Forces at the Shoulder During the Baseball Pitch in Youth and Professional Baseball Throwers

Abstract

PURPOSE: To describe and compare the forces occurring at the shoulder between youth and professional baseball players during a fastball pitch. METHODS: Twenty-five professional and 14 youth baseball pitchers were filmed at 120 Hz using two high-speed video cameras. The locations of 21 body landmarks were digitized and their three-dimensional locations were calculated using the DLT method (Abdel-Aziz & Karara, 1971). Three-dimensional position data was filtered using a fourth order Butterworth filter with a cutoff frequency of 13 Hz. From the marker coordinates, the kinetics (consisting of the three orthogonal forces Fx, Fy and Fz acting at the pitching shoulder) shoulder were calculated using a standard technique (Fleisig et al., 1996; Werner et al., 1993). All kinetic data were normalized to %BW*ht and in time to facilitate comparisons. RESULTS: Ball speed for the professionals was 38.8 m/s and for the youth was 20.7 m/s (p<0.001). Significant differences in the peak magnitudes of all three shoulder forces were noted between the groups. The Fx (anterior/posterior) directed force was 47.4 (12.2) %BW*ht for the professionals compared to 18.3 (3.3) %BW*ht for the youth (p<0.001). The Fy (superior/inferior) directed force was 43.4 (9.9) %BW*ht for professionals and 18.4 (3.8) for the youth (p<0.001). The Fz (distraction/compression) force was the dominant force at the shoulder acting to distract the joint, occurred just prior to bal release and was 131.8 (8.5) %BW*ht for professionals and 50.3 (8.3) %BW*ht for the youth (p<0.001). After accounting for ball speed (as a co-variate), significant difference remained in these forces. CONCLUSIONS: The greatest shoulder force during the pitch in both groups was the Fz-distraction force. In professionals throwers, this force has been linked to SLAP lesions while in youth we suggest it is more associated proximal humeral epiphysitis. After accounting for player body weight, height and ball velocity, significant differences remained in these forces which suggests significant differences in the pitch performance and thus the shoulder mechanics