Context: Glenohumeral external rotation (GH ER) muscle fatigue might contribute to shoulder injuries in overhead athletes. Few researchers have examined the effect of such fatigue on scapular kinematics and muscle activation during a functional movement pattern.Objective: To examine the effects of GH ER muscle fatigue on upper trapezius, lower trapezius, serratus anterior, and infraspinatus muscle activation and to examine scapular kinematics during a diagonal movement task in overhead athletes.Setting: Human performance research laboratory. Design: Descriptive laboratory study. Patients or Other Participants: Our study included 25 overhead athletes (15 men, 10 women; age = 20 ± 2 years, height = 180 ± 11 cm, mass = 80 ± 11 kg) without a history of shoulder pain on the dominant side. Intervention(s):We tested the healthy, dominant shoulder through a diagonal movement task before and after a fatiguing exercise involving low-resistance, high-repetition, prone GH ER from 0° to 75° with the shoulder in 90° of abduction.Main Outcome Measure(s): Surface electromyography was used to measure muscle activity for the upper trapezius, lower trapezius, serratus anterior, and infraspinatus. An electromyographic motion analysis system was used to assess 3-dimensional scapular kinematics. Repeated-measures analyses of variance (phase × condition) were used to test for differences. Results:We found a decrease in ascending-phase and descending-phase lower trapezius activity (F 1,25 = 5.098, P = .03) and an increase in descending-phase infraspinatus activity (F 1,25 = 5.534, P = .03) after the fatigue protocol. We also found an increase in scapular upward rotation (F 1,24 = 3.7, P = .04) postfatigue.Conclusions: The GH ER muscle fatigue protocol used in this study caused decreased lower trapezius and increased infraspinatus activation concurrent with increased scapular upward rotation range of motion during the functional task. This highlights the interdependence of scapular and glenohumeral force couples. Fatigue-induced alterations in the lower trapezius might predispose the infraspinatus to injury through chronically increased activation.Key Words: muscle function, rotator cuff, upper extremity Key Points • Shoulder external rotation muscle fatigue contributed to altered scapular muscle activation and kinematics.• From prefatigue to postfatigue, lower trapezius activation decreased by 4%, infraspinatus activity increased in the descending phase by 4%, and scapular upward rotation motion increased in the ascending phase by 3°.• Upper trapezius and serratus activation did not change from prefatigue to postfatigue.• The force couple between the lower trapezius and infraspinatus was interdependent, and alterations in the lower trapezius due to shoulder external rotation muscle fatigue might predispose the shoulder to injury.

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