Biomechanical risk factors for medial elbow valgus stress in baseball pitchers: the role of Forearm Whip Angle

Medial elbow injuries, particularly ulnar collateral ligament injuries, are a major concern in youth and adolescent pitchers. Elbow varus torque (EVT) is commonly used as an index of medial valgus load. We examined whether the Forearm Whip Angle (FWA)-defined as the sagittal-plane inclination of the throwing-forearm segment relative to the horizontal-relates independently to EVT and which phase-specific lower-extremity/trunk motions determine FWA. Seventeen healthy high-school pitchers were analyzed with 3-dimensional motion capture. Across the pitching cycle (0%-100%, 1% bins), we mapped pointwise Pearson correlations between FWA and 6 kinematic variables and controlled multiplicity using the Benjamini-Hochberg false discovery rate (FDR) with a &#x2265; 3-bin continuity criterion. Two prespecified multiple linear regressions were then fitted: Model 1: EVT &#x223c; FWA + ball velocity; Model 2: FWA &#x223c; phase-averaged kinematics identified a priori from the FDR screen. Mean (&#xb1; standard deviation) values were: FWA 163&#xb0; &#xb1; 17&#xb0;, EVT 57 &#xb1; 13 Nm, and ball velocity 129 &#xb1; 8 km/h (n = 17). After FDR correction, one significant window was identified: a positive association between FWA and pivot-leg hip abduction/adduction (hip_y) at 67%-77% of the cycle. In Model 1, FWA was a strong, independent negative predictor of EVT (standardized &#x3b2; = -0.844; P < .001), whereas ball velocity was not significant (&#x3b2; = 0.195; P = .219). Model fit: R= .690; adjusted R= .646; leave-one-out cross-validated R= .526; root mean square error = 8.62 Nm. In Model 2, hip_y (67-77%) was the only independent predictor of FWA (&#x3b2; = 0.749; P = .002); thorax_x (80%-90%) and pelvis_y (39%-63%) were not significant. Model fit: R= .524; adjusted R= .414; leave-one-out cross-validated R= .284; root mean square error = 18.6&#xb0;. Greater FWA-an index of "whip-like" kinetic-chain coordination-was associated with lower medial elbow valgus load independent of ball velocity. Phase-specific control of pivot-leg hip abduction/adduction (67%-77%) appears to be a key determinant of FWA. These findings support incorporating FWA and targeted hip-control strategies into screening and training; causal inference awaits longitudinal or interventional studies.