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June 28 , 2002


Strength Testing for Employers: A Means
of Reducing Injuries Caused by Overexertion

Gene Carmean, MPA


The need for physical testing for workers in manual materials handling jobs has been recognized by occupational health professionals, risk managers, and ergonomists increasingly over the past few years.  Risk managers have an interest in job safety and reducing workers' compensation costs.  Physicians and occupational health nurses seek to reduce unnecessary injuries and find ways to better predict in advance those most likely to become injured while ergonomists seek to redesign jobs to better match the work to the worker.

Back Injuries: A Continuing Problem

According to the National Academy of Sciences (NAS, 2001) study on musculoskeletal disorders and work, back injuries account for over one million workers losing time from work each year.  NAS conservatively estimates that back injuries cost employers approximately $50 billion dollars each year.  According to this study, the most frequent risk factor associated with back injuries was "lifting and/or carrying loads."  Other risk factors  identified included bending and twisting and whole body vibration.  However, no risk factor came close to the importance of lifting and carrying loads.  Injuries occur when there is a mismatch between the worker and the work.  Workers without sufficient strength to perform their job tasks incur injuries of overexertion.

Reducing Injuries Caused by Overexertion

Since overexertion injuries account for a significant number of all work-related back injuries to employees, it makes sense to reduce the probability of their occurrence.  Hiring workers with the adequate strength to perform the job can reduce these injuries.  One way to determine if a potential employee has the ability to perform the job is by administering a strength test.  Approaches to strength testing must meet two goals:

  1. Include a valid and legally defensible job analysis of the essential, frequently performed, and physically demanding tasks associated with the occupation.

  2. Ensure that the physical ability test is job-related, valid, and reliable that can confidently be used in the selection of individuals for physically demanding jobs. 

Experience with the model described below has resulted in the preplacement testing of more than 10,000 job applicants in California alone.

Strength Tests

Strength tests must be chosen on the basis of safety, reliability, and validity.  Ability tests are safer than job simulation tests because it is preferable to determine how much weight an applicant can lift rather than asking the applicant to lift a heavy weight.  If the applicant does not have the necessary strength to lift the weight an injury may occur during the test.

Valid strength tests should not be confused with Functional Capacity Evaluations (FCE).  FCEs typically involve multiple measurements of various physical dimensions including some assessments of strength and flexibility.  Periodic measurements using the same FCE can document changes in a patient's functional status over time.  However, FCEs typically suffer from the fact that they are basically measures of function with little or no evidence linking them to successful performance on a particular job.  In a comprehensive review of several widely used FCEs, Innes and Straker, (2000), found that "most work-related assessments have limited evidence of validity" and none had sufficient psychometric evidence to support an employment screening decision.  Because employers have legal responsibilities to not discriminate against either females or persons with disabilities, tests such as FCEs are unacceptable choices for physically screening workers prior to placement.  Strength tests must be designed to ensure that the ability test (selection device) is empirically demonstrated to be related to the job.

ADA and Gender Discrimination Concerns

In a series of decisions over the past three years, the US Supreme Court has narrowed the coverage of the Americans with Disabilities Act to such a degree that strength tests should rarely be subject to challenge.  In order to be subject to the provisions of the ADA, a job applicant must be so disabled as to have difficulty performing activities of daily living such as brushing one's teeth, walking, seeing, and so forth.  The disability can no longer be trivial: it must be both serious and enduring.  It will be difficult to find job applicants so disabled that they have difficulty dressing themselves, walking, and brushing their teeth yet be capable of performing a physically demanding job for which a strength test has been validated. 

The primary concern for employers assessing the strength abilities of new hires will be discrimination on account of gender.  Since adverse impact occurs on virtually all tests of physical ability, the need to demonstrate job-relatedness and validity will always be present.  Job-relatedness and validity are demonstrated by job analysis and the demonstration of an empirical relationship between the test and performance on the job.

Job Analysis

To document job-relatedness, it is necessary collect job information from experienced workers.  The job analysis inquiry is directed at collecting tasks from incumbents which require static strength.  Static strength involves the continuous exertion of maximum muscle force for a brief period time.  Tasks that involve the lifting, pulling, pushing, or carrying of objects and materials require static strength.  Following the collection of job information, the work site should be visited to directly examine tools, equipment and materials that had been described by workers during the meeting.  An industrial scale and/or a force gauge is used to directly weigh as many of the relevant objects as possible.  If additional materials or tools are found that are also lifted, these objects are weighed, the weights recorded, and the lifting tasks added to a task listing produced for worker surveys.  In order to measure a job, one needs a measuring tool.  Rating scales are the most useful measuring tools when performing job analysis activities with task inventories.  Rating scales can have a number of customized features depending on the job and specific organizational needs.  To validate a strength test, however, it is important to illicit from workers:

  1. Whether or not the task is performed?

  2. How physically demanding is the task?

  3. How far the object is carried?

  4. How often the task is performed?

  5. How important the task is to the job?

Typically, a sample of employees complete a task inventory.  Employee ratings on the various dimensions are then statistically examined to determine the most critical, frequently performed, and physically demanding tasks. 

Work Sample Development

Having determined which strength tasks are critical for the job, it is next necessary to determine which tasks are suitable for utilization as work samples.  Ideally, the tasks selected should be among the most demanding tasks workers are expected to perform.  Additionally, other criteria should be considered including:

Safety to incumbents.  Tasks selected should be safe to perform in a testing situation.  Some tasks might not be dangerous to experienced workers, but could be to a novice.

Reasonable time to administer.  The tasks selected for work sample development should be those which can be completed in a reasonable amount of time.

Unambiguous scoring and clarity of results.  Tasks selected should be amiable to an unambiguous scoring or rating system.  There should be no disagreement as to what constituted various levels of performance.  Subjective ratings on "style of lifting" or "ease of lifting" are less suitable when objective measures are possible.

Simplicity.  The tasks selected should be as simple as possible from both the point of view of instruction to incumbents and administration of the work sample.

Independence from training and experience.  The tasks selected should be dependent upon strength and not "tricks of the trade" for success.  Novice and experienced individuals with the same level of strength should have the same score.

Commonality.  The tasks selected should be commonly performed by as many workers as possible.

Critical tasks that meet the criteria can be categorized in a variety of ways.  For example, all tasks involving the use of a wheelbarrow or a dolly might form a group or task category called wheelbarrow tasks.  Alternatively, all tasks that involve work at a particular work site, or all tasks performed while repairing heavy equipment could form other groups.  The nature of the job and tasks performed typically lend themselves to the selection of appropriate task categories.  Task categories are important because they help organize the work and ensure that a variety of lifting tasks can be used to construct work samples.   An example of five related tasks that form a category might be:

Five-Gallon Container (Paint, Joint Compound, Floor Sealer) Tasks

1.  Lift/carry a five-gallon can of floor sealer (approx.  weight 46.3 lbs.). 

2.  Carry a five-gallon bucket of paint (55.4 lbs.). 

3.  Lift/handle a five-gallon bucket of joint compound (51 lbs.). 

4.  Lift a five-gallon bucket of paint into the back of a vehicle (55.4 lbs.). 

5.  Lift a five-gallon bucket of paint up onto a stack of other five-gallon paint buckets (55.4 lbs.). 

Work samples may then be developed from these categories of common critical tasks.  For example, a work sample for these tasks might be constructed as:

Five-Gallon Bucket Stack Work Sample

Approach a row of four five-gallon buckets of paint.  Stack three of the buckets on top of one of the bucket furthest to the left.  Next, take the top bucket off the stack and carry it 15 feet to a truck bed.  Set it down and release grip.  Re-grip the bucket and return to the stack of buckets.  Next, place the bucket of paint beside the stack and replace the two remaining cans on the ground in a row, as they were initially. 

Such an event would be timed.  Individuals who struggle with this event will take longer than those who can perform the task with ease.

Selecting Appropriate Static Strength Tests

Depending on the nature of the ability being tested, a wide variety of tests are available.  For strength tests, we have used the Jackson Strength Evaluation System (JSES) and found it to be a valid and reliable predictor of the ability to perform lifting, pushing, pulling, and carrying tasks.  The JSES has several qualities that make it ideal for employment testing.  It has been shown to be safe and practical.  Results should be obtainable within 15 minutes.  The test is highly portable making it ideal for workplace testing.

Empirical Validation

A sample of experienced workers is typically chosen for test validation.  The sample should consist of individuals from various ages, racial groups and both genders.  Field testing consists of a brief medical screening, informed consent, and an explanation of the tests.  Participants are first administered the Jackson Strength Evaluation System which consists of four strength tests.  Participants first exert a constant force for three seconds on three tests which use the lifting bar.  These tests consist of the Arm Lift, the Shoulder Lift and the Trunk Pull tests.  The last test consists of grip strength as measured by a hand dynamometer.  All four tests have an electronic monitor connected to each load cell to record the peak and average amount of force exerted in pounds of force.  Next, the simulations are performed by the participants.  The simulations consist of actual work samples of the job such as the Five-Gallon Bucket Stack described above. Participants are given ample time to rest between events and may decline testing at any time. 

Participants are instructed not to run or to perform the work at an unnatural pace.  Participants are asked to envision a day in which they had a lot of different tasks to perform.  When one task was completed, other important tasks are to follow.  Participants are instructed to work at what might be considered a heavier than average pace, but not one that was unrealistic or unrepresentative of the pace at which they might work on a busy day.  Scores are recorded.

Statistical Analysis

Reliability of the JSES is assessed by comparing the scores of the two recorded trials on each test.  Reliability typically ranges from a low of .97 to a high of .99.  Correlation coefficients are computed for all tests and work samples to determine their interrelationships and lack thereof.  Multiple regression analysis is used to derive equations to predict the performance of individuals on the work sample test who have only taken the JSES.  Evidence for validity is assessed by statistical analysis to determine how well each regression equation is predictive of work sample performance.  The validity coefficients obtained in the MED-TOX studies strongly supports the use of the Jackson Strength Evaluation System as a valid predictor of the ability to lift and carry heavy loads. 

Passing Levels (Cut-off scores)

Setting cut-off scores is a particularly complex area of test construction.  MED-TOX utilizes multiple forms of evidence to arrive a cutoff level that is consistent with business necessity.  Cut-off scores should permit the selection of qualified workers, be based on the results of the job analysis, and account for the performance of currently employed workers.  The final cutoff permits the employment of capable workers while screening out those persons who are likely to be unable to perform or are highly likely to suffer a disabling overexertion injury if placed on the job. 


Strength testing job applicants is the single most cost effective selection device an employer can make to reduce workers' compensation and other injury-related costs.  The relationship between musculoskeletal injuries and lack of strength has been repeatedly demonstrated.  Stronger and more fit workers are more productive and sustain far fewer back injuries than weaker workers.  Strength tests permit the selection of individuals most likely to be able to perform the tasks without undue risk of injury to themselves and to screen-out persons who do not possess sufficient physical ability to adequately perform the job.


National Academy of Sciences (2001).  Musculoskeletal Disorders in the Workplace.  Washington, DC: National Academy Press. 

Innes, E.  & Straker, L.  (2000).  Commercially available work-related assessments: are they reliable and valid? In.  D.  R.  Worth (Ed), Moving in on Occupational Injury.  Boston: Butterworth-Heinemann, 2000.

Copyright © 2002 by WorkCare™ All Rights Reserved

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Original articles © WorkCare™; Orange, California.