Timing-Dependent Trunk Kinematics are Associated with Reduced Shoulder and Elbow Joint Demands During Baseball Pitching

Baseball pitching places substantial mechanical demand on the shoulder and elbow, contributing to a high incidence of upper-extremity injury across competitive levels. Trunk kinematics are known to influence joint moments on distal structures, however it remains unclear how kinematics at different pitch phases influences these moments. PURPOSE: To examine how trunk and pelvis kinematics at different stages of pitching influence peak elbow varus and shoulder internal rotation (IR) moments while controlling for pitch speed and anthropometrics. METHODS: Open access markered motion capture (45 markers, 360 Hz) and ground reaction force (1080 Hz) data were collected from collegiate baseball pitchers (n=100 players, 411 total pitches; age=21.3±2.2 years). Three-dimensional kinematics and kinetics were calculated using inverse dynamics. All data was collected and variables calculated by the OpenBiomechanics Project (Wasserberger et al. 2022). Hip-shoulder separation and trunk orientation (anterior tilt, lateral tilt, and rotation) were quantified at foot plant (FP), maximum external rotation (MER), and ball release (BR). Trunk orientation changes were calculated from FP to MER and from MER to BR. Covariate-adjusted linear mixed-effects models were used to assess associations between trunk and pelvis kinematic changes at different stages and peak shoulder and elbow moments. Pitch velocity, age, height, and body mass were considered covariates, and subject identity was considered a random effect. RESULTS: Greater maximum hip–shoulder separation was positively associated with both peak elbow varus and shoulder internal rotation moments (β=0.777 and 0.847, respectively; p