The influence of excessive ipsilateral trunk tilt on upper-extremity throwing mechanics: a newly characterized parameter for biomechanical evaluation in high school and professional pitchers.

Abstract

Although contralateral trunk tilt has been recognized, the phenomenon of excessive ipsilateral trunk tilt, often observed during earlier portions of the pitch, has not been explored as a potential correlate with throwing-arm kinetics among baseball pitchers. The purpose of this study was to evaluate kinetic and kinematic parameters among high school and professional pitchers related to excessive ipsilateral and contralateral trunk tilt. Professional and high school pitchers were assessed with a 3-dimensional motion capture system (480 Hz) while pitching. Pitchers were grouped as follows: excessive ipsilateral tilt at foot contact (FC), neutral, or excessive contralateral tilt at ball release (BR). Trunk and shoulder kinematics as well as throwing-arm kinetics were compared between subgroups via post hoc regression analysis. Professional pitchers (n=287) had significantly higher ipsilateral trunk tilt at FC (P<.001) than high school pitchers (n=59). High school pitchers with excessive contralateral trunk tilt at BR experienced significantly higher shoulder superior force (27.0%±7.4% body weight [BW] vs. 17.6%±5.1% BW, P<.001) and shoulder anterior force (39.6%±8.2% BW vs. 35.7%±5.4% BW, P<.001) compared with the ipsilateral trunk tilt cohort but had comparable ball velocity (30.2±3.2 m/s vs. 30.4±2.1 m/s, P=.633). For professionals, for every 10° increase in ipsilateral trunk tilt at FC, ball velocity increased by 0.2 m/s (B = 0.02, β = 0.07, standard error [SE] = 0.005, P= .010) whereas elbow varus torque decreased by 0.1% BW × height (B = -0.01, β = -0.08, SE = 0.002, P<.001) and shoulder internal rotation torque decreased by 0.1% BW × height (B = -0.01, β = -0.07, SE = 0.002, P= .005). High school and professional pitchers with excessive ipsilateral trunk tilt at FC consistently demonstrated significantly decreased throwing-arm kinetics (shoulder anterior force and shoulder superior force for high school pitchers and shoulder internal rotation torque and elbow varus torque for professional pitchers) compared with pitchers with excessive contralateral trunk tilt at BR, with equivalent ball velocity. In addition, professional pitchers appeared to engage in significantly greater ipsilateral trunk tilt during early portions of the pitch when compared with high school pitchers, which may represent a kinetically favorable method adopted by pitchers at higher playing levels to maintain adequate ball velocity while concomitantly minimizing throwing-arm kinetics.