Abstract Background: Glenohumeral Internal Rotation Deficit (GIRD) rehabilitation literature lacks comprehensive neuromuscular analysis explaining motor control mechanisms underlying elastic resistance training effectiveness. Current evidence emphasizes range of motion and pain outcomes without addressing rotator cuff and scapular stabilizer activation patterns or co-contraction dynamics essential to shoulder stability. Objective: This randomized controlled trial investigated neuromuscular adaptations in rotator cuff and scapular stabilizer musculature following 12-week TheraBand-based throwing training in symptomatic collegiate volleyball players with GIRD. Methods: Sixty-four male collegiate volleyball players (age 20.4±1.9 years) with documented GIRD (≥10°) and scapular dyskinesis were randomized to intervention (n=32) or control (n=32) groups. The intervention consisted of progressive 12-week TheraBand-based throwing exercises emphasizing external rotator strengthening and scapular stabilization. Primary outcome measures included surface electromyography (%MVIC) of infraspinatus, subscapularis, lower trapezius, and serratus anterior muscles during standardized functional tasks. Secondary measures encompassed muscle co-contraction ratios, calculated shoulder joint stability indices, pain intensity (VAS), disability (DASH), quality of life (WORC), and return-to-play readiness. Results: Intervention group participants demonstrated remarkable neuromuscular adaptations. Infraspinatus EMG activation increased 117% from baseline 22.3%±8.4% MVIC to post-intervention 48.6%±9.2% MVIC (p<0.001, Cohen's d=2.56). Subscapularis activation increased 122% from 18.6%±7.2% to 41.3%±8.6% MVIC (p<0.001, d=2.41). Lower trapezius activation increased 112% from 24.7%±9.1% to 52.4%±10.3% MVIC (p<0.001, d=2.58). Serratus anterior activation increased 107% from 26.4%±10.2% to 54.8%±11.1% MVIC (p<0.001, d=2.47). Critically, subscapularis/ infraspinatus co-contraction ratio improved 19% (0.84 to 0.68, p<0.001), and upper trapezius/lower trapezius ratio improved 39% (1.24 to 0.76, p<0.001), indicating normalized motor control patterns. Shoulder joint stability index nearly doubled from 42.3±12.6 to 73.8±11.4 (p<0.001, d=2.35). Pain decreased 75% from 3.2±1.8 to 0.8±1.1 VAS (p<0.001), DASH scores decreased 77% (18.4 to 4.2 points, p<0.001), and return-to-play readiness improved 52% (58.2% to 88.3%, p<0.001). Control group demonstrated no significant neuromuscular changes or functional improvements. Conclusion: TheraBand-based throwing training produces substantial neuromuscular adaptations in rotator cuff and scapular stabilizer musculature characterized by markedly increased EMG activation, normalized co-contraction ratios reflecting improved motor control, and enhanced dynamic glenohumeral joint stability. These motor control adaptations directly correspond to pain relief, functional disability reduction, and return-to-play achievement, establishing the mechanistic basis for elastic resistance training effectiveness in GIRD management. This investigation advances understanding of how external perturbation and progressive resistance stimulate neuromuscular motor learning and dynamic stabilization in overhead athletes.