Fastballs with greater upward vertical break ("rise") are believed to induce missed swings. However, batters may use learned associations between pitchers` throwing arm angles and ball kinematics to predict the trajectory. Even with identical kinematics, batters may respond differently when ball kinematics deviate from expectations based on arm angle. We tested a hypothesis that fastballs typical for a given arm angle are easier to hit in Major League Baseball. We analyzed ball-tracking data from 297 pitchers throwing >150 fastballs swung at during the 2023-2024 seasons. Preliminary regression analysis showed that arm angle explained 67% of the variance in spin axis, so we used the residuals to define a deviation score, representing the atypicality of spin axis for each arm angle. Multiple regression analysis tested whether the variance in Whiff% (missed swings/total swings) was explained by the deviation score and covariates in each pitcher-batter handedness matchup. All variables together explained 41%-52% of the variance in Whiff% (p < .001). The quadratic terms of the deviation score were insignificant, rejecting our hypothesis. In same-side matchups, spin axis showed a positive association with Whiff% while the deviation score showed a negative association, indicating that pitchers whose fastballs had an axis more inclined toward horizontal had lower Whiff%, but those with an axis more inclined toward horizontal for a given arm angle achieved higher Whiff%. These findings suggest that in recent games, fastballs with an axis atypically inclined toward horizontal, potentially generating greater rise, induce more missed swings than fastballs merely having greater rise.
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