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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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Amir K. Vafadar, Julie N. Cote and Philippe S. Archambault

Context: Joint-position sense (JPS) plays a critical role in the stability of shoulder joint. Restoration of JPS is essential to improve rehabilitation outcomes in individuals with shoulder injury. However, the number of affordable and reliable shoulder-JPS measurement methods for everyday clinical practice is limited. Objective:To estimate reliability and validity of 3 simple shoulder-JPS measurement methods. Design: Cross-sectional study. Participants: 25 healthy men and women. Main Outcome Measure: Absolute-error scores of JPS in 3 ranges of shoulder flexion (low, mid, and high), measured with a laser pointer, an inclinometer, and a goniometer in 2 separate sessions (48 h apart). Results: Overall interrater and intrarater intraclass correlation coefficients were .86 and .78 for the laser pointer, .67 and .70 for the inclinometer, and .60 and .50 for the goniometer, respectively. There was excellent reliability in the low range for the laser pointer and inclinometer methods, but fair to good and poor reliability in mid- and high ranges, respectively. All methods showed strong validity. Conclusion: The laser pointer and inclinometer JPS measurement methods are reliable and can be used by clinicians during rehabilitation of shoulder injuries.

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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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Christopher A. Bailey, Maxana Weiss and Julie N. Côté

Aging affects fatigability and is a risk factor for incurring a fatigue-related injury in the neck/shoulder region. Age-related changes in the electromyographical features of motor control may be partly responsible. Young (N = 17) and older (N = 13) adults completed a reach-and-lift task at their self-selected speed, before and after a fatiguing task targeting the neck/shoulder. Electromyography amplitude (root mean square), amplitude variability (root mean square coefficient of variation [CV]), functional connectivity (normalized mutual information [NMI]), and functional connectivity variability (NMI CV) were extracted from several muscles and analyzed for effects and interactions of age using general estimating equation models. Root mean square CV and deltoid NMI CV increased from pre- to postfatigue (ps < .05). Upper trapezius–deltoid NMI decreased for young, but increased for older adults, while the opposite response was found for lower trapezius–deltoid NMI (ps < .05). Older adults seem to adapt to fatigue in reach-and-lift movement with a cranial shift in control of the scapula.

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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.