The central physiological pathways appear within the shaded area of the figure. It shows, first, the biomechanical relationship between load and the biological response of tissue. Imposed loads of various magnitudes can change the form of tissues throughout the day due to changes in fatigue, work pattern or style, coactivation of muscle structures, etc. Loads within a tissue can produce several forms of response. If the load exceeds a mechanical tolerance or the ability of the structure 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 large enough. Other forms of response may entail such reactions as inflammation of the tissue, edema, and biochemical responses.

Biomechanical studies can elucidate some of these relationships. Biomechanical loading can produce both symptomatic and asymptomatic reactions. Feedback mechanisms can influence the biomechanical loading and response relationship. For example, the symptom of pain might cause an individual to recruit his or her muscles in a different manner, thereby changing the associated loading pattern. Adaptation to a load might lead individuals to expose themselves to greater loads, which they might or might not be able to bear. Repetitive loading of a tissue might strengthen the tissue or weaken it, depending on circumstances. The symptom and adaptation portions of the model can interact with each other as well. For example, symptoms, such as swelling, can lead to tissue adaptations, such as increased lubricant production in a joint. These relationships can be described in mathematical models that distinguish external load (e.g., work exposure) from internal (dose) load and illustrate cascading events, whereby responses to loads can themselves serve as stimuli that increase or decrease the capacity for subsequent responses.

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

To the left of the shaded area in Figure 1, the framework shows environmental factors that might affect the development of musculoskeletal disorders, including work procedures, equipment, and environment; organizational factors; and social context. For example, physical work factors (reaching, close vision work, lifting heavy loads) affect the loading that is experienced by a worker's tissues and structures. Organizational factors can also influence the central mechanism. Although little studied, hypothetical pathways also exist between organizational influences and the biomechanical load-response relationship, as well as the development of symptoms. For example, time pressures to complete a task might induce carelessness in handling a particular load, with consequent tissue damage. The organizational culture can also create an incentive or a disincentive to report a musculoskeletal disorder or to claim that the impairment should be considered a disability. Social context factors, such as a lack of means to deal with psychological stress (e.g., no spousal support), might also influence what a worker reports or even the worker's physiological responses.

To the right in Figure 1, the framework shows the influence of individual physical and psychological factors, as well as non-work-related activities, that might affect the development of musculoskeletal disorders. For example, psychological factors can affect a person's identification of a musculoskeletal disorder or willingness to report it or to claim that the impairment is a disability. Physical factors might involve reduced tissue tolerance due to age or gender or disease states, such as arthritis, which can affect people's biochemical response to tissue loading.



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