Propensity Matched Professional Baseball Pitchers With Early Trunk Rotation Demonstrate Increased Shoulder and Elbow Distractive Forces with Equivalent Ball Velocity.

Abstract

Prior mixed cohort pitching biomechanic analyses have noted improved throwing arm kinetics in pitchers with delayed trunk rotation. To compare throwing arm kinetics and ball velocity in a homogenous professional baseball pitching cohort based on trunk rotation initiation before or after foot contact. 339 professional pitchers instructed to throw 8 to 12 fastball pitches were evaluated with 3-dimensional motion capture (480 Hz). Pitchers were identified as initiating trunk rotation before the moment of foot contact (FC) (EARLY, n=33) and were propensity matched on the basis of height, age, and weight to pitchers who initiated trunk rotation after FC (LATE, n=132) resulting in a 4:1 match. Each EARLY pitcher was paired with each of their four matched LATE pitchers, yielding 132 matched pairs, and between-group differences across 32 pre-specified kinematic and kinetic outcomes were evaluated with paired-samples t-tests using the per-pitcher mean across all available pitches. Effect estimates are reported as mean difference (EARLY - LATE) with 95% confidence intervals. Significance was assessed at Bonferroni-corrected thresholds within outcome sub-domain. Within-pitcher autocorrelation was assessed with the Durbin-Watson statistic. Prior-injury prevalence was compared with Fisher's exact test. EARLY pitchers demonstrated a longer stride length at foot contact (MD +3.7 %BH [95% CI +2.6, +4.8], p<0.001*), a more open pelvis at foot contact (MD -8° [95% CI -11, -5], p<0.001*), greater shoulder external rotation at foot contact (MD +17° [95% CI +10, +24], p<0.001*), greater shoulder horizontal adduction at maximum external rotation (MD +3° [95% CI +1, +5], p=0.002*), greater trunk flexion at ball release (MD +7° [95% CI +5, +9], p<0.001*), greater maximum trunk flexion excursion (MD +5° [95% CI +3, +7], p<0.001*), later foot contact (MD +0.022 ms [95% CI +0.018, +0.026], p<0.001*), and earlier trunk initiation (MD -0.021 ms [95% CI -0.026, -0.016], p<0.001*) than LATE pitchers. EARLY pitchers also showed lower peak pelvic rotation velocity (MD -44°/s [95% CI -67, -22], p<0.001*), greater normalized elbow distractive force (MD +8.3 %BW [95% CI +4.1, +12.6], p<0.001*), greater normalized shoulder distractive force (MD +9.8 %BW [95% CI +5.7, +14.0], p<0.001*), and lower maximum trunk rotational kinetic energy (MD -4.0 J [95% CI -6.2, -1.8], p=0.002*). Ball velocity did not differ between EARLY and LATE pitchers (MD +0.5 m/s [95% CI -0.0, +1.1], p=0.070). Prior shoulder (12.1% vs 9.1%), elbow (18.2% vs 18.9%), and any upper-extremity injury (33.3% vs 31.8%) did not differ between groups (all Fisher exact p>0.5). Professional baseball pitchers who initiated trunk rotation before foot contact demonstrated increased throwing arm kinetics and altered pitching mechanics including increased normalized shoulder and elbow distractive forces, decreased peak pelvic rotation velocity, decreased maximum trunk rotational kinetic energy, and a longer stride with a more open pelvis and greater shoulder external rotation at foot contact, suggesting compensatory motions of the throwing arm to achieve similar ball velocities. Professional pitching staff should be cognizant of the potential mechanical disadvantage of early trunk initiation on the throwing arm. Trunk initiation before or after foot contact gives pitchers a concrete time point by which to adjust or refine their pitching mechanics.