Background: Shoulder external rotation exercises are commonly used to improve the stabilizing ability of the infraspinatus. However, during exercise, excessive activation of the posterior deltoid compared to the infraspinatus causes the humeral head to move anteriorly in an abnormal position. Many researchers have emphasized selective activation of the infraspinatus during shoulder external rotation exercise.
Objects: This study aims to delineate the optimal exercise method for selective activation of infraspinatus by investigating the muscle activities of the infraspinatus and posterior deltoid according to the four shoulder exercise methods and two forearm positions.
Methods: Thirty healthy individuals participated in this study. The participants were instructed to perform shoulder external rotation exercises following four exercise methods: sitting external rotation (SIER); standing external rotation at 90° abduction (STER); prone external rotation at 90° abduction (PRER); side-lying external rotation (SLER), and two forearm positions (neutral, supinated). The electromyography (EMG) signal amplitude was measured during each exercise. Surface EMG signals were recorded from the posterior deltoid, infraspinatus, and biceps brachii. Results: EMG results of the infraspinatus and posterior deltoid in PRER, were significantly higher than that of the other exercises (p < 0.01). The EMG ratio (infraspinatus/posterior deltoid) in SIER was significantly higher than that of the other exercises. EMG activation of the posterior deltoid in SIER, PRER, and SLER was significantly higher in neutral than in supinated (p < 0.01). Furthermore, the EMG of the infraspinatus in SIER was significantly higher in neutral than in supinated (p < 0.01). The EMG ratio (infraspinatus/ posterior deltoid) in SIER was significantly higher in neutral than in supinated (p < 0.05.) Contrarily EMG ratios in PRER and SLER were significantly higher in supinated than in neutral (p < 0.05).
Conclusion: The results show that clinicians should consider these exercise methods and forearm positions when planning shoulder external rotation exercises for optimal shoulder rehabilitation.

INTRODUCTION
MATERIALS AND METHODS
    1. 연구대상자
    2. 연구 설계 및 절차
    3. 실험방법
    4. 분석 방법
RESULTS
    1. 연구대상자의 일반적인 특성
    2. 어깨 가쪽돌림 운동 시 운동방법에 따른 근육별 근활성도비교
    3. 어깨 가쪽돌림 운동 시 아래팔의 자세에 따른 근육별 근활성도 비교
    4. 어깨 가쪽돌림 운동 시 운동방법에 따른 가시아래근/뒤어깨세모근 근활성도 비 비교
    5. 어깨 가쪽돌림 운동 시 아래팔의 자세에 따른 가시아래근/뒤어깨세모근 근활성도 비 비교
DISCUSSION
CONCLUSIONS
FUNDING
ACKNOWLEDGEMENTS
CONFLICTS OF INTEREST
AUTHOR CONTRIBUTIONS
ORCID
REFERENCES

• 손명기(대전대학교 대학원 물리치료학과) | Myeong-gi Son (Department of Physical Therapy, The Graduate School, Daejeon University)
• 김선엽(대전대학교 보건의료과학대학 물리치료학과) | Suhn-yeop Kim (Department of Physical Therapy, College of Health and Medical Science, Daejeon University, Daejeon, Korea) Corresponding author