Purpose: To describe school-level differences in ball velocity and spin rate in youth baseball players assessed with a portable tracking device and to examine cross-sectional associations with body size. Methods: A total of 438 players were grouped as elementary (n = 29), junior high (n = 154), or senior high (n = 255). Each performed 5 maximal-effort throws, and ball velocity (in kilometers per hour) and spin rate (in revolutions per minute) were measured with Rapsodo (mean of 5 trials). Testing differed by level (elementary: rubber ball, 16 m; junior/senior high: hardball, 18.44 m). Groups were compared using 1-way analysis of variance with Bonferroni-adjusted post hoc pairwise comparisons. Pearson correlations assessed associations between anthropometrics (height, body mass) and performance within each group. Results: Ball velocity and spin rate differed across levels (P < .001). For ball velocity, correlations with height were r = .688 (elementary), .763 (junior high), and .124 (senior high), and correlations with body mass were r = .391, .690, and .218, respectively. For spin rate, correlations with height were r = -.383, .541, and .106, and correlations with body mass were r = -.332, .446, and .087, respectively. The velocity-spin correlation differed by level, showing a negative association in the elementary group (r = -.524) but strong positive associations in the junior high (r = .776) and senior high (r = .712) groups. Conclusions: Under these field-based testing conditions, ball velocity and spin rate were higher in older school-level groups, and cross-sectional association patterns differed by level; longitudinal studies under standardized constraints are needed to characterize within-player changes over time.
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