Biomechanical risk factors for medial elbow valgus stress in baseball pitchers: The role of the Forearm Whip Angle (FWA).

Abstract

Medial elbow injuries, particularly ulnar collateral ligament (UCL) 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-D motion capture. Across the pitching cycle (0-100%, 1% bins), we mapped pointwise Pearson correlations between FWA and six kinematic variables and controlled multiplicity using the Benjamini-Hochberg false discovery rate (FDR) with a ≥3-bin continuity criterion. Two prespecified multiple linear regressions were then fitted: (Model 1) EVT ∼ FWA + ball velocity; (Model 2) FWA ∼ phase-averaged kinematics identified a priori from the FDR screen. Mean (±SD) values were: FWA 163° ± 17°, EVT 57 ± 13 Nm, and ball velocity 129 ± 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 β = -0.844; P < .001), whereas ball velocity was not significant (β = 0.195; P = .219). Model fit: R² = .690; adjusted R² = .646; LOOCV R² = .526; RMSE = 8.62 Nm. In Model 2, hip_y (67-77%) was the only independent predictor of FWA (β = 0.749; P = .002); thorax_x (80-90%) and pelvis_y (39-63%) were not significant. Model fit: R² = .524; adjusted R² = .414; LOOCV R² = .284; RMSE = 18.6°. 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.