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Onset of Shoulder Muscle Activation During Internal and External Shoulder Rotation

Tomohito Ijiri, Yukio Urabe, Noriaki Maeda, Junpei Sasadai and Toshiaki Suzuki

Background: Scapulae activation and the order of muscle activation tends to be changed in patients with shoulder disorder and abnormal muscle activate pattern is the risk for shoulder disability. However, previous studies have only explored shoulder elevation in regard to the order of shoulder muscle activation. Hence, the order of muscle activation during other shoulder movements is unknown. The purpose of this study was to investigate the order of shoulder muscle activation during internal and external shoulder rotation.
Methods: Eighteen healthy subjects participated in this study. The onset of muscle activation was measured by electromyography under two conditions. Initially, the subjects started the movement of internal or external rotation at their own discretion, and then they performed isometric contraction of internal or external rotation against an external load.
Results: Following the two test conditions, results of internal rotation task showed that activation of the middle trapezius was significantly later than that of the pectoralis major and the serratus anterior. External rotation indicated that the serratus anterior was recruited after the infraspinatus. The onset of muscle activation of the middle trapezius was the same as that of the infraspinatus.
Conclusion: This result showed that the onset among shoulder muscles during internal and external rotation wasn’t the same. Especially, there was the difference of onset time among each scapular muscle. From this difference, it is thought that each scapular muscle may have different role in shoulder isometric contraction.