Why is the humeral retroversion of throwing athletes greater in dominant shoulders than in nondominant shoulders?

Nobuyuki Yamamoto MD, Eiji Itoi MD, Hiroshi Minagawa MD, Masakazu Urayama MD, Hidetomo Saito MD, Nobutoshi Seki MD, Takenobu Iwase MD, Shinji Kashiwaguchi MD and Tetsuya Matsuura MD

A rotation angle of the proximal humerus relative to the elbow (bicipital-forearm angle) was measured by use of ultrasonography to determine the relationship between humeral retroversion and growth in dominant and nondominant shoulders of 66 elementary and junior high school baseball players. The subjects were aged 12 years on average. The bicipital-forearm angle was significantly smaller in dominant shoulders than in nondominant shoulders. This indicated that the retroversion angle was greater in dominant shoulders than in nondominant shoulders. Furthermore, there was a moderately positive correlation between age and the bicipital-forearm angle in both dominant and nondominant shoulders. From these data, the authors conclude that the humeral retroversion angle decreases with age, and the decrease is much smaller in dominant shoulders. They assumed that the repetitive throwing motion does not increase the retroversion of the humeral head but rather restricts the physiologic derotation process of the humeral head during growth.

Journal of Shoulder and Elbow Surgery, Volume 15, Issue 5, September-October 2006, Pages 571-575

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