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

Tomoyuki Matsui; Kazuya Seo; Tetsuya Miyazaki; Yoshikazu Azuma; Yudai Kikuchi; Noriyuki Kida; Toru Morihara

doi:10.1016/j.jse.2025.11.008

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

J Shoulder Elbow Surg. 2025 Dec 13:S1058-2746(25)00851-1. doi: 10.1016/j.jse.2025.11.008. Online ahead of print.

Authors

Tomoyuki Matsui¹, Kazuya Seo², Tetsuya Miyazaki¹, Yoshikazu Azuma¹, Yudai Kikuchi¹, Noriyuki Kida³, Toru Morihara⁴

Affiliations

¹ Marutamachi Rehabilitation Clinic, Kyoto, Japan.
² University Hospital, Kyoto Prefectural University of Medicine, Kyoto, Japan.
³ Kyoto Institute of Technology, Kyoto, Japan.
⁴ Marutamachi Rehabilitation Clinic, Kyoto, Japan. Electronic address: toru4271@koto.kpu-m.ac.jp.

PMID: 41397514
DOI: 10.1016/j.jse.2025.11.008

Abstract

Background: 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.

Methods: 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 ≥ 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.

Results: Mean (± standard deviation) 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; leave-one-out cross-validated R² = .526; root mean square error = 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; leave-one-out cross-validated R² = .284; root mean square error = 18.6°.

Conclusion: 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.

Keywords: Forearm whip angle; adolescents; biomechanics; elbow varus torque; injury prevention; kinetic chain; pitching mechanics; ulnar collateral ligament.