CONTEXT:
Isolated infraspinatus atrophy (IIA) is a common condition among overhead activity athletes, which affects the hitting shoulder and is caused by suprascapular nerve injury. The loss of infraspinatus function could lead to altered activity of the glenohumeral and scapulothoracic (ST) muscles and compromise the optimal shoulder function.

OBJECTIVE:
To assess the surface electromyographic (sEMG) activity patterns, relationships, and response latencies of relevant shoulder girdle muscles in professional volleyball players with IIA and in healthy control players.

DESIGN:
Cross-sectional study.

SETTING:
Research laboratory.

PATIENTS OR OTHER PARTICIPANTS:
Twenty-four male professional volleyball players (12 players with diagnosed IIA and 12 healthy players) recruited from local volleyball teams.

INTERVENTION(S):
sEMG activity of anterior, middle, and posterior deltoid; upper, middle, and lower trapezius; and serratus anterior (SA) was recorded and evaluated during a movement of shoulder abduction in the scapular plane, monitored with an optoelectronic motion capture system.

MAIN OUTCOME MEASURE(S):
sEMG activity, relationships, and response latencies of the selected muscles were analyzed with analysis of variance models to highlight statistical differences within and between groups.

RESULTS:
Athletes with IIA demonstrated significant higher deltoid and trapezius muscles activity and lower SA activity compared with the contralateral shoulder and healthy athletes. The shoulder with IIA also showed a higher activity ratio between the upper trapezius and other ST muscles in addition to anticipated activation of the upper trapezius and delayed activation of the SA, with regard to the onset of shoulder movement.

CONCLUSIONS:
This study highlighted altered shoulder muscle activity levels, ST muscles imbalances, and abnormal ST recruitment patterns in the hitting shoulder of professional volleyball players with IIA, secondary to suprascapular nerve neuropathy. Such shoulder girdle muscles' impairments may compromise the optimal scapulohumeral rhythm and function, increasing the risk of acute and overuse shoulder injuries.