Question 3: What is the state of scientific knowledge, characterized by the degree of certainty or lack thereof, with regard to occupational and nonoccupational activities causing such conditions?

The relationships among work factors, biomechanical loads, and responses are supported by mathematical models and direct measurements. The mathematical models are widely accepted and applied to design mechanical structures in aircraft and automotive design. Direct measurements have been used to a lesser extent than modeling because they are potentially injurious to human subjects; however, when they have been used they generally support the biomechanical models.

It has been shown that the load forces encountered over time in normal work activities often approach the physiological and mechanical tissue limits. Limits may be exceeded as a result of a single high force or as a result of repeated loads over time. Some tissues have a greater ability to adapt to repeated loads if there is sufficient recovery time between successive loads, while other tissues, e.g., nerves, are less able to adapt.

Biomechanical loads are encountered in activities of work, daily living, and recreation. The contribution of these activities to tissue response is related to their relative duration and intensity. For most people, their main exposure is at work. There is a substantial body of epidemiological literature that shows a disproportionately high incidence of musculoskeletal disorders of all types among persons exposed to high biomechanical loads. Although there can be debate about acceptable exposure limits, there can be little disagreement about the fundamental relationship between extreme work exposures and musculoskeletal morbidity.

Question 4: What is the relative contribution of any causal factors identified in the literature to the development of such conditions in (a) the general population; (b) specific industries; and (c) specific occupational groups?

Data at the population and industry level have been collected for a variety of purposes by different groups using different methods. Some data are based on survey results, some on clinical or medical diagnoses, and some on compensation claims. In the judgment of the steering committee, it is not possible to make useful comparisons on the basis of these data.

The incidence of musculoskeletal disorders in any specific occupational group can be expected to reflect the tissue loads imposed by the work, the tissue tolerances of the mix of individuals doing it, and the other activities in their lives imposing related loads. The relative contribution of the different factors in any occupational group depends on (1) the strength of these relationships across the ranges of individuals and activities in that group and (2) the variability of the individuals and activities. For example, gender will not be an important predictive factor if men and women do not, on average, have different tissue tolerances for the loads imposed in that occupational group or if workers in the group are overwhelmingly men or women. The relative contribution in a specific industry will, similarly, depend on the mix of individuals and tasks in it. Unfortunately, measurements of the relevant features of individuals and tasks are typically unavailable, limiting our ability to assess the relative contribution of different factors across groups. The evidence shows that the incidence of musculoskeletal

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