Background High shoulder external rotation range of motion (ROM) in baseball players is associated with a higher risk of shoulder and elbow injuries. Subjective shoulder discomfort can impair performance in these athletes. While shoulder stabilization surgery is effective in non-throwing athletes, including those in collision sports, its efficacy is limited in throwing athletes. Understanding the appropriate degree of anterior and posterior shoulder structural translation is crucial for treating shoulder instability in baseball players. This study aimed to determine whether subjective shoulder discomfort in baseball players is influenced by anterior or posterior translation through quantitative assessment using ultrasound imaging. Methods Fifty-one healthy college students were recruited, including: 40 baseball players and 11 controls who had never participated in an overhead sport. The humeral head translation was measured using ultrasonography. The translations were calculated separately for the anterior and posterior directions. Measurement parameters included shoulder rotation ROM and Oxford Shoulder Instability Score (OSIS), and Scapular dyskinesis test (SDT). Statistical analyses compared humeral head translation and ROM between the groups. Multiple regression analysis was used to examine the relationship between external rotation ROM and anterior translation in baseball players. Baseball players were classified into asymptomatic and non-asymptomatic groups based on the median OSIS values collected. Multiple regression analysis was conducted using anterior and posterior translation as independent variables and OSIS as the dependent variable. Another model used external rotation ROM as the dependent variable, with anterior translation as the main predictor. All regression models used forced entry. In addition, the association between SDT classification and both OSIS-based grouping and humeral head translation was examined using chi-square and t-tests, respectively. Results Baseball players exhibited significantly higher external rotation ROM and anterior translation than controls (p < 0.05). Multiple regression analysis revealed that higher anterior translation in baseball players was associated with a higher external rotation ROM (p < 0.001). The findings of the two-group comparison and multiple regression analysis indicated that higher posterior translation was associated with increased OSIS in baseball players (p < 0.001). Positive SDT was significantly associated with OSIS grouping (p < 0.05), and was also associated with posterior translation of the humeral head (p < 0.05). Conclusion While anterior translation appears to be an adaptation to support external rotation, posterior translation may related to subjective feelings of instability in baseball players with scapular dyskinesis. Ultrasound imaging enabled precise quantification of subtle translation differences, offering a valuable tool for detecting early signs of shoulder dysfunction. These findings highlight the importance of assessing posterior humeral head translation in athletes reporting discomfort, which may aid in developing more targeted rehabilitation strategies and prevent progression to clinically evident instability.
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