Mechanisms of Shoulder Range of Motion Deficits in Asymptomatic Baseball Players

BACKGROUND: Shoulder range of motion (ROM) deficits have been identified as injury risk factors among baseball athletes. Despite the knowledge surrounding these risk factors, there is a lack of consensus regarding the specific tissues responsible for these deficits in ROM. PURPOSE/HYPOTHESIS: The purpose of this study was to elucidate the primary mechanisms of posterior shoulder tightness (capsular, musculotendinous, bony) by examining the tissue responses that occur with the application of an acute intervention in baseball players with ROM deficits. The hypothesis was that posterior rotator cuff stiffness, not glenohumeral joint mobility, would be primarily responsible for ROM gains observed within an acute treatment setting. STUDY DESIGN: Controlled laboratory study. METHODS: Through use of ultrasound elastography, electromagnetic motion analysis, and ultrasound imaging, posterior rotator cuff stiffness, glenohumeral joint translation, and humeral torsion were examined in 60 asymptomatic baseball players (age, mean ± SD, 19 ± 2 years) with shoulder ROM deficits. Tissue mechanisms were examined concurrently, with the ROM gains elicited by an acute application of instrument-assisted soft tissue mobilization plus self-stretching (n = 30) versus self-stretching only (n = 30). Separate 3-way analyses of variance (group × arm × time) and linear regression analyses were used to determine the treatment effects and relationships between tissue mechanisms and ROM gains. RESULTS: ROM gains were associated with decreases in rotator cuff stiffness (internal rotation: r = 0.35, P = .034; horizontal adduction: r = 0.44, P = .008) and increased humeral retrotorsion (internal rotation: r = -0.35, P = .034), not joint translation (P > .05). Players receiving instrument-assisted soft tissue mobilization plus stretching displayed greater shoulder ROM gains (internal rotation, +5° ± 2° P = .010; total arc of motion, +8° ± 6° P = .010; horizontal adduction, +7° ± 2° P = .004; and decreased posterior rotator cuff stiffness, -0.2 ± 0.3 kPa P = .050) compared with players receiving self-stretching alone. CONCLUSION: Decreases in rotator cuff stiffness were associated with acute ROM gains in baseball players. The study results show that changes in rotator cuff stiffness, not glenohumeral joint mobility or humeral torsion, are most likely associated with the ROM deficits observed in adolescent baseball players. CLINICAL RELEVANCE: Reducing rotator cuff stiffness may be beneficial in improving the ROM deficits associated with injury risk in overhead athletes.