Recent studies have documented an increase in humeral retroversion significantly correlates with the changes of increased external rotation and decreased internal rotation in the throwing shoulder of baseball players. However, the time course over which this changes occurs is unknown. The purpose of this study was to assess the humeral retroversion in young baseball players. Both shoulders of 77 young baseball players(elementary school: 25, junior high school: 25, high school: 27) were evaluated and the humeral retroversion was assessed by using the ultrasonographic technique of Ito et al. The mean humeral retroversion in elementary school players was 11.9° in the throwing shoulder and 18.0° in the contralateral shoulder. The mean humeral retroversion in junior high school players was 13.8° in the throwing shoulder and 31.1° in the contralateral shoulder. The mean humeral retroversion in high school players was 15.6° in the throwing shoulder and 25.1° in the contralateral shoulder. A paired t-test showed that the difference in measurements of the humeral retroversion between the throwing side and the contralateral side ( P < 0.01 ). In the throwing shoulder of baseball players there is an increase in humeral retroversion when compared with the contralateral shoulder in the throwing shoulder significantly increased since the period of primary childhood with a slight duration of their active participation in baseball. We thought that a change of humeral retroversion by a throw occurred from the early stages when they started baseball.
