Increased Tendon Thickness in Healthy Collegiate Baseball Pitchers Is Associated With Fewer Years of Pitching and Less External Rotation Motion Adjusted for Humeral Torsion

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Background: Mechanistic models of rotator cuff (RC) tendinopathy implicate repetitive loading as a driver of tendon structural changes. Baseball pitchers experience repetitive loading and physical adaptations in shoulder range of motion (ROM) and humeral torsion (HT) that may affect tendon structure and lead to tendinopathy. Purpose: To (1) characterize the effect of repetitive loading by determining dominant-to-nondominant arm differences in shoulder ROM, HT, and RC tendon thickness in healthy collegiate pitchers and (2) determine whether side-to-side differences in tendon thickness are associated with shoulder ROM, HT, and self-reported years of pitching experience. Study Design: Cross-sectional. Level of Evidence: Level 3. Methods: A total of 71 healthy, Division I pitchers were assessed bilaterally for shoulder internal rotation (IR), external rotation (ER), and flexion. HT and RC tendon thickness were measured via ultrasound. ROM measures were adjusted for HT to account for individual differences in bony morphology. Years of pitching was recorded. Paired t tests compared dominant vs nondominant arms (Aim 1). Multivariable linear regression, following LASSO selection, predicted side-to-side differences in tendon thickness using ER adjusted for HT and years pitching (Aim 2). Results: Aim 1: dominant arms showed greater tendon thickness and ER, and less IR, flexion, and HT (greater retrotorsion) ( P < 0.01). Aim 2: greater tendon thickness was associated with fewer years pitching (–0.1 mm per year; P = 0.02) and had a limited, nonsignificant relationship with less ER ROM-adjusted for HT ( P = 0.14) (model: adjusted r -squared = 0.08; P = 0.04). Conclusion: There are ROM, tendon structure, and HT adaptations in pitchers’ dominant arm. Greater tendon thickness was moderately associated with fewer years of pitching and was not significantly associated with less difference in ER ROM-adjusted for HT. Clinical Relevance: These findings are hypothesis-generating and support future longitudinal work incorporating exposure to pitching with bony morphology to understand their combined relationship with tendon morphology and potential injury risk.

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