Sidearm Deliveries Are Associated With Lower Shoulder and Elbow Injury Burden and a Favorable Performance Profile in Major League Baseball: A Cohort Study of 927 Pitchers (2020-2025).

Background: Pitching-related shoulder and elbow injuries continue to rise in baseball despite workload restrictions and biomechanics-informed coaching. Purpose: To evaluate associations between arm slot and shoulder/elbow injured list (IL) time-loss burden and performance in Major League Baseball (MLB) pitchers. Study Design: Cohort study; Level of evidence, 3. Methods: Publicly available data were aggregated for 927 MLB pitchers who threw ≥1000 pitches during 2020 to 2025 and had arm angle data available from the MLB Hawk-Eye tracking system. Arm slot was defined as the throwing-arm angle relative to horizontal at ball release and averaged per pitcher (mean ± SD, 39.0°± 10.9°). Based on the distribution of arm slot values in the cohort, pitchers were categorized as sidearm (<33.5°; n = 255), three-quarters (33.5°-44.4°; n = 373), or overhand (>44.4°; n = 299). Primary outcomes were cumulative shoulder+elbow, shoulder, and elbow IL days accrued during the 2020 to 2025 study period. Performance outcomes included run prevention metrics, FanGraphs "plus" metrics (Stuff+ reflects pitch quality indexed to league average), strikeout and walk rates, hard-hit percentage (frequency of high-exit-velocity contact), and expected fielding independent pitching (xFIP; pitcher-controlled run prevention estimate). Group comparisons used 1-way analysis of variance with Tukey testing. Continuous associations with performance metrics reaching group-level significance were assessed with linear regression. Injury burden was modeled using negative binomial regression with log (total pitches) and percentage of games started as covariates; the primary analysis additionally adjusted for Stuff+ and 4-seam fastball velocity to test for independent associations after accounting for established risk factors. Results: Pitchers accumulated 86,412 combined IL days (25,710 shoulder; 60,702 elbow). Shoulder injury burden increased linearly with more overhand arm slots, whereas combined and elbow burden were nonlinear, peaking within the overhand group (45°-57°). Per 1 SD (10.9°) toward a more overhand delivery, injury burden increased for combined (incidence rate ratio [IRR] = 1.09; Conclusion: In this MLB cohort, sidearm deliveries were associated with lower shoulder/elbow time-loss burden, superior pitch quality, and lower hard-hit rates. Shoulder injury burden increased linearly with more overhand deliveries, whereas elbow burden followed a nonlinear pattern that peaked within the overhand range. These associations were independent of workload, velocity, and pitch quality. Arm slot may represent a clinically relevant marker of pitcher injury risk that warrants prospective study.