Relationships between throwing mechanics and shoulder anterior force in high school and professional baseball pitchers

Background: Excessive shoulder anterior force has been implicated in pathology of the rotator cuff in little league and professional baseball pitchers; in particular, anterior laxity, posterior stiffness, and glenohumeral joint impingement. Distinctly characterized motions associated with excessive shoulder anterior force remain poorly understood. Methods: High school and professional pitchers were instructed to throw fastballs while being evaluated with 3D motion capture (480 Hz). A supplementary random forest model was designed and implemented to identify the most important features for regressing to shoulder anterior force, with subsequent standardized regression coefficients to quantify directionality. Results: 130 high school pitchers (16.3 +/- 1.2 yrs; 179.9 +/- 7.7 cm; 74.5 +/- 12.0 kg) and 322 professionals (21.9 +/- 2.1 yrs; 189.7 +/- 5.7 cm; 94.8 +/- 9.5 kg) were included. Random forest models determined nearly all the variance for professional pitchers (R-2 = 0.96), and less than half for high school pitchers (R-2 = 0.41). Important predictors of shoulder anterior force in high school pitchers included: trunk flexion at maximum shoulder external rotation (MER) (X.IncMSE = 2.4, beta = -0.23, p < 0.001), shoulder external rotation at ball release (BR)(X.IncMSE = 1.7, beta = -0.34, p < 0.001), and shoulder abduction at BR (X.IncMSE = 3.1, beta = 0.17, p < 0.001). In professional pitchers, shoulder horizontal adduction at foot contact (FC) was the highest predictor (X.IncMSE = 13.9, beta = 0.50, p < 0.001), followed by shoulder external rotation at FC (X.IncMSE = 3.6, beta = 0.26, p < 0.001), and maximum elbow extension velocity (X.IncMSE = 8.5, beta = 0.19, p < 0.001). Conclusion: A random forest model successfully selected a subset of features that accounted for the majority of variance in shoulder anterior force for professional pitchers; however, less than half of the variance was accounted for in high school pitchers. Temporal and kinematic movements at the shoulder were prominent predictors of shoulder anterior force for both groups. Clinical relevance: : Our statistical model successfully identified a combination of features with the ability to adequately explain the majority of variance in anterior shoulder force among high school and professional pitchers. To minimize shoulder anterior force, high school pitchers should emphasize decreased shoulder abduction at BR, while professionals can decrease shoulder horizontal adduction at FC.