Interindividual factors influencing on the relationship between elbow varus torque and varus angular acceleration during baseball pitching

To clarify the interindividual factors influencing on relationship between EVT and EVAA during baseball pitching.

Thirty-five healthy collegiate baseball pitchers participated in this study. Twenty to twenty-one pitches per pitcher recorded using a motion capture system (360 Hz, OptiTrack) were used in the following analyses. Multi-level models were used, where maximum EVT was the dependent variable and EVAA was the independent variable for level 1 and length from elbow to the center of mass of a composite segment (forearm, hand, and ball), a mass of the composite segment, and elbow flexion angle were independent variables to explain the variance of the intercept and the slope at level 2. The values of the independent variables were used at the timing of maximum EVT. Several models were created by varying the fixed and random effects for intercept and slope. The concordance correlation coefficient (CCC) between the measured EVT and the estimated EVT was calculated to select the best-fitting model. To examine the impact on EVT, the change in EVT when each variable increased by 1 SD was calculated.

The random slope and intercept model was selected as the best-fit model. The model excluding the length (fixed effect of intercept, p=0.27) was adopted as the final model (CCC: 0.955). As mass and flexion angle increased, both intercept and slope increased. The mean EVT was 76.2 Nm, and the changes in EVT for a 1 SD increase in each factor were 14.0 Nm (EVAA), 12.1 Nm (mass), and 8.9 Nm (flexion angle), respectively.

It was confirmed that the greater EVAA, the higher EVT in within individual analysis. In addition to EVAA, in between individual analysis, the mass and the elbow flexion angle contributed to the increased EVT. 

Greater EVAA, mass, and flexion angle are prone to have higher EVT, which leads to greater cumulative load and have higher risk of elbow injury. Although players with greater mass, regardless of throwing motion, are potentially at higher risk for elbow injury due to increased EVT, the mass should not be decrease. EVAA and flexion angle are modifiable factors in the throwing motion, and their improvement is important for injury prevention.