component of musculoskeletal disorders. Loads of various magnitudes are imposed on bone or tissue in various ways. The impact of loads on tissues may change in the course of a day because of changes in fatigue, work pattern or style, coactivation of muscle structures, or other factors. The impact of loads on tissues can provoke several responses. If the load exceeds a mechanical tolerance or the ability of the tissue to withstand the load, tissue damage will occur. For example, damage to a vertebral end plate will occur if the load borne by the spine is heavy enough. Other tissue responses may include such reactions as inflammation, edema, and biochemical responses.

Systematic responses of the body to biomechanical loading may produce both symptomatic and asymptomatic reactions. Some of these reactions contain feedback mechanisms that can influence the tissue loading and response relationship, as illustrated in the gray box (physiological pathways) of Figure 1. For example, pain might cause an individual to use muscles in a different way, thus changing the associated loading pattern. Repetitive loading of a tissue might lead an individual to ignore discomfort signals and expose himself to greater loads, which he might or might not be able to bear. The symptom and adaptation portions in Figure 1 can interact with each other as well. For example, swelling might lead to a tissue adaptation, such as increased production of lubricant in a joint.

These symptoms, responses, and adaptive behaviors can lead to functional impairment. In the workplace, this might be reported as a work-related musculoskeletal disorder. If severe enough, the impairment could be considered a disability, and lost or restricted workdays could result.

In the box to the right, the framework recognizes the influence of individual factors, including physical and psychological factors, that might affect the musculoskeletal disorder sequence. For example, psychological factors can affect one's willingness to report a musculoskeletal disorder or claim that the impairment is a disability. Physical factors might involve reduced tissue tolerance due to age, gender, and overall physical condition. Disease states, such as arthritis, also can affect a person's biochemical response to tissue loading.

In the boxes to the left of the central physiological pathways, Figure 1 identifies environmental factors that might affect the development of musculoskeletal disorders, including physical work, organizational factors, and social context. For example, physical work factors (such as lifting heavy boxes or equipment) can affect the loading that is experienced by a worker's tissues and structures, and can also influence the symptoms and reported incidence of a musculoskeletal disorder. Organizational factors can also influence each component of the musculoskeletal disorder sequence. For example, time pressures to complete a task might frustrate a worker and provide an incentive or disincentive to report a musculoskeletal disorder or to claim that an impairment should be considered a disability. Although little studied, pathways exist between organizational influences and the biomechanical and body reaction (symptoms and response) components of the musculoskeletal disorder sequence.



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