There is a high prevalence of overuse injuries in volleyball, which can result in participation time loss. The game of volleyball involves repetitive overhead movements such as serving and spiking. Overhead motions rely on effective movement of the scapula, produced by the shoulder muscles. Muscle fatigue may affect scapula thoracic (ST) muscle function, and in turn affect scapula kinematics. Overuse injuries, such as subacromial impingement syndrome (SIS), have been linked to dysfunctional scapula kinematics and ST muscle activity among overhead athletes. The causal nature of this link is unclear, that is, does injury lead to altered kinematics and muscle function, or vice versa? If injury arises due to dysfunctional scapula control, then states of fatigue could increase the risk of injury by causing changes in muscle function. The aim of this study was to investigate the effect of fatigue, caused by the repetitive overhead action of hitting (spiking) in volleyball, on shoulder muscle activation. Surface electromyography (EMG) was used on ST muscles to evaluate peak amplitude, time to peak, frequency, and area under the EMG curve. Thirteen trained healthy volleyball players were tested before and after a repetitive spiking intervention in a controlled laboratory setting. In the post-intervention measurement compared to the pre, a reduction in peak amplitude (p < 0.05) and area under the EMG curve (p < 0.05) was observed in the Anterior Deltoid (AD). Lower Trapezius (LT) was found to have increased area under the curve (p < 0.01), increased frequency (p < 0.01), and decreased time to peak (p < 0.05). A decreased time to peak was also observed for the MT (p < 0.01). The results indicate that the spiking intervention does affect the scapulothoracic muscle activation. An explanation for these results is that the system acts to maintain stability of the shoulder girdle during rapid tasks, through anticipatory postural adjustments in the scapulothoracic muscles.These changes in muscle activation were observed in healthy participants, which suggests that muscle dysfunction is a precursor to the development of injury, as opposed to a result of it.
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