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Amirhossein K. Vafadar, Julie N. Côté, and Philippe S. Archambault

The purpose of this study was to estimate the extent to which muscle fatigue can impact on the position sense in the upper limb. Twelve healthy volunteers were asked to do a reaching task while grasping a wooden block and match the block’s position with a corresponding target displayed on a flat screen, without vision. Following that, subjects performed resistive exercises with Thera-band strips until fatigue was induced and then the position sense task was repeated. A significant change in the endpoint position was observed after fatigue, in the up/down direction (p ≤ .001). The variability of endpoint positions in up/down direction was also significantly increased after fatigue (p ≤.03). There was no significant change in endpoint orientation but there was a significant fatigue × orientation effect on endpoint rotational variability. In a follow-up experiment, a group of subjects repeated the same protocol, but with a period of quiet rest between the two position sense tasks. In that group, there were no differences in endpoint position, orientation or variability. Muscle fatigue is an important factor that should be taken into consideration during the treatment of musculoskeletal injuries as well as athletic training.

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Zachary R. Weber, Divya Srinivasan, and Julie N. Côté

The objectives of this study were to assess the sex-specific relationships between motor and sensory adaptations to repetitive arm motion–induced neck/shoulder fatigue, and to measure how additional sensory stimulation affects these adaptations. Twenty-three participants performed two sessions of a repetitive pointing task until scoring 8 on the Borg CR10 scale for neck/shoulder exertion or for a maximum of 45 min, with and without sensory stimulation (i.e., light touch) applied on the fatiguing shoulder. Just before reaching the task termination criteria, all participants showed changes in mean and variability of arm joint angles and experienced a fivefold increase in anterior deltoid sensory threshold in the stimulus-present condition. Women with the greatest increases in anterior deltoid sensory thresholds demonstrated the greatest increases in shoulder variability (r = .66), whereas men with the greatest increases in upper-trapezius sensory thresholds demonstrated the greatest changes in shoulder angle (r = −.60) and coordination (r = .65) variability. Thus, sensory stimulation had no influence on time to termination but affected how men and women differently adapted, suggesting sex differences in sensorimotor fatigue response mechanisms.

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Julie N. Côté, Anatol G. Feldman, Pierre A. Mathieu, and Mindy F. Levin

Fatigue affects the capacity of muscles to generate forces and is associated with characteristic changes in EMG signals. It may also influence interjoint and intermuscular coordination. To understand better the global effects of fatigue on multijoint movement, we studied movement kinematics and EMG changes in healthy volunteers asked to hammer repetitively. Movement kinematics and the activity of 20 muscles of the arm, trunk, and leg were recorded before and after subjects became fatigued (as measured using a Borg scale). When fatigue was reached, maximal grip strength and elbow range of motion decreased while the EMG amplitude of the contralateral external oblique muscle was increased. Fatigue did not affect shoulder and wrist kinematics or movement frequency. Results suggest that fatigue influences motion at both local and global levels. Specifically, interjoint and intermuscular coordination adapt to compensate for local effects of fatigue and to maintain key movement characteristics, such as the trajectory of the end effector and the movement frequency. Nonlocal compensations may be a focus of future studies of how fatigue affects complex movements such as those typically performed in the workplace.