Electromyographic analysis of the shoulder during isokinetic external rotation in baseball players with pitching-impaired shoulders

ABSTRACT Context: Studies have shown that shoulder rotator strength is lower in athletes with impaired throwing shoulders; however, the specific muscles that exhibit lesser activity in these athletes remain unclear. Objective: This study aimed to determine the muscle activity characteristics during isokinetic external shoulder rotation in baseball players with throwing-impaired shoulders. Design: Cross-sectional study. Setting: University laboratory Patients or Other Participants: A total of 18 college baseball players were assigned to the pain group and 24 to the control group. Main Outcome Measures: Isokinetic shoulder external rotation torque, at 90° abduction of the shoulder joint, was measured in concentric and eccentric directions. Surface electromyography of the infraspinatus, posterior deltoid, and middle trapezius muscles was also performed. The maximum and average torques, as well as electromyography of each muscle, were determined. Additionally, infraspinatus muscle thickness was measured. Results: No differences were observed between the two groups in maximum torque (P = .231), average torque (P = .835), maximum muscle activity for each muscle, or average muscle activity (P = .071; .806). Eccentric external rotation was analyzed in 13 and 14 individuals in the pain and control groups, respectively. The maximum (P = .026) and average (P = .025) torque were significantly lower in the pain group. The pain group also exhibited significantly lower maximum (P = .009) and mean (P = .012) muscle activity in the posterior deltoid. In contrast, no difference in muscle activity was observed between the two groups for the infraspinatus and middle trapezius (P = .115; .402). Furthermore, infraspinatus muscle thickness was significantly lower in the pain group (P = .021). Conclusions: Shoulder external-rotation torque during eccentric motion was lower in athletes with throwing-impaired shoulders, which may be associated with reduced posterior deltoid activity and infraspinatus atrophy.