Intra-player movement pattern variability in baseball pitching has been demonstrated to affect metrics of performance and injury risk. It is currently unknown how many pitches are necessary to properly accommodate for intra-player variability for an appropriately representative analysis of a pitcher`s throwing mechanics. The purpose of this study was to establish a standard for how many fastball pitches are necessary to account for within and between-individual pitching variability for fastballs thrown by youth athletes. Motion capture data were collected for seven fastballs thrown by 17 male youth pitchers (age = 13.5 ± 1.07 years) using a 3D motion analysis system, incorporating 10-cameras. Maximum shoulder distraction force (MSDF) and maximum elbow valgus torque (MEVT) were compiled from pitching reports previously generated using Visual 3D and the Qualisys baseball module. Spearman-Brown prophecy formula was used to calculate how many pitches were required to account for within and between subject variability for throwing arm metrics analyzed. Results showed that 2, 4, and 19 fastballs are needed to accurately report MSDF and MEVT metrics with 90%, 95%, and 99% reliability, respectively. The results of this study provide the first methodological guidelines for how to account for the factor of fastball pitch variability in youth baseball biomechanics studies.
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