Context: In a coordinated throwing motion, peak pelvis rotation velocity is reached before the peak upper torso rotation velocity, so that angular momentum can be transferred effectively from the proximal (pelvis) to distal (upper torso) segment. Despite the fact that upper extremity kinematics and kinetics during throwing are highly influenced by trunk kinematics, the effects of proper vs. improper trunk rotation sequence on baseball pitching performance (ball speed) and upper extremity biomechanics linked to common pitching-related upper extremity injuries have not been investigated. Objective: To investigate the effects of trunk rotation sequence on ball speed and upper extremity kinematics and kinetics linked to injuries in high school baseball pitchers.

Design: Cross-sectional study.

Setting: Biomechanics laboratory.

Patients or Other Participants: Seventy-three healthy high school baseball pitchers (dominance=57-right/16-left, age=15.5±1.2years, height =179.3± 7.3cm,mass=72.7±9.8kg).

Interventions: Three-dimensional kinematics of the pitching motion were captured using a motion capture system. The pitches were performed until at least 3 strike-pitches were captured.

Main Outcome Measures: Timings of peak pelvis and upper torso rotation velocity (% pitching time = time from stride foot contact to ball release), maximal shoulder external rotation angle (MaxER), elbow extension angle at ball release (ElbEXT), peak shoulder proximal force, shoulder internal rotation moment, and elbow varus moment were calculated from the marker coordinate data. Subjects were considered to have a proper vs. improper trunk rotation sequence when the peak pelvis rotation velocity reached before vs. after the peak upper torso rotation velocity beyond the margin of standard error of measurement (±3.7% pitching time), respectively. Subjects in whom the difference in timing of peak pelvis and upper torso velocities was within the margin of error were excluded. Dependent variables were compared between groups using independent t-tests (˜<.05).

Results: Subjects with improper trunk rotation sequence (n=33) demonstrated greater MaxER (mean difference=7.2°±2.9°, 95%CI =1.39°-13.0°, t54=2.5, p=.016) and greater shoulder proximal force (mean difference=9.2±3.9%body weight, 95%CI=1.4-16.9%, t54=2.4, p=.021) compared to those with proper trunk rotation sequence (n=23). There were no group differences in ElbEXT, ball speed, or the other joint kinetic variables (p>.05).

Conclusions: No significant difference observed in ball speed or joint moments suggest that other factors may have a larger influence on these variables. Anatomically, the greater MaxER is associated with increased risk of posterior impingement and superior labral anterior-posterior lesion. Additionally, increased shoulder proximal force that resists distraction of the shoulder during arm-acceleration is linked to increased tensile stress on the rotator cuff and biceps-labral complex. Therefore, improper trunk rotation sequence may be harmful to the throwing shoulder. Methods to identify pitchers with improper trunk rotation sequence without a motion capture system, and to correct the improper technique need to be investigated in order to apply this observation to prevention of pitching-related upper extremity injuries.