Abstract
BACKGROUND: While ulnar collateral ligament (UCL) injuries have been widely studied, no investigations have characterized the kinematics of acutely presenting UCL injuries or examined the pitch-by-pitch sequence leading up to ligamentous failure. PURPOSE: To use Statcast pitch-tracking metrics from Major League Baseball (MLB) games to identify potential warning signs for acute UCL injury. STUDY DESIGN: Retrospective case-control; Level of evidence, 3. METHODS: Seven MLB pitchers who sustained acute UCL injuries on a single pitch between 2020 and 2025 were retrospectively identified. Statcast data from the injury game and 4 prior outings were analyzed for each player. Five pitch-tracking metrics (velocity, spin rate, release extension, arm angle, and acceleration magnitude) were extracted for each pitch. For each pitcher and pitch type, a principal component model was trained on control outings; Mahalanobis distance (MD) was calculated to quantify multivariate deviation from baseline. To assess short-term variability, MD values from the 5 fastballs before injury were summed and compared with control-derived thresholds. Pitch-to-pitch trends in MD and individual features were analyzed for progressive changes. Matched controls were selected according to age, handedness, starting versus relief pitcher, and season pitch count. The same process was then completed for a 5-game sample window of pitches for these matched controls. Outlier pitches and periods of elevated short-term variability were identified when MD values exceeded a 95th percentile threshold derived from each pitcher's control data; statistical significance for performance trends was set at P < .05. RESULTS: Principal component analysis and MD analysis revealed 3 principal findings: (1) every injury pitch was a mechanical outlier based on MD exceeding the 95th percentile of the control distribution (all P < .05), (2) velocity was suppressed by a mean of 2.1 SD and arm angle reduced by a mean of 1.5 SD at the injury pitch, and (3) 86% of pitchers (6 of 7) showed elevated cumulative deviation across the 5 fastballs preceding injury, compared with 7% of matched controls (1 of 14) (P < .001). CONCLUSION: This study demonstrated that acute UCL injuries in MLB pitchers are often preceded by identifiable changes in key pitch-tracking metrics. Rather than a prolonged deterioration across outings, acute UCL failure may reflect a distinct biomechanical pattern marked by short-term variability and abrupt decompensation within the injury game. These findings provide the first pitch-level characterization of UCL failure and may support future in-game risk-monitoring strategies. However, future studies are needed to help understand these warning signs and prevent UCL injury.