Effects of precision and time on task on shoulder muscles during repetitive tapping tasks
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Epidemiological studies have associated occupational tasks that required precision with neck and shoulder musculoskeletal disorders. To identify potential injury mechanisms, the effects of repetitively moving a probe to a small target were studied. The research questions were: (1) Do muscle activation patterns, joint movements or the movement times to targets differ during tasks of different levels of precision? (2) Do muscle activation patterns, joint movements, or movement times change during the tasks? (3) Do changes, if they occur, depend on task precision? Participants sat a table and repetitively tapped between a Home (19 mm diameter) and a precision Target for seven minutes. The Target was 3.2, 10.0, 19.0, or 47.0 mm during the high, medium, low, or no precision tasks. The time to move from the Home to the Target, pronation/supination acceleration, and surface electromyograms from the descending trapezius and anterior deltoid were monitored continuously. During analyses, movements to the Target were divided into two phases. Phase I was ballistic and moved the probe to the vicinity of the Target. Phase II was slower and ended when the probe touched the Target. During Phase I the movement time and pronation/supination acceleration did not depend on task precision. During Phase I, movement time, the acceleration, and muscle activation patterns did not change as tasks continued. During Phase II, movement time increased (P < 0.001) and pronation/supination acceleration decreased (P < 0.001) with increased task precision. Two mechanisms, "improved control" and the "inability to sustain control" described changes during Phase II as tasks continued. During improved control movement time decreased, the pronation/supination acceleration increased (P ≤ 0.019), and the RMS EMG of the descending trapezius declined (P = 0.042). During the inability to sustain control movement time increased and the RMS EMG of the anterior deltoid increased (P = 0.018). Improved control developed more frequently during low than high precision tasks and the inability to sustain control developed more frequently during high than low precision tasks (P = 0.019). Therefore, changes in movement time, joint movements, and muscle activation patterns developed during tasks, and those changes depended on task precision.