Effects of a Shoulder Brace on Deltoid Activation and Scapular Coordination During Internal–External Arm Rotation in Healthy Adults

Background: Early rehabilitation after rotator cuff repair requires controlled stress and optimal neuromuscular balance.This study evaluated whether a standardized mechanical shoulder brace alters muscle activation and scapular coordination during standing rotational range of motion (ROM) at 30°, 60°, and 90° of abduction.Methods: Surface electromyography was recorded from 12 shoulder-girdle muscles of healthy participants during standing external-internal rotation; Muscle activation, normalized to reference voluntary contraction, and scapular balance indices (middle trapezius/upper trapezius, MT/UT; lower trapezius/upper trapezius, LT/UT; serratus anterior/upper trapezius, SA/UT) of the braced side were compared to the contralateral unbraced reference.Results: Across all tested angles, activity consistently decreased in the anterior deltoid, upper trapezius, and upper latissimus dorsi, while substantially increasing in the middle deltoid, posterior deltoid, and middle trapezius (all adjusted P < 0.05).For scapular balance, the MT/ UT ratio demonstrated the most substantial brace-induced improvement, increasing by +132.1% to +150.3% across angles (P < 0.001);Conversely, the LT/UT and SA/UT ratios exhibited angle-specific modulation with neither showing uniform improvement and both demonstrating non-significant trends toward reduction at 60° and 90°, respectively. Conclusions:The standardized shoulder brace functions as a motor-strategy modifier that effectively recalibrates scapular force couples, largely by suppressing UT dominance and facilitating MT engagement.However, the robust increase in posterior deltoid activity and angle-specific mechanical constraints suggest that the brace redistributes rather than universally unloads neuromuscular demand.These findings support clinical utility primarily as a positioning and strategy-guiding device during early, lower-angle ROM retraining.