This study revealed how high school pitchers generated momenta during fastballs and changeups at a whole-body level. Baseball pitchers control ground reaction forces to generate whole-body momentum. Pitchers attempt to throw as fast and accurately as possible during fastballs but also need to throw off-speed pitches like changeups to deceive batters. To understand whole-body momenta generation in fastballs and changeups, this study aimed to determine (a) the roles of each leg in impulse generation in high school pitchers, (b) if these impulses differed between fastballs and changeups, and (c) if net impulses related to ball speed. Linear mixed models revealed that the back leg generated significantly more forward linear impulse (p <​ 0.0001) and more angular impulse about the leftward (p <​ 0.0001) and upward axes (p <​ 0.0001) than the lead leg in fastballs and changeups as a group and within each pitcher. However, when comparing fastballs and changeups, there were no significant differences in net forward linear impulse (p = 0.71) or net angular impulse about the leftward axis (p = 0.42) or about the upward axis (p = 0.72) at the group-level, despite significantly greater ball speeds during fastballs (p <​ 0.0001). This study did not detect any significant group-level associations between ball speed and net forward linear impulse (p = 0.6 for fastballs, p = 0.81 for changeups) or net angular impulse about leftward axis (p = 0.52 for fastballs, p = 0.35 for changeups) or about the upward axis (p = 0.76 for fastballs and changeups) during fastballs or changeups, though some participants exhibited trends between impulses and ball speed individually.