Effects of Different Load Intensities During Shoulder External Rotation Resistance Exercise on Shoulder and Trunk Muscle Activation in Healthy Young Men

Purpose: This study aimed to investigate the changes in scapular upward rotator muscles and erector spinae activity according to different relative load conditions (0%, 1%, 2%, 3% of body weight) during prone shoulder external rotation exercise.Methods: Twenty healthy adult males without shoulder pathology voluntarily participated in this study.Surface electromyography (sEMG) was used to measure muscle activity of the lower trapezius (LT), upper trapezius (UT), serratus anterior (SA), and erector spinae (ES) during prone shoulder external rotation under four load conditions (0%, 1%, 2%, 3% of body weight).EMG signals were normalized to a reference voluntary contraction (%RVC).A randomized cross-over design was applied to minimize order and fatigue effects.Statistical analysis was conducted to compare differences in muscle activation across the load conditions. Results:The LT showed significantly greater activation at 1% body weight compared with 0%, without significant increases in the SA or ES, indicating selective recruitment.At 3%, although LT activation further increased, co-activation of the UT, SA, and ES was observed, suggesting compensatory strategies.The UT exhibited significant increases between 0% and 1%, 0% and 2%, 0% and 3%, 1% and 2%, and 1% and 3% conditions, demonstrating a progressive stepwise activation pattern.The SA showed significant increases from 2% and higher, while the ES demonstrated significantly higher activation at 3% load compared with 0%.