Scapular muscle activation and co-activation following a fatigue task.

Scapular muscles precisely move the scapulothoracic articulation and if fatigued may contribute to pathology. Fatigue of serratus anterior may be a mechanism for shoulder pathology by altering scapula motions and requiring compensation by other shoulder muscles. A total of 28 asymptomatic subjects performed a task to fatigue the serratus anterior, while muscle activity was recorded from three muscles. Mean normalized activation levels and activation ratios were examined before and after the fatigue task during arm elevation and lowering. All muscles demonstrated meaningful declines in the median frequency of the electromyographic signal during the task. Following the task, only the upper trapezius had higher mean activation levels (mean difference 10.79% MVIC), while the serratus anterior/lower trapezius activation ratio was altered (mean difference -0.3). Higher mean upper trapezius activation may be compensatory for fatigue of other shoulder muscles and may reflect fiber type or central control mechanisms. Serratus anterior eccentric endurance training may be beneficial for the prevention of shoulder pathology.