The effects of shoulder abduction angle and elastic band resistance on shoulder muscle activity during external rotation exercises

Elastic band exercises are widely used in clinical and field settings to improve shoulder muscle function due to their portability and accessibility. The purpose of this study was to investigate the effects of different humeral abduction (ABD) angles and levels of elastic band resistance on shoulder muscle activity during ER exercises. Thirteen healthy male participants (age: 24.1 ± 2.9 years; height: 176.8 ± 5.9 cm; mass: 70.9 ± 9.1 kg) were recruited for this study. Surface and needle electromyography (EMG) were used to record activity from seven shoulder and rotator cuff muscles: posterior deltoid (PD), upper trapezius (UT), lower trapezius (LT), serratus anterior (SA), supraspinatus (SSP), infraspinatus (ISP), and teres minor (TM) during standing ER exercises. Participants performed exercises at three shoulder ABD angles (0°, 45°, and 90°) and two levels of elastic band resistance (low and moderate). UT and SA activity significantly increased at 90° shoulder ABD compared to 0° and 45° (both p < 0.001). SSP activity increased at both 45° and 90° shoulder ABD compared to 0° (p = 0.007, p = 0.015). SA/UT, LT/UT, and SSP/ISP co-contraction indices (CCIs) increased at 45° shoulder ABD compared to 0° (p = 0.036, p < 0.001, p < 0.001). Muscle activity in the UT, LT, SSP, and ISP increased with moderate compared to low band resistance (p ≤ 0.028). Additionally, CCIs among shoulder muscles was greater with moderate compared to low band resistance (p ≤ 0.012). These findings provide foundational insights for optimizing shoulder ER exercises and may inform future research and rehabilitation program design.