Since musculoskeletal disorders of the upper extremity associated with repetitive work may be caused by cumulative exposure, a physiologic marker that indicates a level of collective change in the muscle due to exposure is needed. Muscle fatigue of long duration, also known as low frequency fatigue, increases with exposure and can persist for more than 24 hours, into the next workday. We therefore propose to test the hypothesis that fatigue of long duration may be used as a measure of cumulative exposure to repetitive work. Our approach for this four-year study is to conduct a series of hypothesis-driven laboratory-based experiments, which will simulate repetitive wrist fiexion work tasks for eight-hour workdays. These experiments will determine a relationship between parameters describing exposure to repetitive work with changes in a physiological response, the level of muscle fatigue. Traditional isometric muscle stimulation techniques will provide objective measurements of fatigue of long duration created by the wrist flexion task throughout the workday and into the next morning. Since fatigue of long duration can persist into the next morning, the first set of experiments will test whether muscle fatigue accumulates over consecutive workdays, and whether accumulated fatigue -is equivalent to fatigue from a single bout of work. The second set of experiments will test which quantitative measure of exposure to repetitive work dominates the physiological response: is it cumulative activation, force or work? The third set of experiments will determine the effects of several proposed job task interventions, including training, mini-breaks and different repetition rates, on the level of fatigue. Ultimately, this study will link aspects of exposure to repetitive work with changes in muscle physiology, an important step in the etiology of work-related musculoskeletal disorders, and will aid in the redesign of repetitive working tasks.