The Association Among Trunk Rotation, Ball Velocity, and the Elbow Varus Moment in Collegiate-Level Baseball Pitchers

Abstract

Background: The incidence of upper extremity injuries in baseball pitchers is increasing. Over the past decade, research has attempted to elucidate the cause of these injuries, focusing mainly on pitching arm mechanics with little examination of other important segments, such as the trunk. This is surprising, as trunk motion has been shown to have significant effects on pitching mechanics. Purpose: To determine the associations between trunk rotation, ball velocity, and the moments about the elbow joint. Study Design: Descriptive laboratory study. Methods: Data collected using 3-dimensional motion analysis techniques from 99 collegiate pitchers (18.0-24.8 years) were analyzed. A random intercept mixed-effects regression model was used to determine if significant associations existed between trunk rotation and ball velocity or elbow varus moment. Results: Significant associations were found between trunk rotation angle at ball release and elbow varus moment (P = .019, β = 0.254) as well as ball velocity (P = .016, β = 0.060). For every 10° increase over the average trunk rotation angle at ball release, the elbow varus moment increased by 2.54 N·m and the ball velocity increased by 0.60 m/s. Additionally, the maximum rotational velocity of the trunk was positively associated with elbow varus moment (P < .001, β = 0.029) and ball velocity (P < .001, β = 0.007). For every 100 deg/s increase over the average maximum rotational velocity of the trunk, the elbow varus moment increased by 2.90 N·m and the ball velocity increased by 0.70 m/s. Conclusion: In collegiate pitchers, trunk rotation angle at ball release was significantly associated with ball velocity and elbow varus moment. Also, an increase in maximum rotational velocity of the trunk was significantly associated with an increase in the ball velocity and elbow varus moment. This work demonstrates the importance of trunk mechanics in the kinetic chain of the pitch cycle. Clinical Relevance: Pitching coaches and trainers can use the results to stress the importance of trunk mechanics in pitching, specifically, combining adequate core function with increased trunk rotational velocity in an effort to increase pitching velocity without increasing elbow joint stress.