The Relationship of Throwing Arm Mechanics and Elbow Varus Torque: Within-subject Variation for Professional Baseball Pitchers Across 81,999 Throws

OBJECTIVES: Likely due to the high level of strain exerted across the elbow during the throwing motion, elbow injuries are on the rise in baseball. In order to identify at risk athletes and guide post-injury return to throw programs, a better understanding of the variables that influence elbow varus torque is desired. The purpose of this work was to describe the within-subject relationship between elbow varus torque and arm slot, arm speed, and shoulder rotation in professional baseball pitchers. METHODS: A total of 81 professional pitchers performed 81,999 throws while wearing a motusBASEBALL sensor and sleeve (Motus Global, Massapequa, NY, USA). These throws represented a combination of throw types, such as warm-up/catch, structured long-toss, bullpen throwing from a mound, and live game activity. Variables recorded for each throw included arm slot, arm speed, shoulder rotation, and elbow varus torque. Linear mixed-effects models and likelihood ratio tests were utilized to estimate to the relationship between elbow varus torque and arm slot, arm speed, and shoulder rotation within individual pitchers. RESULTS: All three metrics, arm slot (χ(2) = 428, p<0.01), arm speed (χ(2) = 57683, p<0.01), and shoulder rotation (χ(2) = 1392, p<0.01), were estimated to have a significant relationship with elbow varus torque. Within individual athletes, a 1-Nm increase in elbow varus torque was associated with a 13° decrease in arm slot, a 116°/s increase in arm speed, and an 8° increase in shoulder rotation. CONCLUSION: Elbow varus torque increased significantly as pitchers increased shoulder rotation during the arm cocking phase and rotational velocity of the arm during the arm acceleration phase of throwing. Thus shoulder flexibility, arm speed, and elbow varus torque (and likely injury risk) are inter-related and should be considered collectively when treating pitchers.