Application of the Strain Index: An Advance in Exposure Assessment and Analysis

Kurt T. Hegmann (A),
Arun Garg (B),
and J. Steven Moore ©

(A) Department of Preventive Medicine
Medical College of Wisconsin
8701 Watertown Plank Road
Milwaukee, Wisconsin 53226

(B) Department of Industrial and Manufacturing Engineering
The University of Wisconsin-Milwaukee
P.O. Box 784
Milwaukee, Wisconsin 53201

(C)Department of Occupational and Environmental Medicine
University of Texas Health Center
Tyler Tyler, Texas 75710

ABSTRACT

Ergonomic tools that have predictive validity, are easy to use and quantifiable are needed to be able to measure risks of a job without subjective bias. The Strain Index is a substantial advancement and has been devised to analyze ergonomic risks for distal upper extremity neuromusculoskeletal disorders. This semiquantitative tool allows for the measurement of hazards and does not require unduly lengthy training to begin to use it accurately.

Uses of The Strain Index include analysis of a current job to assess whether it is safe or hazardous,

quantification of the risks, and assistance in the initial design of a job or in the redesign of a job. By utilizing quantitative methods, one of the assets of the model is to assist the user in identifying the specific aspect of the job that is driving the model towards a hazardous assessment and therefore, allows for a targeted approach to best eliminate or lower the risk.

There is no currently available similar model to analyze shoulder risks. A new research investigation has been started that will accomplish a number of goals. First, this research will analyze shoulder risks in detail, and a model, similar to The Strain Index is anticipated. Second, the full validation of The Strain Index, and refinement if needed, will be accomplished. Also, it is believed that there may eventually be a need for models for each major neuromusculoskeletal disorder with relationships to work. Data from this project are anticipated to assist in further defining risks for the various upper extremity disorders.

INTRODUCTION

Current assessment of the ergonomic risks of a job often rely upon expert opinion. While at times an expeditious and satisfactory method, it may be subject to substantial biases, including information biases at the time of the assessment. More importantly, there are unlikely to be sufficient experts to evaluate all jobs in question. A satisfactory resolution could be modeled upon the industrial hygiene assessments of toxicological hazards.

With the quantification of toxicologic exposures and the analysis of morbidity data, it became possible to develop safe and hazardous levels of chemical exposure, since there were valid and reproducible methods. With additional data, some of these levels have then needed further refinement. Quantification of ergonomic risks for distal upper extremity neuromusculoskeletal disorders should proceed based upon this template. Once quantification is possible, widespread measurement and redesign of hazardous jobs are possible.

Currently available methods for distal upper extremity disorders include those using the Silverstein model. Applying such information would require measuring for high forces in combination with high rates of repetition to assign the 'hazardous' label to a job. However, other factors, such as duration of the exertion, posture and vibration would not be included in determining the safety of a job. To utilize concepts of Nathan, one would conclude that there is no reason to evaluate a job for safety when a case of carpal tunnel syndrome arises, as it is not an occupational problem. In our opinion, an approach that integrates the risk factors in a model is attractive, particularly when the model was derived from morbidity data, as well as it relies upon fatigue theory and biomechanical models.

No such models are available for the shoulder. Further, the epidemiologic data are so weak that basic questions are unanswered. For example, it is not known whether force is more important than posture, or whether force has any relevance in the generation of a case of supraspinatus tendinitis/tear.

METHODS AND RESULTS
Principles Needed for Ergonomic Tools

Methods of quantification of ergonomic risk should ideally have the following characteristics: reliability, validity, ease of measurement, and simplicity of calculation.

The first of these criteria indicate that the tool or the independent analyses arrive at the same conclusion, be it safe, or hazardous. The second requires that the measuring tool or analysis be shown to truly measure the target effect or risk.

The latter two concepts, however are infrequently discussed. Yet, they too are necessary, particularly initially. The tool must utilize easily obtained measurements, such as weight of a tool. If the measurements are too difficult to obtain, or require estimation, such as force at the A-1 pulley, then the tool is unlikely to be widely adopted at the current time, other than research circles, will not be utilized in job (re)design, nor for potential regulatory enforcement.

Similarly, the fourth criteria is also necessitated for parallel reasons. Complex mathematical formulas are less likely to be widely adopted. Regardless, computerization of complex formulas would likely be used in the future.

The Strain Index

The Strain Index was modeled from morbidity data in a pork processing facility. Besides relying upon the morbidity data, it also relies upon biomechanical principles, and fatigue theory. Thus, it incorporates multiple principles, with the particular strength of relying upon epidemiologic data.

There are six task variables utilized to assess the risk: intensity of exertion, duration of exertion per cycle, efforts per minute, wrist posture, speed of exertion, and duration of the task per day. The Strain Index score is the product of six multipliers. Of these variables, the intensity of the exertion is the most significant, and at higher levels of force, will typically drive the model. (See Tables 1 and 2.) Of these variables, all of them are quantifiable, with the exceptions of the intensity of exertion and the speed of the work. Intensity of exertion has verbal anchors to separate the different ratings, such as "relaxed effort" from "noticeable effort." Speed of the work is relatively easily assigned, such as "extremely relaxed pace" from "rushed, but able to keep up." Applying The Strain Index requires collecting data, assigning a rating value, determining multipliers and calculating a Strain Index score.

The Strain Index likely has content validity as it is a model to measure ergonomic risk factors. It also has additional benefits of quantifying the risk and yields information to target the redesign of jobs. Exertional demands of the jobs are measured and this is believed to be a key component of ergonomic risk (fatigue theory, biomechanical modeling and epidemiologic data).

Uses of The Strain Index include:

• Job design
• Job analysis
  Safe vs. Hazardous determination
  Quantification of risk
• Targeted risk factor modification

The following situation is not incorporated in the model: it does not account for extrinsic compression. Also, it does not measure force, but force is assessed in a semiquantitative manner. Force has typically driven the differentiation between safe and hazardous designations more than the other factors. Particularly, the differentiation between light and moderate force is in some situations the critical decision. There are some anchors that have been developed to help with that differentiation. Additional information from the shoulder study may also help in that regard.

It has not been fully validated, but has been applied in an intervention study in an unblinded fashion. In the study to validate the OSHA checklist, there was 80% agreement (reliability) between two observers. Full validation is anticipated as one endpoint of the shoulder study.

The Shoulder Study

Shoulder disorders of the rotator cuff complex (supraspinatus tendinitis/tears, rotator cuff tendinitis/tears, subacromial bursitis, impingement syndrome and bicipital tendinitis) constitute significant morbidity and cost, often accounting for the second most vexing problem after back disorders. Yet, epidemiologic-ergonomic investigations are extraordinarily weak methodologically. Typically, such studies have been cross sectional with crude measures of self reported exposure. To date, the understanding of the risks for shoulder disorders is crude and whether variables are risk factors or not is unclear. Also, the relative importance of the various risk factors is completely undecided. There is likewise not a model of the same type as The Strain Index with which to evaluate shoulder exposures.

An epidemiologic-ergonomic investigation of shoulder disorders has been started at multiple plant sites of different manufacturers, involving hundreds of workers, with the goals of obtaining the epidemiologic and ergonomic information with which to clarify the hazards of shoulder exposures and develop a model, analogous to The Strain Index, to assist in the design and analysis of jobs. This study will include data derived from questionnaires: identification of potential personal risk factors, accident history data, past medical history information, and information on activities that increase or relieve the symptoms. Physical measurement data will also be obtained, including: weights of tools/parts, and postural variables. Videotapes will be the primary source for other exposure data including: cycle times, duration of exertion, and most postural variables. From the exposure data, quantitative job stress information will be calculated and assessed. Morbidity data will be obtained from both OSHA 200 logs and medical records. Case definitions will be utilized. A model will be derived from these combined sources of data, analogous to The Strain Index. A later goal is to validate this model.

Due to the ease of obtaining the exposure information, additional data will be obtained for other disorders, such as cervical radiculopathy, lateral epicondylitis, ulnar neuropathy at the elbow, carpal tunnel syndrome, etc. Additional models for these disorders are possible. Lastly, this study will serve as a study on a completely different population of workers from which The Strain Index was derived, with which to validate The Strain Index.

CONCLUSIONS

A useful model, The Strain Index, has been developed for the evaluation of distal upper extremity neuromusculoskeletal ergonomic risks. While currently undergoing full validation, it has proven useful in evaluating jobs. Additional refinements will be made and the future likely includes specific models for different disorders, including supraspinatus tendinitis, a study for which has begun. Additional participants in the Shoulder Study are being sought.

REFERENCES

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Keyserling, W.M., D.S. Stetson, B.A. Silverstein, and M.L. Brouver: A checklist for evaluating risk factors associated with upper extremity cumulative trauma disorders. Ergonomics 36(7):807-831 (1993).

Mathiowetz, V., N. Kashman, G. Vollard, K. Wever, M. Dowe, and S. Roger: Grip and pinch strength: Normative data for adults. Arch. Phys. Med Rehabil. 66:69-74 (1985).

Moore, J.S., and A. Garg: The Strain Index: A proposed method to analyze jobs for risk of distal upper extremity disorders. Am. Ind. Hyg. Assoc. J. 56:443-458 (1995).

Nathan, P.A., K.D. Meadows, and L.S. Doyle: Occupation as a risk factor for impaired sensory conduction of the median nerve at the carpal tunnel. J. Hand Surg. 13-B(2):167-170 (1988).

Rohmert, W.: Physiologische grundlagen der erholungszeitbestimmung. Zbl. Arb. Wiss. 19:1-28 (1965).

Silverstein, B.A., L.J. Fine, and T.J. Armstrong: Occupational factors and carpal tunnel syndrome. Am. J. Ind. Med. 11:343-358 (1987).

Table 1. Rating values to assign for calculating The Strain Index.

Table 2. Determining multipliers to calculate The Strain Index.

Keywords: Ergonomics, Musculoskeletal Diseases, Occupational Injuries, Occupational Diseases, Shoulder Tendinitis, Risk Assessment