Performing repetitive manual tasks can lead to muscle fatigue, which may induce changes in motor coordination, movement stability, and kinematic variability. In particular, movements performed at or above shoulder height have been associated with increased shoulder injury risk. The purpose of this study was to determine the effects of repetitive motion-induced muscle fatigue on posture and on the variability and stability of upper extremity movements. Ten healthy subjects performed a repetitive task similar to sawing continuously until volitional exhaustion. This task was synchronized with a metronome to control movement timing. Subjects performed the sawing task at shoulder ("High") and sternum height ("Low") on two different days. Joint angles and muscle activity were recorded continuously. Local and orbital stability of joint angles, kinematic variability (within subject standard deviations), and peak joint angles were calculated for five bins of data spaced evenly across each trial. Subjects fatigued more quickly when movements were performed at the High height. They also altered their kinematic patterns significantly in response to muscle fatigue. These changes were more pronounced when the task was performed at the High height. Subjects also exhibited increased kinematic variability of their movements post-fatigue. Increases in variability and altered coordination did not lead to greater instability, however. Shoulder movements were more locally stable when the task was performed at the High height. Conversely, shoulder and elbow movements were more orbitally unstable for the High condition. Thus, people adapt their movement strategies in multi-joint redundant tasks and maintain stability in doing so.
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