Context: Researchers have shown that shoulder-rotator strength is lower in athletes with impaired throwing shoulders; however, the specific muscles that exhibit lower activity in these athletes remain unclear. Objective: To compare isokinetic concentric and eccentric shoulder external-rotation torque and muscle activities between baseball players with pitching-related shoulder impairments and healthy players. Design: Cross-sectional study. Setting: University laboratory. Patients or Other Participants: A total of 42 collegiate baseball players were assigned to the pain group (n = 18; age = 19.3 ± 1.0 years, height = 1.73 ± 0.05 m, mass = 75.1 ± 10.0 kg) and the control group (n = 24; age = 19.8 ± 1.0 years, height = 1.73 ± 0.04 m, mass = 69.7 ± 6.1 kg). Main Outcome Measure(s): Isokinetic shoulder external-rotation torque at 90° of shoulder-joint abduction was measured in concentric and eccentric directions. Surface electromyography of the infraspinatus (ISP), posterior deltoid (PD), and middle trapezius (MT) muscles was performed. The maximum and mean torques, as well as electromyography of each muscle, were determined. Additionally, ISP muscle thickness was measured. Results: No differences were observed between the pain (n = 15) and control (n = 19) groups during concentric shoulder external rotation in maximum (t32 = 1.22, P = .23) or mean (t32 = 0.21, P = .84) torque or maximum (P range, .19-.81) or mean (P range, .07-.77) muscle activity for each muscle. During eccentric external rotation, maximum (t25 = 2.37, P = .03) and mean (t25 = 2.37, P = .03) torques were lower in the pain (n = 13) than control (n = 14) group. The pain group also exhibited lower maximum (U = 34.00, P = .009) and mean (U = 36.00, P = .01) muscle activity in the PD. In contrast, no difference in muscle activity was observed between the 2 groups for the ISP and MT (P range, .12-.40). Furthermore, ISP muscle thickness was smaller in the pain group (t40 = -2.46, P = .02; Cohen d = 0.83). Conclusions: Shoulder external-rotation torque during eccentric motion was lower in athletes with throwing-impaired shoulders, which may be associated with reduced PD activity and ISP atrophy. Key Points Eccentric shoulder external-rotation torque was lower in baseball players with throwing impairments. In baseball players with throwing-impaired shoulders, decreased posterior deltoid muscle activity may be associated with eccentric shoulder external-rotation movements.
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