27.7

40 122 55.4 65 29.5

50 127 57.6 68 30.9

60 133 60.3 71 32.2

70 140 63.5 75 34.0

80 150 68.1 78 35.4

90 160 72.6 85 38.6

95 171 77.6 90 40.8

99 197 89.4 100 45.4

Mean 129 58.6 68 30.7

S.D. 25 11.2 14 6.3

Minimum 50 22.7 <40 <18.1

Maximum >200 >90.7 100 49.9

Number 1066 605

McDaniel et al.(5) also performed a test of isoinertial endurance. This involved

holding a 31.8-kg weight at elbow height for the duration the subject could perform the task. Male basic trainees were able to hold the weight for an average

of 53.3 seconds (±22.11), while female basic trainees managed to hold the

weight an average of 10.3 seconds (±10.5).

When developing the SAT, the Air Force examined more than 60 candidate

tests in an extensive, 4-year research program and found the incremental

weight lift to 1.83 m to be the single best test of overall dynamic strength capability that was both safe and reliable.(14) This finding was confirmed by an independent study funded by the U.S. Army.(15) This study compared the SAT to a

battery of tests developed by the Army (including isometric and dynamic tests),

and then compared these with representative heavy-demand tasks performed

within the Army. Results showed the SAT to be superior to all other tests in

predicting performance on the criterion tasks.

Virginia Tech Data

Kroemer(1,3) described results of a study using an apparatus similar to the one

used by the U.S. Air Force. The sample consisted of 39 subjects (25 male)

recruited from a university student population. The procedures were similar to

those of McDaniel et al.,(5) except that the minimum starting weight was 11.4

kg and maximal lifting limits were established to prevent overexertion. These
27

Table II

Results of Maximal Isoinertial Strength Tests for 25 Male and 14 Female

University Students.(2)

All Male Female

Mean SD CV N Mean SD CV N Mean SD CV N

Overhead

Liftest (kg) 27.0 10.3 3.5% 33 34.8 5.2 3.2% 19 16.3 3.7 3.9% 14

Lift >45.5 kg — — — 6 — — — 6 — — — 0

Knuckle

Liftest (kg) 53.9 13.4 6.9% 22 62.2 7.8 5.2% 8 49.1 13.7 7.8% 14

Lift > 77 kg — — — 17 — — — 17 — — — 0

were 77.1 kg for floor to knuckle height tests, and 45.4 kg for floor to overhead

reach tests. The following procedure was used to establish the maximal load: if

the initial 11.4 kg weight was successfully lifted, the weight was doubled to

22.7 kg. Additional 11.4-kg increments were added until an attempt failed or

the maximal lifting limit was reached. If an attempt failed, the load was

reduced by 6.8 kg. If this test weight was lifted, 4.5 kg was added; if not, 2.3

kg was subtracted. This scheme allowed quick determination of the maximal

load the subject could lift. 

In Kroemer’s study, 6 of 25 male subjects exceeded the cut-off load of 100

pounds in overhead reach lifts.(1,3) All 14 females stayed below this limit. The

19 remaining male subjects lifted an average of 27 kg. The female subjects lifted an average of 16 kg. In lifts to knuckle height, 17 of the 25 male (but none

of the female) subjects exceeded the 77.1 kg cut-off limit. The remaining subjects lifted an average of about 54 kg, with males averaging 62 kg and females

49 kg. The coefficients of variation for all tests were less than 8%. Summary

data for this study are given in Table II. 

The Progressive Isoinertial Lifting Evaluation (PILE) 

Another variety of MIST has been described by Mayer et al.(10,16) Instead of

using a weight rack, as shown in Figure 4, the Progressive Isoinertial Lifting

Evaluation (PILE) is performed using a lifting box with handles; weight is

increased in the box as it is lifted and lowered. Subjects perform two isoinertial

lifting/lowering tests: one from floor to 30 in. (lumbar) and one from 30 to 54

in. (cervical). Unlike the isoinertial procedures described earlier, this test has

three possible criteria for termination: (1) voluntary termination due to fatigue,

excessive discomfort, or inability to complete the specified lifting task; (2)

achievement of a target heart rate (usually 85% of age-predicted maximal heart

rate); or (3) when the subject lifts a “safe limit” of 55%–60% of his or her

body weight. Thus, in contrast with the previous tests, the PILE test is
terminated as a result of cardiovascular factors rather than when an acceptable

load limit is reached.

Since the PILE was developed as a means of evaluating the degree to which

functional capacity has been restored in individuals complaining of chronic low

back pain (LBP), the initial weight lifted by subjects using this procedure is

somewhat lower than in the previous tests. The initial starting weight is 3.6 kg

for women and 5.9 kg for men. Weight is increased upwards at a rate of 2.3 kg

every 20 seconds for women, and 4.6 kg every 20 seconds for men. During

each 20-second period, four lifting movements (box lift or box lower) are performed. The lifting sequence is repeated until one of the three endpoints is

reached. The vast majority of subjects are stopped by the “psychophysical”

endpoint, indicating a perception of fatigue or overexertion. The target heart

rate endpoint is typically reached in older or large individuals. The “safe limit”

endpoint is typically encountered only by very thin or small individuals.

Mayer et al.(10) developed a normative database for the PILE, consisting of 61

males and 31 females. Both total work (TW) and force in pounds (F) were normalized according to age, gender, and a body weight variable. The body weight

variable, the adjusted weight (AW), was taken as actual body weight in slim

individuals but as the ideal weight in overweight individuals. This was done to

prevent skewing the normalization in overweight individuals. Table III presents

the normative database for the PILE. 

28

Table III

Normative Database for the Progressive Isoinertial Lifting Evaluation.(10)

Males (n = 61)

AW LW/AW LTW/AW CERF/AW CERTW/AW

Means 161.3 .50 22.8 .40 12.3

Standard 19.6 .10 7.8 .10 5.1

Deviations

Standard 2.51 .01 1.0 .01 .81

Error of the

Mean

Females (n = 31)

Means 121.6 .35 17.04 .25 7.32

Standard 10.65 .07 7.0 .04 2.4

Deviations

Standard 1.98 .01 1.3 .01 .56

Error of the

Mean

L = lumbar; CER = cervical; TW = total work in feet–pounds; AW = adjusted

weight in pounds; F = final force in pounds.
29

Evaluation According to 

Physical Assessment Criteria 

Is Isoinertial Strength Testing Safe to Administer?

The MIST procedures described here appear to have been remarkably free of

injury. Isoinertial procedures have now been performed many thousands of

times without report of verifiable injury. However, reports of transitory muscle

soreness have been noted.(10) The temporary muscle soreness associated with

isoinertial testing has been similar to that experienced in isokinetic tests but has

been reported less frequently than in isometric strength tests.

The following list summarizes the recommendations made by McDaniel et

al.(5) for designing safe isoinertial weight lift testing procedures:

1. Weight-lifting equipment should be designed so that the weights and handle move only in a vertical direction.

2. Sturdy shoes should be worn, or the subject may be tested barefoot.

Encumbering clothing should not be worn during the test.

3. The initial weight lifted should be low — 20 to 40 pounds. Weights in this

range are within the capability of almost everyone. Weight increments

should be small.

4. The upper limit should not exceed the largest job-related requirement or

160 pounds, whichever is less.

5. The starting handle position should be 1 to 2 feet above the standing surface. If the handle is lower, the knees may cause obstruction. If the handle

is too high, the subject will squat to get his or her shoulders under it

before lifting. A gap between the handles allows them to pass outside the

subject’s knees during lifting, allowing a more erect back and encouraging

the use of leg strength.

6. The recommended body orientation before lifting should be (a) arms

straight at the elbow, (b) knees bent to keep the trunk as erect as possible,

and (c) head aligned with the trunk. The lift should be performed smoothly, without jerk.

7. A medical history of the subject should be obtained. If suspicious physical

conditions are identified, a full physical examination should be performed

prior to testing. Subjects over 50 years of age or pregnant should always

have a physical before testing.

8. All sources of overmotivation should be minimized. Testing should be

done in private and results kept confidential. Even the test subject should

not be informed until the testing is completed.

9. If the subject pauses during a lift, the strength limit has been reached, and

the test should be terminated. Multiple attempts at any single weight level

should not be allowed.

10. The testing should always be voluntary. The subject should be allowed to

stop the test at any time. The subject should not be informed of the criteria prior to or during the test.

It is noteworthy that, as of 1994, more than 2 million subjects have been tested on the SAT without any back injury or overexertion injury. (14)
Does Isoinertial Strength Testing Give

Reliable, Quantitative Values?

Kroemer et al.(3) reported LIFTEST coefficients of variation (measures of intraindividual variability in repeated exertions) of 3.5 for all subjects in overhead

lifts, and 6.9 in lifts to knuckle height. The same study showed somewhat higher variability in tests of isometric strength (coefficient of variations ranging

from 11.6 to 15.4). Test-retest reliability was not reported by McDaniel et al.(5)

Mayer et al.(10) reported correlation coefficients of a reproducibility study of the

PILE that demonstrated good test-retest reliability for both floor to 30 in. lifts

(r = .87, p < .001) and 30 to 54 in. lifts (r = .93, p < .001). Thus, the reliability

of isoinertial procedures appears to compare favorably with that demonstrated

by other strength assessment techniques. 

Is Isoinertial Strength Testing Practical? 

Isoinertial techniques generally appear practical in terms of providing a test

procedure that requires minimal administration time and minimal time for

instruction and learning. Even in a worst case scenario, the isoinertial procedures used by Kroemer(2) would take only a few minutes to determine the maximal weight-lifting capability of the subject for a particular condition. The

McDaniel et al. (5,14) procedure can be performed in approximately three to five

minutes. The PILE test administration time is reported to last on the order of

five minutes. (10)

Practicality is determined in part by cost of the equipment required — and

the cost of isoinertial techniques is quite modest. In fact, the PILE test requires

no more hardware than a lifting box, some sturdy shelves, and some weight.

The equipment needed to develop the LIFTEST devices used by McDaniel et

al.(5) and Kroemer(1-3) are slightly more expensive, but are not prohibitive for

most applications. In fact, Kroemer(2) states that the device is easily dismantled

and transported to different sites in a small truck or station wagon, or perhaps

in a mobile laboratory vehicle. 

Is Isoinertial Strength Testing Related to 

Specific Job Requirements? 

Since industrial lifting tasks are performed dynamically, isoinertial strength

tests do appear to provide some useful information related to an individual’s

ability to cope with the dynamic demands of industrial lifting. McDaniel (14) has

reported that these tests are predictive of performance on a wide range of

dynamic tasks, including asymmetric tasks, carrying, and pushing tasks.

Furthermore, Jiang et al.(11) demonstrated that the isoinertial lifting test to six

feet was more highly correlated with psychophysical tests of lifting capacity

than isometric techniques. The PILE test possesses good content validity for

industrial lifting tasks, as subjects are able to use a more “natural” lifting technique when handling the lifting box.

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31

Does Isoinertial Strength Testing Predict Risk 

of Future Injury or Illness?

The ability of a strength test to predict risk of future injury or illness depends

on performance of prospective epidemiological studies. As of this writing, no

such studies have been conducted on the isoinertial techniques described here.

Summary

Isoinertial strength tests are defined as those in which mass properties of an

object are held constant, as in lifting a given weight over a predetermined distance. Several types of strength tests fit this rather broad definition, including

the classic psychophysical technique. However, several distinctions can be

made between psychophysical strength assessments and other isoinertial procedures. Maximal Isoinertial Strength Tests (MIST) are typically characterized as

techniques designed to quickly establish an individual’s maximal strength

through a systematic adjustment of weight by the experimenter. Psychophysical

strength assessments typically are designed to establish an acceptable (not maximal) workload over a relatively longer duration, with the subject being allowed

to freely adjust the weight. Isoinertial techniques have typically been used as a

worker selection tool, whereas psychophysical tests are most often used for

ergonomic job design.

Two primary MIST assessment techniques have been established. One

involves use of a modified weight lifting device with which the subject lifts a

rack of weights to a prescribed height (the LIFTEST technique). The other test

uses a lifting box, into which weights are placed at specified times until the lifting limit is achieved (the Progressive Isoinertial Lifting Evaluation or PILE).

Both types of MIST have been shown to be safe, reliable, and practical methods of strength assessment. None of the MIST techniques discussed in this section have demonstrated the ability to predict risk of future injury or illness.

References

1. Kroemer, K.H.E.: Development of LIFTEST: A Dynamic Technique to

Assess the Individual Capability to Lift Material, Final Report (NIOSH

Contract 210–79–0041. Blacksburg, Va.: Ergonomics Laboratory, IEOR

Department, Virginia Polytechnic Institute and State University, 1982.

2. Kroemer, K.H.E.: An Isoinertial Technique to Assess Individual Lifting

Capability. Hum. Factors 25(5):493–506 (1983).

3. Kroemer, K.H.E.: Testing Individual Capability to Lift Material:

Repeatability of a Dynamic Test Compared with Static Testing. J. Safety

Res.16(1):1–7 (1985).

4. Kroemer, K.H.E., W.S. Marras, J.D. McGlothlin, D.R. McIntyre, and

M. Nordin: On the Measurement of Human Strength. Intl. J. Ind. Ergon.

6:199–210 (1990).

5. McDaniel, J.W., R.J. Shandis, and S.W. Madole: Weight Lifting

Capabilities of Air Force Basic Trainees (AFAMRL–TR–83–0001).
Wright-Patterson AFBDH, Air Force Aerospace Medical Research

Laboratory, 1983.

6. Snook, S.H.: The Design of Manual Handling Tasks. Ergonomics

21:963–985 (1978).

7. Parnianpour, M., M. Nordin, N. Kahanovitz, and V. Frankel: The

Triaxial Coupling of Torque Generation of Trunk Muscles during Isometric

Exertions and the Effect of Fatiguing Isoinertial Movements on the Motor

Output and Movement Patterns. Spine 13(9):982–992 (1988).

8. Ayoub, M.M., and A. Mital: Manual Materials Handling. London: Taylor

and Francis, 1989. pp. 241–242.

9. Champ, D.B., and G.B.J. Andersson: Occupational Biomechanics. New

York: John Wiley & Sons, 1991. pp. 152–153.

10. Mayer, T. G., D. Barnes, N.D. Kishino, G. Nichols, R.J. Gatchell, H.

Mayer, and V. Mooney: Progressive Isoinertial Lifting Evaluation — I. A

Standardized Protocol and Normative Database. Spine 13(8):993–997

(1988a).

11. Jiang, B.C., J.L. Smith, and M.M. Ayoub: Psychophysical Modeling of

Manual Materials-Handling Capacities Using Isoinertial Strength

Variables. Hum. Factors 28(6):691–702 (1986).

12. Ostrom, L.T., J.L. Smith, and M.M. Ayoub: The Effects of Training on

the Results of the Isoinertial 6-Foot Incremental Lift Strength Test. Intl. J.

Ind. Ergon. 6:225–229 (1990).

13. Stevenson, J.M., J.T. Bryant, S.L. French, D.R. Greenhorn, G.M.

Andrew, and J.M. Thomson: Dynamic Analysis of Isoinertial Lifting

Technique. Ergonomics 33(2):161–172 (1990).

14. McDaniel, J.W.: “Strength Aptitude Tests.” 1994. [Personal communication to Sean Gallagher, National Institute for Occupational Safety and

Health/Pittsburgh Research Center.]

15. Myers, D.O., D.L. Gebhardt, C.E. Crump, and E.A. Fleishman:

Validation of the Military Entrance Physical Strength Capacity Test (MEPSCAT). U.S. Army Research Institute Technical Report 610, NTIS No.

AD-A142 169 (1984).

16. Mayer, T.G., D. Barnes, G. Nichols, N.D. Kishino, K. Coval, B. Piel, D.

Hoshino, and R.J. Gatchell: Progressive Isoinertial Lifting Evaluation —

II. A Comparison with Isokinetic Lifting in a Chronic Low-Back Pain

Industrial Population. Spine 13(8):998–1002 (1988b).

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33

4. Psychophysical Strength

Theory and Description of the 

Psychophysical Methodology

According to contemporary psychophysical theory, the relationship between the

strength of a perceived sensation (S) and the intensity of a physical stimulus (I)

is best expressed by a power relationship.(1)

S = kIn (1)

This psychophysical principle has been applied to many practical problems,

including the development of scales or guidelines for effective temperature,

loudness, brightness, and ratings of perceived exertion. Based on the results of

a number of experiments using a variety of scaling methods and a number of

different muscle groups, the pooled estimate of the exponent for muscular

effort and force is 1.7.(2)

When applying this principle to work situations, it is assumed that individuals

are capable and willing to consistently identify a specified level of perceived

sensation (S). For manual materials handling tasks, this specified level is usually the maximum acceptable weight or maximum acceptable force. These phrases are defined by the instructions given to the test subject:(3)

You are to work on an incentive basis, working as hard as you can

without straining yourself, or becoming unusually tired, weakened,

overheated, or out of breath.

If the task involves lifting, the experiment measures the maximum acceptable

weight of lift. Similarly, there are maximum acceptable weights for lowering

and carrying. Such tests are isoinertial in nature; however, in contrast to the

tests described in Chapter 3, they are typically used to test submaximal, repetitive handling capabilities. Data are also available for pushing and pulling.

These are reported as maximum acceptable forces and include data for initial as

well as sustained pulling or pushing. 

Why Use Psychophysical Methods?

Snook identified several advantages and disadvantages to using psychophysical

methods for determining maximum acceptable weights.(4) The advantages

include:

● Realistic simulation of industrial work (face validity);

● Ability to study intermittent tasks (physiological steady state not

required);

● Results are consistent with the industrial engineering concept of “a fair

day’s work for a fair day’s pay”;

● Results are reproducible; and

● Results appear to be related to low-back pain (content validity).
Disadvantages include:

● Tests are performed in a laboratory;

● It is a subjective method that relies on self-reporting by the subject;

● Results for very high frequency tasks may exceed recommendations for

energy expenditure; and

● Results are insensitive to bending and twisting.

Liberty Mutual preferred to use the data derived from these studies to design

a job to fit the worker since this application represented a more permanent,

engineering solution to the problem of low-back pain in industry.(5) This

approach not only reduces the worker’s exposure to potential low-back pain

risk factors, but also reduces liability associated with worker selection.(5)

Published Data

Liberty Mutual

Snook and Ciriello at the Liberty Mutual Insurance Company have published

the most comprehensive tables for this type of strength assessment.(6) The most

recent data are summarized in nine tables, organized as follows:(6)

1. Maximum acceptable weight of lifting for males.

2. Maximum acceptable weight of lifting for females.

3. Maximum acceptable weight of lowering for males.

4. Maximum acceptable weight of lowering for females.

5. Maximum acceptable forces of pushing for males (initial and sustained).

6. Maximum acceptable forces of pushing for females (initial and sustained).

7. Maximum acceptable forces of pulling for males (initial and sustained).

8. Maximum acceptable forces of pulling for females (initial and sustained). 

9. Maximum acceptable weight of carrying (males and females).

Other Sources

Ayoub et al.(7) and Mital(8) have also published tables for maximum acceptable

weights of lift. Even though their tables are similar in format and generally in

agreement with those from Liberty Mutual, there are some differences. Possible

sources for these differences may be differences in test protocol, differences in

task variables, and differences in subject populations and their characteristics.

Experimental Procedures and Methods

For the sake of simplicity and convenience, the Liberty Mutual protocol for lifting or lowering and an excerpt from the lifting table will be used as examples

for this section. The protocols used by Ayoub et al.(7) and Mital(8) were similar,

but not exactly the same. The reader should refer to the original publications

for details.

The Liberty Mutual experimental procedures and methods were succinctly

reviewed in their most recent revision of the tables.(6) The data reported in these

revised tables reflect results from 119 second-shift workers from local industry

(68 males, 51 females). All were prescreened to ensure good health prior to

participation. These subjects were employed by Liberty Mutual for the duration

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35

of the project (usually 10 weeks). All received 4 to 5 days of conditioning and

training prior to participation in actual test sessions. 

Test subjects wore standardized clothing and shoes. The experiments were

performed in an environmental chamber maintained at 21°C (dry bulb) and

45% relative humidity. Forty-one anthropometric variables were recorded for

each subject, including several isometric strengths and aerobic capacity. 

A single test session lasted approximately 4 hours and consisted of five tasks.

Each task session lasted 40 minutes, followed by 10 minutes rest. Most subjects

participated in at least two test sessions per week for 10 weeks. In general, a subject’s heart rate and oxygen consumption were monitored during the sessions.

Lifting or Lowering Tasks

In a lifting or lowering task session, the subject was given control of one variable, usually the weight of the box. The other task variables would be specified

by the experimental protocol. These variables include:

1. Lifting zone — whether the lift occurs between floor level to knuckle

height (low), knuckle height to shoulder height (center), or shoulder

height to arm reach (high).

2. Vertical distance of lift — the vertical height of the lift within one of these

lifting zones. The specified values for distance of lift in the tables are 25

cm. (10 in.), 51 cm. (20 in.), and 76 cm. (30 in.). It is possible to use linear extrapolation for lift distances not exactly equal to one of these values. 

3. Box width — the dimension of the box away from the body. The three

values of box width are 34 cm. (13.4 in.), 49 cm. (19.3 in.), and 75 cm.

(29.5 in.). It is possible to use linear extrapolation between these values. 

4. Frequency of lift — expressed as one lift per time interval, including intervals of 5 seconds, 9 seconds, 14 seconds, 1 minute, 2 minutes, 5 minutes,

and 8 hours.

These definitions apply to a lowering task, except the word “lower” is substituted for “lift.” The test protocol for lowering was essentially identical to that

for lifting, and the results are reported in a similar format. It should be noted,

however, that the test protocols for lifting and lowering involved using a special

apparatus that returned the box to its original specified location, so that the subject only lifted or lowered, not both.

The subject was instructed to adjust the weight of the box, according to his or

her own perceptions of effort or fatigue, by adding or removing steel shot or

welding rods from a box. The box had handles and a false bottom to eliminate

visual cues. Each task experiment was broken into two segments so that the initial weight of the box could be randomly varied between high versus low so

that the subject approached his or her maximum acceptable weight from above

as well as below. If the results met a 15% test–retest criterion, the reported

result was the average of these two values. If the results did not meet this criterion, they were discarded and the test repeated at a later time. 

In reporting the results, it was assumed that the gender-specific maximum

acceptable weights for a particular task were normally distributed. As a consequence, the results were reported as percentages of population, stratified by

gender. The Liberty Mutual tables are organized around the following
percentages: 90%, 75%, 50%, 25%, and 10%.(6) The 90th percentile refers to a

value of weight that 90% of individuals of that gender would consider a maximum acceptable weight (90% “acceptable”), while the 10th percentile refers to

a value of weight that only 10% of individuals of that gender would find

acceptable (10% “acceptable”). 

Important Caveats

Snook and Ciriello have identified several important caveats that should be

remembered when using the Liberty Mutual tables.(6)

1. The data for each experimental situation were assumed to be normally

distributed when the maximum acceptable weights and forces acceptable

to 10%, 25%, 50%, 75%, and 90% of the industrial population were

determined.

2. Not all values in the tables are based on experimental data. Some values

were derived by assuming that the variation noted for a particular variable

for one type of task would be similar to that observed for another task,

e.g., the effects on lowering would be similar to that on lifting.

3. The tables for lifting, lowering, and carrying are based on boxes with handles that were handled close to the body. They recommend that the values

in the tables be reduced by approximately 15% when handling boxes

without handles. When handling smaller boxes with extended reaches

between knee and shoulder heights, they recommend reducing the values

by approximately 50%.

4. Some of the reported weights and forces exceed recommended levels of

energy expenditure if performed for 8 or more hours per day. These data

are italicized in the tables.

5. The data in the tables give results for individual manual materials handling tasks. When a job involves a combination of these tasks, each component should be analyzed separately, and the component with the lowest

percent of capable population represents the maximum acceptable weight

or force for the combined task. It should be recognized, however, that the

energy expenditure for the combined task will be greater than that for the

individual components.

Some recent data suggest that persons performing lifting tasks are relatively

insensitive to the perception of high disc compression forces on the spine.(9) As

a result, there may be some tasks in the tables that exceed recommended levels

of disc compression.

Related Research

Task and Subject Variables

A variety of researchers have examined the effects of other task and subject

variables using the psychophysical protocol. Most of these studies involve a

small number (<10) of college students as test subjects. Some experiments used

the Liberty Mutual protocol; others used the protocol described by Ayoub et

al.(7) and Mital.(8) These “refinements” are summarized in Table IV.

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37

Recommended Applications

Job Evaluation

The Liberty Mutual tables were developed for the purpose of evaluating work,

not workers.(10) In particular, the tables are intended to help industry in the evaluation and design of manual materials handling tasks that are consistent with

worker limitations and abilities.(6) The explicit goal is the control of low-back

pain through reductions in initial episodes, length of disability, and

recurrences.(10)

To apply the tables in the context of job evaluation, it is first necessary to

specify the task variables of the job. For a lifting task, this includes the lift

zone, distance of lift, box width, frequency of lift, and the presence or absence

of box handles. In addition, it is necessary to measure the weight of the object

Table IV

Miscellaneous Task Variables Evaluated Using the Psychophysical

Methodology.

Task Variable(s) Reference(s)

Zone of lift 5–8, 21–23

Distance of lift 5–8, 21–23

Frequency of lift 5–8, 21–23

Box width 5–8, 21–24

Extended work shifts 8

Combinations of lift, carry, and lower 11, 12

Angle of twist 23

Box length 23, 24

Material density 25

Location of center of gravity 25

Center of gravity relative to preferred hand 25

Sleep deprivation 26

Bag versus box 26

Fullness of bag (same weight) 26

Bag ± handles 26

Day 1 to day 5 of work week 19

Asymmetrical loads 28–30

Asymmetrical lifting 28–31

Emergency scenario 32

Handle position 33

Handle angle 33

Duration of lifting 34, 35 

Overreach heights 36

Restricted vs. unrestricted shelf opening clearances 37

Experienced vs. inexperienced workers 38

Nonstandard or restricted postures 20, 39–41
Table V

Excerpt from the Liberty Mutual Tables for Maximum Acceptable Weight

of Lift (kg) for Males and Females.

Floor Level to Knuckle Height

One Lift Every

Gender Box Distance Percent 5 9 14 1 2 5 30 8

Width of Lift Capable sec sec sec min min min min hr

(cm) (cm)

90 7 9 10 14 16 17 18 20

75 10 13 15 20 23 25 25 30

Males 49 51 50 14 17 20 27 30 33 34 40

25 18 21 25 34 38 42 43 50

10 21 25 29 40 45 49 50 59

90 6 7 8 9 10 10 11 15

75 7 9 9 11 12 12 14 18

Females 49 51 50 9 10 11 13 15 15 16 22

25 10 12 13 16 17 17 19 26

10 11 14 15 18 19 20 22 30

Italicized values exceed 8-hour physiological criteria (energy expenditure).

to be handled, perhaps using a scale or dynamometer. Once these variables are

specified, the measured weight can be compared to the data in the table to

determine the percent of capable population for males and females. The procedure is similar for pulling or pushing. The required force can be measured with

a dynamometer.

Consider the following example. The task is to lift a 49-cm wide box that

weighs 20 kg once every minute between floor level to knuckle height for a

distance of 51 cm. In Table V, excerpted from the Liberty Mutual tables, the

weight of the box, 20 kg, is exactly equal to the maximum acceptable weight of

lift for 75% of males, that is, 75% of males would consider this task “acceptable.” By contrast, the highest maximum acceptable weight of lift reported for

females is 18 kg. As a result, this task is “not acceptable” to more than 90% of

females. 

38

Job Design

To apply the tables in the context of job design, the process is essentially identical. All task-specific parameters must be identified, except the required weight

or force (that is what you are determining). You select a desired percent of

capable of population, noting gender effects, then identify the maximum

acceptable weight or force that corresponds to that desired percent. This is the

value recommended for job design.
39

As an example, suppose you wish to design a lifting task that requires a 49-

cm wide box that must be lifted 51 cm once per minute within the floor-toknuckle zone. You desire to design this job to accommodate 75% of females.

According to the data in Table V, you would recommend that the box weigh no

more than 11 kg. This weight would be acceptable to 75% of females and over

90% of males.

Multiple task analysis, consisting of lifting, carrying, and lowering, has also

been investigated for the Liberty Mutual data.(11) In this circumstance, it was

observed that the maximum acceptable weight for the multiple task was less

than that for only the carrying task when performed separately, but not significantly different from the lifting or lowering maximum acceptable weights when

performed separately. For this type of a multiple task, the maximum acceptable

weight for the task should be the lowest maximum acceptable weight of the

lifting or lowering task as if it were performed separately. One should be careful, however, because the energy expenditure for the multiple task is probably

underestimated when compared to performing the tasks separately. Similar

results were reported by Jiang et al.(12)

Validation

Content Validity

The concept of content validity, also called face validity, addresses whether the

content of the test is identical or highly similar to the content of the job. This is

one of the major advantages of the psychophysical methodology, but it is

important for the user to realize the limitations of the data, especially the

caveats noted earlier. 

It is noted that a 40-minute test protocol is used to predict an 8-hour maximum acceptable weight or force. The researchers at Liberty Mutual examined

this assumption by having subjects select their maximum acceptable weight

according to the usual protocol, then having them continue to work, adjusting

the weight or force as desired, for a total of 4 hours.(10) No statistically significant difference was found between the values selected after 40 minutes and

those selected after 4 hours. Karwowski and Yates reported similar results.(13)

Mital also examined this issue relative to the Ayoub et al. data.(14) Mital found

that the test subjects’ estimates of their 8-hour maximum acceptable weights of

lift were significantly greater than that selected at the end of an actual 8-hour

period of work (an average 35% reduction). He “corrected” for this effect in his

tables for 8-hour maximum acceptable weights of lift.(8)

Criterion-Related Validity

Criterion-related validity, also called predictive validity, deals with the question

of whether the results of the this type of job analysis predict risk of future

injury or illness. This is generally demonstrated by the presence of a statistically significant correlation between a test “score” and a particular outcome in an

appropriately conducted epidemiological study.

There are two such studies relevant to the criterion-related validity of the psychophysical methodology.
Liberty Mutual Data. In 1978, Snook, Campanelli, and Hart published an

investigation of three preventive approaches to low-back injuries in industry.(15)

They distributed 200 questionnaires to Liberty Mutual Loss Prevention representatives throughout the United States. These representatives were asked to

complete the questionnaire for the most recent compensable back injury. If the

specific act or movement associated with the injury were some form of manual

handling task, a task evaluation was completed to estimate the percent of capable working population that could perform the task without overexertion, e.g.,

what percent of the population could perform the task without exceeding their

maximum acceptable weight or force. 

The investigators received 192 questionnaires, one with incomplete data.

They observed that 70% of these 191 low-back injuries were associated with

manual materials handling tasks. They also compared the observed number of

injuries to an expected number of injuries according to whether the percent

capable population was greater than or less than 75%. This analysis is summarized as follows:

$ 75% capable < 75% capable

Observed 98 93

Expected* 145.9 45.1

* The expected values were derived from control data that

revealed that 23.6% of jobs involve handling tasks that less

than 75% of the population could perform without overexertion.

X2 = 66.6; p < .01

Based on these results, the authors concluded:

1. A worker is three times more susceptible to low-back injury if he or she

performs a job that less than 75% of the working population can perform

without overexertion. 

2. At best, the ergonomic approach could reduce low-back injuries associated with manual material handling tasks by 67% by designing the jobs so

that percent capable population were 75% or greater. The remaining 33%

of back injuries will occur regardless of the job demands.

3. Since only 50% of the industrial back injuries are related to manual materials-handling tasks where the percent capable population is less than

75%, the overall reduction in in low-back injuries would be 33%. This

reduction would be higher if the percent capable population were raised to

90%.

Ayoub et al. Data. Ayoub and co-workers proposed the use of a severity index,

called the Job Severity Index (JSI), for purposes of validation.(16) The JSI is a

ratio of job demands to worker capability. Since a job may consist of multiple

tasks, they defined the JSI as a time- and frequency-weighted average of the

maximum weight required by each task divided by the task-specific worker

capacity. Their validation studies included 101 jobs, performed by 385 males

40
41

and 68 females, and involved four steps:

1. Selection of candidate jobs.

2. Analysis of candidate jobs in terms of lifting requirements and morbidity

data.

3. Determination of the JSI for jobs and operators.

4. Determination of the relationship between JSI and observed morbidity.

Individual JSIs were calculated for each worker that were subsequently grouped

in to four categories: .00 # JSI < .75; .75 # JSI < 1.5; 1.5 # JSI < 2.25; and JSI

$ 2.25. 

The morbidity data were classified into five groups: musculoskeletal injuries

to the back; musculoskeletal injuries to other parts of the body; surface-tissue

injuries due to impact; other surface-tissue injuries; and miscellaneous injuries.

These data were reported as incidence rates per 100 workers per year. Data for

severity (days lost) and cost were also collected. 

The results revealed that the incidence of back injuries and the incidence of

disabling back injuries increased substantially if the JSI was greater than or

equal to 1.5. The relationships were nonlinear. The severity for disabling back

injuries was increased if the JSI was greater than 2.25. The authors did not

report any statistical analyses.

Another aspect of their validation involved classifying jobs according to the

percent of capable population. Each job was categorized according to the percentage of the population “overstressed,” that is, JSI greater than 1.5. The

ranges were: % > 75; 5 < % # 75, and % # 5. They observed that the incidence of back injuries, incidence of disabling injuries, days lost per injury, and

total cost increased as the percent of population “overstressed” increased. The

authors did not report any statistical analyses.

Both Sets of Data. Another study that examined the predictive validity of the

psychophysical methodology was published by Herrin, Jaraiedi, and

Anderson.(17) These investigators performed detailed biomechanical and psychophysical evaluations on 55 industrial jobs from five major industries. The

psychophysical analyses involved determining the minimum percent of capable

population from the Liberty Mutual tables for each individual task (PSY.MIN)

as well as an average percent of capable population when the job involved multiple tasks (PSY.AVG). Additional comparison variables included the Job

Severity Index (JSI) and Lifting Strength Ratio (LSR). These investigators

modified the definition of JSI to represent a frequency- and time-weighted ratio

of weights lifted compared to the average task-specific lifting strength of males

and females, averaged across all tasks. By contrast, the LSR represented the

worst case scenario in that it was the largest single ratio identified among all

the tasks.

After the jobs were characterized as described above, injury and illness data

for 6912 incumbent workers were monitored for 2 years retrospectively and 1

year prospectively (> 12.6 million man-hours). Morbidity was categorized as

contact incidents, musculoskeletal disorders (excluding the back), and back

incidents, and expressed as incidence rates (number of incidents per 100 workers per year). Severity data were also examined (lost-time vs. no-lost-time).
The results revealed a significant negative correlation between the minimum

percent capable population (PSY.MIN) and all three incidence rates, that is, the

incidence rates increased as the percentage capable population decreased. A

similar correlation was noted between PSY.MIN and severity. There was no

correlation between the average percentage capable population (PSY.AVG)

with any incidence rate or severity. The incidence rates for musculoskeletal disorders and back disorders were positively and significantly correlated with the

LSR. LSR was also correlated with severity. The JSI only correlated with

severity, not incidence.

The authors offered the following conclusions:

1. Overexertion injuries can be related to physical job stresses.

2. Indices representing the extremes of the job requirements (PSY.MIN and

LSR) are generally more predictive of risk than indices representing averages (PSY.AVG and JSI).

3. The percentage of capable population for the most stressful aspect of the

job, either isometric or psychophysical, is the simplest index of this type.

Evaluation According to Physical 

Assessment Criteria 

Is Psychophysical Strength Testing Safe to Administer?

According to Snook, there was one compensable injury among the 119 industrial worker test subjects.(18) This single episode involved a chest wall strain

associated with a high lift. It was also associated with 4 days restricted activity,

but no permanent disability.

Does Psychophysical Strength Testing Give Reliable

Quantitative Values?

The Liberty Mutual protocol incorporates a criterion for test–retest reliability

(maximum difference of 15%). Legg and Myles reported that 34% of their data

did not meet this criterion.(19) In contrast, Gallagher and coworkers reported that

only 3% of tests in their study had to be repeated because the 15% test–retest

criterion was violated.(20) Clearly, the maximum acceptable weights and forces

are quantitative.

Is Psychophysical Strength Testing Practical?

There are two major sources of impracticality associated with this type of

strength assessment: 1) it is conducted in a laboratory, and 2) the duration of

testing is somewhat prolonged compared to other strength assessment methods.

It is possible, however, to have the subjects use objects that are actually handled in the workplace. Equipment is not very costly.

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43

Is Psychophysical Strength Testing Related to 

Specific Job Requirements (Content Validity)?

The content validity of this method of strength assessment is one of its greatest

assets. One potential weakness, however, is its insensitivity to bending and

twisting.

Does Psychophysical Strength Testing Predict Risk of

Future Injury or Illness (Predictive Validity)?

The results of two epidemiological studies suggest that selected indices derived

from the psychophysical data are predictive of risk for contact injury, musculoskeletal disorders (excluding the back), and back disorders.(15,16) These indices

are correlated to the severity of these injuries. A third study demonstrated predictive value.(17) It should be noted, however, that at high frequencies, test subjects selected weights and forces that often exceeded consensus criteria for

acceptable levels of energy expenditure. In addition, test subjects may also

select weights and forces that exceed consensus levels of acceptable disc compression.

Summary

The psychophysical methodology, as applied to strength, has been used to

determine the maximum acceptable weights and forces associated with manual

materials-handling tasks for healthy adult male and female industrial workers.

The results of these studies have been published in a series of tables for lifting,

lowering, pushing, pulling, and carrying. The data were primarily developed for

the assessment of the strength requirements of such tasks relative to the abilities of a population of healthy adult workers. As a result, a job is analyzed by

comparing the required weight or force to the percent of capable population.

Applied in this manner, the job analysis results correlate with observations of

morbidity, especially related to the low back.

This technique was neither developed nor standardized for the purpose of

worker selection. At this time, the use of psychophysical methods of strength

assessment for predicting capability or future risk of injury, illness, impairment,

or disability for an individual has not been validated. In the context of a preemployment evaluation, job-specific psychophysical testing might be considered for testing ability to perform critical job tasks; however, the motivation of

the test subject may affect the results. For example, an individual who is highly

motivated to demonstrate capability may select a “maximum acceptable weight

or force” greater than what would be selected in a different context. In terms of

a preplacement evaluation, the issue may be direct threat. At this time, no evidence indicates that this testing can predict risk of future injury for an individual. The assessment of human strength by psychophysical methods therefore

has limited application to the assessment of individuals. As Snook and associates state, the available data should rather be used to analyze jobs.(10)
References

1. Stevens, S.S.: On the Psychophysical Law. Psychol. Rev. 64:153–181

(1957).

2. Jones, L.A.: Perception of Force and Weight: Theory and Research.

Psychol. Bull. 100(1):29–42 (1986).

3. Snook, S.H.: Psychophysical Acceptability as a Constraint in Manual

Working Capacity. Ergonomics 28(1):331–335 (1985).

4. Snook, S.H.: Psychophysical Considerations in Permissible Loads.

Ergonomics 28(1):327–330 (1985).

5. Snook, S.H.: The Design of Manual Handling Tasks. Ergonomics 21:963–

985 (1978).

6. Snook, S.H., and V.M. Ciriello: The Design of Manual Handling Tasks:

Revised Tables of Maximum Acceptable Weights and Forces. Ergonomics

34(9):1197–1213 (1991).

7. Ayoub, M.M., N.J. Bethea, S. Devanayagam, S.S. Asfour, G.M.

Bakken, D. Liles, A. Mital, and M. Sherif: Determination and Modeling

of Lifting Capacity, Final Report (HEW [NIOSH] Grant No.

5–R01–OH–00545–02). 1978.

8. Mital, A.: Comprehensive Maximum Acceptable Weight of Lift Database

for Regular 8-Hour Shifts. Ergonomics 27:1127–1138 (1984).

9. Thompson, D.D., and D.B. Chaffin: Can Biomechanically Determined

Stress Be Perceived? Human Factors and Ergonomics Society, Proceedings

of the 37th Annual Meeting, Seattle Wa., 1993. pp. 789–792.

10. Snook, S.H.: Approaches to the Control of Back Pain in Industry: Job

Design, Job Placement, and Education/Training. Spine: State of the Art

Reviews 2:45–59 (1987).

11. Ciriello, V.M., S.H. Snook, A.C. Blick, and P.L. Wilkinson: The Effects

of Task Duration on Psychophysically Determined Maximum Acceptable

Weights and Forces. Ergonomics 33:187–200 (1990).

12. Jiang, B.C., J.L. Smith, and M.M. Ayoub: Psychophysical Modeling for

Combined Manual Materials-Handling Activities. Ergonomics

29(10):1173–1190 (1986).

13. Karwowski, W., and J.W. Yates: Reliability of the Psychophysical

Approach to Manual Materials Handling Activities. Ergonomics

29:237–248 (1986).

14. Mital, A.: The Psychophysical Approach in Manual Lifting — A

Verification Study. Human Factors 25(5):485–491 (1983).

15. Snook, S.H., R.A. Campanelli, and J.W. Hart: A Study of Three

Preventive Approaches to Low Back Injury. J. Occup. Med. 20(7):478–481

(1978).

16. Ayoub, M.M., J.L. Selan, and D.H. Liles: An Ergonomics Approach for

the Design of Manual Materials-Handling Tasks. Hum. Factors

25(5):507–515 (1983).

17. Herrin, G.D., M. Jaraiedi, and C.K. Anderson: Prediction of

Overexertion Injuries Using Biomechanical and Psychophysical Models.

Am. Ind. Hyg. Assoc. J. 47(6):322–330 (1986).

44
45

18. Snook, S.H.: “Assessment of Human Strength: Psychophysical Methods.”

Roundtable Presentation at the American Industrial Hygiene Conference &

Exposition, Boston, 1992.

19. Legg, S.J., and W.S. Myles: Metabolic and Cardiovascular Cost, and

Perceived Effort Over an 8 Hour Day When Lifting Loads Selected by the

Psychophysical Method. Ergonomics 28(1):337–343 (1985).

20. Gallagher, S.: Acceptable Weights and Psychophysical Costs of

Performing Combined Manual Handling Tasks in Restricted Postures.

Ergonomics 34(7):939–952 (1991).

21. Ciriello, V.M., and S.H. Snook: A Study of Size, Distance, Height, and

Frequency Effects on Manual Handling Tasks. Hum. Factors

25(5):473–483 (1983).

22. Mital, A., and M.M. Ayoub: Effect of Task Variables and Their

Interactions in Lifting and Lowering Loads. Am. Ind. Hyg. Assoc. J.

42:134–142 (1981).

23. Asfour, S.S., M.M. Ayoub, and A.M. Genaidy: A Psychophysical Study

of the Effect of Task Variables on Lifting and Lowering Tasks. J. Hum.

Ergol. 13:3–14 (1984).

24. Garg, A., A. Mital, and S.S. Asfour: A Comparison of Isometric and

Dynamic Lifting Capability. Ergonomics 23(1):13–27 (1980).

25. Mital, A., and I. Manivasagan: Maximum Acceptable Weight of Lift as a

Function of Material Density, Center of Gravity Location, Hand

Preference, and Frequency. Hum. Factors 25(1):33–42 (1983).

26. Legg, S.J., and D.R. Haslam: Effect of Sleep Deprivation on SelfSelected Workload. Ergonomics 27(4):389–396 (1984).

27. Smith, J.L., and B.C. Jiang: A Manual Materials Handling Study of Bag

Lifting. Am. Ind. Hyg. Assoc. J. 45(8):505–508 (1984).

28. Mital, A., and H.F. Fard: Psychophysical and Physiological Responses to

Lifting Symmetrical and Asymmetrical Loads Symmetrically and

Asymmetrically. Ergonomics 29(10):1263–1272 (1986).

29. Mital, A.: Maximum Weights of Asymmetrical Loads Acceptable to

Industrial Workers for Symmetrical Lifting. Am. Ind. Hyg. Assoc. J.

48(6):539–544 (1987).

30. Mital, A.: Psychophysical Capacity of Industrial Workers for Lifting

Symmetrical Loads and Asymmetrical Loads Symmetrically and

Asymmetrically for 8 Hour Work Shifts. Ergonomics 35(7/8):745–754

(1992).

31. Drury, C.G., J.M. Deeb, B. Hartman, S. Wooley, C.E. Drury, and S.

Gallagher: Symmetric and Asymmetric Manual Materials Handling. Part

1. Physiology and Psychophysics. Ergonomics 32(5):467–489 (1989).

32. Legg, S.J., and C.M. Pateman: Human Capabilities in Repetitive Lifting.

Ergonomics 28(1):309–321 (1985).

33. Drury, C.G., and J.M. Deeb: Handle Positions and Angles in a Dynamic

Lifting Task. Part 2. Psychophysical Measures and Heart Rate. Ergonomics

29(6):769–777 (1986).

34. Mital, A.: Maximum Acceptable Weights of Lift Acceptable to Male and

Female Industrial Workers for Extended Work Shifts. Ergonomics

27(11):1115–1126 (1984).
35. Fernandez, J.E., M.M. Ayoub, and J.L. Smith: Psychophysical Lifting

Capacity Over Extended Periods. Ergonomics 34(1):23–32 (1991).

36. Mital, A., and F. Aghazadeh: Psychophysical Lifting Capabilities for

Overreach Heights. Ergonomics 30(6):901–909 (1987).

37. Mital, A., and L-W Wang: Effects on Load Handling of Restricted and

Unrestricted Shelf Opening Clearances. Ergonomics 32(1):39–49 (1989).

38. Mital, A.: Patterns of Differences Between the Maximum Weights of Lift

Acceptable to Experienced and Inexperienced Materials Handlers.

Ergonomics 30(8):1137–1147 (1987).

39. Smith, J.L., M.M. Ayoub, and J.W. McDaniel: Manual Materials

Handling Capabilities in Non-Standard Postures. Ergonomics

35(7/8):807–831 (1992).

40. Gallagher, S., W.S. Marras, and T.G. Bobick: Lifting in Stooped and

Kneeling Postures: Effects on Lifting Capacity, Metabolic Costs, and

Electromyography of Eight Trunk Muscles. Int. J. Ind. Erg. 3:65–76

(1988).

41. Gallagher, S., and C.A. Hamrick: Acceptable Workloads for Three

Common Mining Materials. Ergonomics 35(9):1013–1031 (1992).

46
47

5. Isokinetic Strength

Theory and Description of 

Isokinetic Strength Measurement

The concept of isokinetic measurement of strength was originally related by

Hislop and Perrine.(1) Characteristics of an isokinetic exertion are constant

velocity throughout a predetermined range of motion. Strictly speaking, a

means of speed control, and not a load in the usual sense, is applied in isokinetic exertions.(1) However, load and resistance are definitely present in this technique. In this case, the load is a result of the energy absorption process performed by the device to keep the exertion speed constant. The device prevents

energy from being dissipated through acceleration in isokinetic exercise. The

energy is instead converted into a resistive force, which varies in relation to the

efficiency of the skeletal muscle. 

Since the speed of motion is held constant in isokinetic exercise, the resistance experienced during a contraction is equivalent to the force applied

throughout the range of motion. For this reason, the technique of isokinetic

exercise has sometimes been referred to as accommodating resistance exercise.

This type of exercise allows the muscle to contract at its maximum capability at

all points throughout the range of motion. At the extremes of the range of

motion of a joint, the muscle has the least mechanical advantage, and the resistance offered by the machine is correspondingly lower. Similarly, as the muscle

reaches its optimal mechanical advantage, the resistance of the machine

increases proportionally. It must be understood, however, that while isokinetic

devices control the speed of the exertion, this does not assure a constant speed

of muscle contraction.

The speed of isokinetic contractions is constant during individual exertions;

however, it is also possible to compare muscular performance over a wide

range of isokinetic velocities. Increasing the isokinetic speed of contraction will

place increasing demands on Type II muscle fibers (fast twitch and fast oxidative glycolytic).

Workplace Assessment

It is clear that isometric strength testing cannot substitute for dynamic strength

assessment when examining highly dynamic occupational job demands. As

most industrial work tasks contain a significant dynamic component, analysis

of isokinetic strength capabilities appears to offer some advantage to isometric

testing in this regard. However, it must be recognized that isokinetic devices are

not entirely realistic compared with free dynamic lifting in which subjects may

use rapid acceleration to gain a weight-lifting advantage. 
Most isokinetic devices available on the market focus on quantifying strength

about isolated joints or body segments, for example, trunk extension and flexion (see Figure 7). This may be useful for rehabilitation or clinical use, but isolated joint testing is generally not appropriate for evaluating an individual’s

ability to perform occupational lifting tasks. One should not assume, for

instance, that isolated trunk extension strength is representative of an individual’s ability to perform a lift. In fact, lifting strength may be almost entirely

unrelated to trunk muscle strength. Strength of the arms or legs (and not the

trunk) may be the limiting factor in an individual’s lifting strength. For this reason, machines that measure isokinetic strengths of isolated joints or body segments should not be used as a method of evaluating worker capabilities related

to job demands in most instances.

Published Data

Several investigators have used dynamic isokinetic lifting devices designed to

measure whole-body lifting strength.(2-5) These devices typically consist of a

handle connected by a rope to a winch, which rotates at a specified constant

48

Figure 7—Many isokinetic devices are designed to evaluate isolated joint muscle strengths. Such devices can be of great benefit in a clinical setting, but may

not be as conducive to workplace assessment procedures. (Cybex Medical,

Division of Henley HealthCare, Inc., Sugarland, Texas.) [Photo courtesy of

Henley HealthCare.]
velocity when the handle is pulled (Figure 8). The amount of force generated

by the subject is thus evaluated over a specified range of motion, and the peak

or average force generated during the test is recorded. As detailed below, some

investigators have been able to demonstrate that the results of certain isokinetic

strength tests (for example, an isokinetic exertion from floor to chest height)

appear to be correlated with the amount of weight individuals were willing to

lift for infrequently performed tasks.(2)

49

Figure 8—An isokinetic whole-body strength measurement system. This device

allows the experimenter to assess various muscular strengths (such as those

shown) at a constant velocity. (From Kamon, E., and Pytel, J.L.: Dynamic

Strength Test as a Predictor for Maximal and Acceptable Lifting. Ergonomics

24(9):663–672 (1981). Reprinted with permission of Taylor and Francis Ltd.)

Pytel and Kamon(2) analyzed various types of isokinetic strength in relation to

maximal dynamic lifting capacity in their initial study. These investigators

developed three isokinetic strength techniques and compared them to actual

lifting capabilities of their subjects, consisting of 10 male and 10 female subjects. Each of the isokinetic exertions were performed at two speeds: .73

meters/second and .97 meters/second. Figure 8 illustrates the three isokinetic

exercises evaluated in this study, which were named Dynamic Lift Strength

(DLS), Dynamic Back Extension Strength (DBES), and Dynamic Elbow

Flexion Strength (DEFS). These were compared with two tests that evaluated

actual lifting capacity. The Maximal Dynamic Lift (MDL) was the amount that

a subject estimated was the maximum he or she could safely lift from the floor
50

Table VI

Means and Standard Deviations of the Maximal Dynamic Lift (MDL) and

Peak Forces for the Dynamic Lift Strength (DLS), Dynamic Back

Extension Strength (DBES), and the Dynamic Elbow Flexion Strength

(DEFS) Performed at Two Speeds.(2)

Test Speed (m/sec) Strength (N)

Women (n = 10) Men (n = 10)

MDL 250 ± 54 544 ± 109

DLS .73 379 ± 95 601 ± 129

DBES .73 315 ± 87 540 ± 101

DEFS .73 167 ± 33 323 ± 55 

DLS .97 260 ± 99 398 ± 113

DBES .97 210 ± 95 339 ± 102

DEFS .97 120 ± 38 233 ± 39

to a level 113 cm above the floor. The Maximal Acceptable Lift was defined as

the weight subjects felt they could lift safely at a rate of six lifts per minute for

a regular work day. 

Results of this investigation are provided in Table VI. The investigators found

that the DLS (measured at .73 m/sec), in combination with the gender of the

subject, was highly correlated with the MDL selected by the subject (R2 =

.941). Isokinetic tests performed at .97 m/sec did not correlate as well. Strength

values obtained at this speed were consistently lower than those obtained at the

slower isokinetic velocity, a finding regularly reported in tests of isokinetic

strength. 

Other investigators have taken the same type of device and devised methods

of mounting it in different orientations to evaluate isokinetic strength in a variety of orientations.(3,5) Various handles attached to the end of the rope have been

used to evaluate tasks such as short distance carrying or pulling in the horizontal, vertical, or transverse planes. The variations described above have been

used to measure isokinetic lifting strengths using horizontal exertions at heights

of 81 cm and 152 cm in a sample of male and female university students. These

subjects were instructed to exert as hard as possible without jerking. Table VII

provides data on the isokinetic strength of male and female students in both

vertical and horizontal planes. 
51

Evaluation According to Physical 

Assessment Criteria 

Is Isokinetic Strength Testing Safe to Administer?

Given proper procedures and supervision, isokinetic musculoskeletal testing

appears to be a reasonably safe method of evaluating muscular strength and

endurance. Certain risks associated with use of free weights, weight machines,

and other isotonic methods of assessing strength are not present in isokinetic

testing. In addition, since the resistance or load experienced by the subject is

directly related to the force the subject voluntarily applies, risk of injury due to

overloading of the musculature would decrease, because the subject can control

his or her own effort. However, it should be noted that some investigators have

reported that lower velocity isokinetic exertions may be painful.(6,7)

Certain precautions have been suggested to reduce injury risk in performance

of isokinetic musculoskeletal evaluations:

1. Warm-up and stretching of the involved muscle groups.

2. Performance of 5 to 10 submaximal trial repetitions to assess proper

alignment, subject comfort, and subject familiarization with the test

requirements.

3. Postexercise stretching.

4. Ice/compression/elevation any time postexercise effusion or swelling

occurs.

In addition, subjects should wear tennis or running shoes during isokinetic

muscle testing when performing standing exertions.

The American Academy of Orthopaedic Surgeons has established guidelines

to meet when testing dynamic muscle performance.(8) The following summarize

the guidelines developed by the AAOS Human Performance Testing Task

Force:

1. Equipment must be determined safe for both the subject and the tester.

2. The reliability and validity of the equipment should be documented.

3. Equipment should be designed to ensure freedom of movement with subTable VII

Means and standard deviations for isokinetic strengths (N) of males and

females in the vertical and horizontal planes.(5) Isokinetic tests include

dynamic lift strength (DLS), dynamic elbow flexion strength (DEFS), and

horizontal isokinetic strengths at 81 cm (DS81) and 152 cm (DS152).

Test Strength

Females Males

DLS 632 ± 251 1083 ± 297

DEFS 269 ± 132 741 ± 327

DS81 223 ± 122 344 ± 93

DS152 312 ± 156 594 ± 172
ject comfort, and isolation of the motion should be achieved via proper

stabilization techniques.

4. Training and education in correct use of the equipment should be available.

Does Isokinetic Strength Testing Give Reliable,

Quantitative Values?

Several studies have reported on the reliability of values obtained using isokinetic devices. Results have generally indicated high reliability for isokinetic

equipment. In a study examining the isokinetic movement of the knee extensors

using a CYBEX II dynamometer, Johnson and Siegel(9) found reliability coefficients ranged from .93–.99. Furthermore, these authors reported that reliability

appears to be affected more by testing over days than when comparing different

trials performed on the same day. Pipes and Wilmore(10) reported test reliability

in isokinetic exertions of a similar magnitude (r = .92–.99) in tests of bench

press strength and leg press strength. Moffroid et al.(11) performed a test of reliability for torque measurements at various velocities with a CYBEX device and

found that peak torque was reliably measured (r = .999) at velocities ranging

from 4 to 12 rpm. Intratest, intertest, and intertester reliability of isokinetic

strength measurements were examined in a study quantifying strength in children using a CYBEX dynamometer.(12) The authors concluded that none of

these sources of measurement error were a significant source of inaccuracy. 

While good reliability for the CYBEX dynamometer has been reported, some

authors have expressed concern about a torque “overshoot” artifact that may

appear in CYBEX torque measurements.(13) This artifact is evidenced as an initial prominent spike in the torque output curve, which is then followed by a

series of progressively diminishing secondary oscillations. The cause of this

phenomenon appears to be a result of “overspeeding” of the dynamometer’s

input lever during a free acceleration period before its resistance mechanism is

engaged. The authors concluded that the prominent initial spikes represent inertial forces and should not be confused with actual muscle tension development.

Proper signal damping procedures may suppress this “overshoot”; however,

damping should not be used when absolute torque values are required.

Many other isokinetic devices have been developed since the introduction of

the CYBEX in 1980. Most of these devices have demonstrated reliability similar to the CYBEX. Klopfer and Greij(6) analyzed the reliability of torque production on the Biodex B-200 at high isokinetic velocities (300 deg/s–450 deg/s)

and found that coefficients of correlation ranged from .95–.97, reflecting a high

degree of reliability of the test equipment. Other authors reported reliability of

between .94 and .99 with the same equipment.(14) A study analyzing the reliability of the Kinetic Communicator (KINCOM) device reported intraclass correlation coefficients of .94–.99.(15) Reliability of the Lido isokinetic system

appears somewhat lower than the others reported here, ranging from .83–.94.(16)

The reliability of the Mini-Gym (the isokinetic device best suited to analysis of

occupational tasks) does not appear to have been reported in the literature. 

52
53

The foregoing data suggest that isokinetic strength testing equipment generally exhibits a high degree of reliability. However, it should be noted that results

obtained using one system may not be comparable to results collected on other

systems. Several studies have attempted to compare results between systems,

and all have found significant differences. Torque values may vary as much as

10%–15% between different systems.(17,18) These discrepancies indicate that

data collected on different devices cannot be compared, and normative data

generated on one system cannot be used on other systems.

Is Isokinetic Strength Testing Practical?

Several issues may impact the practicality of using isokinetic devices to examine an individual’s muscular capabilities. Not the least of these is the significant

cost of purchasing an isokinetic measurement system. Many of the systems discussed in this section cost tens of thousands of dollars, which may render such

systems impractical for many applications. Another important issue related to

practicality for job-specific strength assessment is the ability of these devices to

easily simulate a variety of occupational tasks. Although certain isokinetic

devices have been specifically designed to mimic lifting tasks,(2) many are

designed simply for quantifying of strength of isolated muscle groups in a clinical setting without regard to accurate simulation of work tasks. 

Is Isokinetic Strength Testing Related to 

Specific Job Requirements?

The answer to this question depends upon the type of isokinetic device and

how it is used. As discussed previously, isokinetic machines that test isolated

muscle groups do not meet this criterion if the job requires use of many muscle

groups or body segments. On the other hand, the Mini-Gym can be used to

evaluate the dynamic strength necessary to perform many types of occupational

tasks, and results of strength tests using this device appear to be related to lifting capacity, at least under certain conditions.(2) However, many industrial tasks

are clearly too complicated to be evaluated using current isokinetic technologies. Great care must be taken to ensure that isokinetic strength measurements

are appropriate for analysis of strength requirements associated with specific

occupational tasks.

Does Isokinetic Strength Testing Predict Risk 

of Future Injury or Illness?

A recent prospective epidemiological investigation analyzed whether isokinetic

lifting strength could predict who would be at risk of occupational low-back

pain.(19) Subjects were required to perform maximal whole-body lifting exertions using an isokinetic linear lift task device, and were then followed for 2

years to evaluate whether this measure of strength predicted who would experience LBP. Results of this study indicated that isokinetic lifting strength was a

poor predictor of subsequent LBP or injury. It should be noted, however, that
no attempt was made in this study to compare job strength requirements to

individual strength capabilities. Whether isokinetic strength tests can be used to

predict future LBP when a careful comparison of job demands and individual

strength capacity is made has yet to be determined.

Summary

Isokinetic strength assessment is a technique of assessing dynamic muscle

function where the velocity of motion is constant. Numerous isokinetic devices

are available on the market, most of which focus on quantifying strength about

isolated joints or body segments. Devices that perform isolated joint assessment

are typically quite expensive and may be well-suited to clinical and rehabilitative use. However, such devices may be limited in their ability to assess occupational demands at the workplace. This is because isolated joint or segment

strengths may be unrelated to a person’s ability to perform a specified occupational task. For example, the ability to perform a lifting task may be unrelated

to isokinetic trunk strength; rather, the ability to perform such a task may be

limited by strength capabilities of other muscle groups (such as those of the

arms or legs). 

A different sort of isokinetic device has been used by some to measure

whole-body lifting strength. These devices typically have a handle connected

by a rope to a winch, which rotates at a specified isokinetic velocity when the

handle is pulled. Good correlations have been reported between isokinetic lifting strength (typically a lift from floor to chest height) and psychophysical lifting results of tasks having similar vertical displacement. However, while validity of whole-body isokinetic strength has been demonstrated for relatively simple lifting tasks, the more complex lifting tasks often seen in industry are not

well simulated using current isokinetic apparatus.

Isokinetic muscle testing appears to be a relatively safe and highly reliable

technique of assessing dynamic muscle function. The practicality of using isokinetic systems may depend heavily on their substantial cost. The limitations of

assessing job-specific strength demands by such systems has been noted above.

When assessing isokinetic strength, one must always bear in mind that this

mode of contraction is not quite physiologic, that is, isokinetic movements are

not used in everyday human motion. The ability of isokinetic muscle testing to

predict risk of future injury or illness has not yet been demonstrated. Thus far,

prospective studies have shown that generic isokinetic strength tests (like

generic static strength tests) do not predict those who might experience low

back pain. Whether isokinetic strength tests can be used to predict injury or illness when careful comparisons of job demands and individual strength capabilities are performed has not yet been investigated.

54
55

References

1. Hislop, H., and J.J. Perrine: The Isokinetic Concept of Exercise. Phys.

Ther. 47:114–117 (1967).

2. Kamon, E., and J.L. Pytel: Dynamic Strength Test as a Predictor for

Maximal and Acceptable Lifting. Ergonomics 24(9):663–672 (1981).

3. Mital, A., W. Karwowski, A.K. Mazouz, and E. Orsarh: Prediction of

Maximum Weight of Lift in the Horizontal and Vertical Planes Using

Simulated Job Dynamic Strengths. Am. Ind. Hyg. Assoc. J. 47:288–291

(1986).

4. Kishino, N.D., T.G. Mayer, R.J. Gatchel, M. M. Parish, C. Anderson,

L. Gustin, and V. Mooney: Quantification of Lumbar Function: Part 4:

Isometric and Isokinetic Lifting Simulation in Normal Subjects and LowBack Dysfunction Patients. Spine 10(10):921–927 (1985). 

5. Mital, A., and R. Vinayagormoothy: Three-Dimensional Dynamic

Strength Measuring Device: A Prototype. Am. Ind. Hyg. Assoc. J.

45:B9–B12 (1984).

6. Klopfer, D.A., and S.D. Greij: Examining Quadriceps/Hamstrings

Performance at High Velocity Isokinetics in Untrained Subjects. J. Orthop.

Sports Phys. Ther. 10:18–22 (1988).

7. Elsner, R.C., L.R. Pedegana, and J. Lang: Protocol for Strength Testing

and Rehabilitation of the Upper Extremity. J. Orthop. Sports Phys. Ther.

4:229 (1983).

8. American Academy of Orthopaedic Surgeons (AAOS): Human

Performance Testing Task Force, October 1988.

9. Johnson, J., and D. Siegel: Reliability of an Isokinetic Movement of the

Knee Extensors. Res. Quar. 49:88–90 (1978).

10. Pipes, T.V., and J.H. Wilmore: Isokinetic vs. Isotonic Strength Training

in Adult Men. Med. Sci. Sports Exer. 7:262–271 (1975).

11. Moffroid, M., R. Whipple, J. Hofkosh, et al.: A Study of Isokinetic

Exercise. Phys. Ther. 49:735 (1969).

12. Molnar, G.E., J. Alexander, and N. Gutfield: Reliability of Quantitative

Strength Measurements in Children. Arch. Phys. Med. Rehabil. 60:218

(1979).

13. Sapega, A.A., J.A. Nicholas, D. Sokolow, and A. Saraniti: The Nature of

Torque “Overshoot” in CYBEX Isokinetic Dynamometry. Med. Sci. Sports

Exer. 14(5):368–375 (1982).

14. Wilk, K.E., and R.E. Johnson: The Reliability of the Biodex B-200

(Abstract). Physical Therapy 68:792 (1988).

15. Farrell, M., and J.G. Richards: Analysis of the Reliability and Validity of

the Kinetic Communicator Exercise Device. Medicine and Science in

Sports and Exercise 18:44–49 (1986).

16. Lord, J., S. Aitkins, M. McCrory, et al.: Reliability of the Lido Digital

Isokinetic System for the Measurement of Muscular Strength (Abstract).

Phys. Ther. 67:757 (1987).
17. Wilk, K.E., R.E. Johnson, et al.: A Comparison of Peak Torque Values of

Knee Extensor and Flexor Muscle Groups Using Biodex, Cybex, and KinCom Isokinetic Dynamometers. Phys. Ther. 67:789 (1987).

18. Timm, K.E.: Comparison of Knee Extensor and Flexor Group

Performance Using the Cybex 340 and the Merac Isokinetic

Dynamometers. Phys. Ther. 69:389 (1989). 

19. Mostardi, R.A., D.A. Noe, M.W. Kovacik, and J.A. Porterfield:

Isokinetic Lifting Strength and Occupational Injury: A Prospective Study.

Spine 17(2):189–193 (1992).

56
57

6. Conclusion

In spite of advances in measurement techniques and an explosive increase in

the volume of research, our understanding of human strength remains in its

introductory stages. It is clear that muscle strength is a highly complex and

variable function that depends on a large number of factors. It is not surprising,

therefore, that large differences in strength exist not only between individuals,

but even within the same individual tested repeatedly on a given piece of equipment. The issue is compounded by the fact that correlations of strength among

different muscle groups in the same individual are generally low, and tests of

isometric strength do not necessarily reflect the strength an individual might

exhibit in a dynamic test. As a result of these and other influences, great care

needs to be exercised in designing, evaluating, reporting, and interpreting muscular strength assessments.

Traditionally, tests of muscular strength were in the domain of the

orthopaedist, physical therapist, and exercise physiologist. Such tests are also

an important tool for the ergonomist, however, because of the high strength

demands on workers in manual materials-handling tasks. In some cases, task

demands may approach or even exceed the strength that an individual is voluntarily willing to exert in a test of strength. In such cases, evidence suggests

that the likelihood of injury is significantly greater than when the task

demands lie well within an individual’s strength capacity. Because the relationship among strength capabilities, job demands, and musculoskeletal injury

has been established, it is apparent that tests of muscular strength may benefit

the ergonomist both in designing jobs and in ensuring that individuals have

sufficient strength to safely perform physically demanding jobs. Several

strength assessment techniques have been employed for these purposes, each

possessing unique characteristics and applicability to job design and worker

selection procedures. Our main purpose has been to elucidate these strengths

and weaknesses, so that tests of strength may be properly applied in designing

jobs and selecting workers.

One of the crucial points we have emphasized is that any test of strength used

in job design or worker selection must be directly related to the demands of the

job.

(1) For example, if an occupational lifting task has a high dynamic component, a test of isometric strength is not likely to provide the data necessary for

proper design of the job. Of course, dynamic strength tests would also be misapplied in assessing a job requiring isometric exertions. Another potential pitfall is using tests of strength on isolated muscle groups and assuming that they

are indicative of whole-body strength. For example, one might mistakenly

assume that dynamic trunk extension strength represents a person’s capability

to perform a lifting task. However, an individual’s lifting capacity may be

entirely unrelated to trunk extension strength. It may, instead, be limited by an

individual’s arm or leg strength, depending on the task being performed.

A final point on strength assessment should be made. An individual’s strength

capability cannot be considered a fixed human attribute. Strength training regimens can increase an individual’s strength capability by 30%–40%. Whether
such changes have a preventive effect when a person performs heavy physical

work has yet to be established in epidemiologic studies; however, some anecdotal evidence supports the possibility.(1)

It should be clear from this publication that muscular strength tests are tools

that can be used to prevent occupational musculoskeletal disease. However, if

these techniques are to be applied successfully, it is imperative that they be

applied with a clear understanding of the advantages and limitations associated

with each strength assessment procedure. 

Reference

1. Chaffin, D.B., and G.B.J. Andersson: Occupational Biomechanics, 2nd

Ed. New York: John Wiley & Sons, 1991. p. 518.

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59

Appendix 

Typical Written Instructions 

for Isometric Strength Testing

“The strength tests you will participate in today will be isometric — that is,

there will be no actual movement of the body during the test. Instead, you will

be exerting a force against [here enter the description of the interface between

the testee and the testing hardware]. The attached photographs (sketches) show

examples of people doing the tests.”

“The test procedure will be conducted as follows:

1. Each test will be explained to you.

2. Each motion you will be asked to perform will be demonstrated. If there

is anything you do not understand, please ask questions.

3. You will be placed into position at the [interface].

4. The test motion you are to perform will be demonstrated again.

5. You will try the test motion. If you have an questions about the test

motion, please ask questions.

6. You will be given a 1-minute rest.

7. You will perform the first trial of the test. 

8. You will be a given a rest of at least 2 minutes.

9. You will perform the second trial of the test. In some cases, you will

receive additional 2-minute rests and perform additional test trials.”

“During each test, I will position you for the test. I will tell you to ‘begin

when you are ready.’You can begin your exertion any time after that. While

you are performing the test, I will be giving you a slow four count ‘one, two,

three, four, relax.’ Do not stop your exertion until I tell you to relax.”

“Do not jerk against the interface. Gradually increase your strength to the

maximum you feel comfortable exerting, and then hold that force level until

you are asked to relax. It is important for you to maintain your maximum exertion during the entire test. When the relax command is given, you may relax as

fast as you want to.”