Adaptations of the shoulder to overhead throwing in youth athletes

Astolfi, Matthew
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University of Delaware
Introduction: Due to the high repetition and force associated with overhead throwing anatomical adaptations are observed in elite and professional level baseball athletes however, little is known about their origin and progression. This is especially concerning because the incidence of chronic shoulder injuries in youth baseball is on the rise and may precipitate lifelong biomechanical alterations and associated pathologies. Arm dominance and throwing have been correlated with structural changes in older throwers including humeral retroversion (HR), and posterior capsule thickness (PCT) yet the influence of age on these adaptations is unknown. Purpose: To investigate the relationship of age and arm dominance on measures of HR, PCT, subacromial space (SAS), glenohumeral internal rotation (GHIR), external rotation (GHER), posterior shoulder tightness (PST). Methods: Thirty-five subjects ages eight to twelve years old, participating in organized youth baseball underwent testing using Diagnostic Ultrasound and measurements of glenohumeral internal rotation, external rotation, and posterior shoulder tightness were obtained. Results: The dominant arms had significantly less HR (p < .001), and GHIR (p< .001), but greater PCT (p< .01), and GHER (p< .001) than the non-dominant arm. Dominant IR was significantly different between the under 10-year-old group and the over 10-year-old group. There were no differences with regard to PST between shoulders or age groups. A significant, strong, negative correlation between dominant GHIR and GHER (r= -.395, p< .05), a significant, strong, negative correlation between HR and GHIR (r= -.431, p=.01), and a significant, strong, positive correlation between HR and GHER (r= .448, p< .01) were observed. Dominant PCT was strongly, positively correlated with GHER (r= .322, p= .059) was observed. Conclusions: The alterations in a youth baseball sample are similar to those observed in older baseball athletes. This is the first study to demonstrate greater PCT in the dominant arm of youth baseball athletes. The magnitude of HR differences in youth was shown to be similar to older baseball athletes. Alterations in glenohumeral ROM displayed similar trends of increased GHER, and decreased GHIR. The youth baseball population is developing adaptations consistent with older baseball athletes, indicating further research is needed to determine the developmental mechanisms in youth, and the potential injury risks associated with such adaptations in youth.