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groups and positioning of the whole person could provide a pattern of motion that may partially offset the insulting exposure in the real work environment. Therefore, the degree to which the results from these more focused studies can be generalized to workers in the context of their ongoing tasks must be carefully interpreted. Third, the degree to which an animal model is analogous to the human must be considered. How experiments that involve loading of mouse tails are relevant to human musculoskeletal disorders is a meaningful question that scientists need to clarify for others. While seemingly distant from human experience, this animal model is relevant from the perspective of comparative anatomy and provides a quantifiable aspect to measurement of the impact of physical stresses. Ultimately, the basic science studies contribute important information about the mechanisms by which injury can occur following a prescribed set of exposures, but the application of the findings to the whole human depends on the extent to which the results are congruent with data from human studies.

Observational Epidemiology

From the perspective of observational epidemiology, studies of human populations are designed that measure both exposures (e.g., repetitive motion) and outcomes (e.g., back disorders). A key feature of measurement in observational epidemiologic studies is that the results are generated from populations of humans. There are limits to the measurements that persons (as opposed to cells or tissues) will allow or tolerate. Because the comparisons involve groups, there are levels of detail that remain desirable but are not feasible to measure (e.g., the cost may be prohibitive). However, surveys provide information, such as symptoms, that cannot be obtained in the same way from certain types of laboratory study; for example, back pain may be disabling even in the absence of objective diagnostic test findings, and the frequency of this condition is important to ascertain.

The presence of symptoms in the absence of objective findings needs careful attention. Symptoms are often the first presentation of an illness and may represent changes that are due to physical damage (due, in this case, to repetitive lifting) that has not progressed enough to be measured by physiological tests or physical exam. Self-reports of symptoms may also reflect other factors, such as psychological stress. These trade-offs (e.g., in detail versus feasibility of measurement) are inherent in observational epidemiologic studies.

The basic strategy in an epidemiologic study is to identify the extent to which the outcome (e.g., back disorder) occurs more frequently in the exposed group than the unexposed group. The strength of this type of



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