Playing level achieved, throwing history, and humeral torsion in Masters baseball players.

Humeral torsion is thought to be beneficial for throwing. To examine this hypothesis, the throwing and non-throwing arms of 84 Masters baseball players over 35 years of age were measured for humeral torsion, and the highest playing level they achieved and their playing history were recorded. Regression analyses were used to obtain predictors of the highest playing level achieved, throwing arm humeral torsion, and side-to-side difference in humeral torsion. Equations accounting for 36%, 46%, and 12% of the variance respectively were produced. Achieving representative level playing status was associated with the number of seasons played under 16 years of age and having greater throwing arm humeral retrotorsion. Throwing arm humeral retrotorsion was associated with non-throwing arm humeral retrotorsion and an increased number of seasons played before the age of 16 years. A model in which repeated throwing develops an adaptive unilateral bone torsion along with growth in baseball expertise is proposed. Thus dominant arm humeral retrotorsion is a predictor of highest baseball playing level achieved, which in turn is influenced by genetic endowment (as seen in the torsion of the non-throwing arm) and amount of throwing activity, particularly prior to the age of 16.