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Pelvis and trunk counter-rotation are key factors in pitching, where energy or momentum is transferred from the lower extremities through to the throwing hand. Axial pelvic-trunk separation (PTS) angle, calculated by subtracting trunk position from pelvis position, is known to effect throwing arm kinematics. Perhaps altering stride length and drive leg propulsion influences axial PTS which may increase risk of throwing arm injury. Therefore, this retrospective analysis investigated stride length influences on; i) transverse PTS and ii) sequencing (trunk-to-pelvis transverse angular velocity ratio, otherwise known as the proximal plyometric effect (PPE)) during fastball pitching. Secondary analysis of 19 healthy-skilled, competitive pitchers motion capture data previously collected and post-processed was undertaken to test the a-priori hypotheses that ±25% changes from desired stride length, respective of over-stride (OS) and under-stride (US), impacted PTS angle and PPE. The experimental design was a blinded randomized crossover design where pitchers threw 2 simulated games at ±25% changes from desired stride length. Comparisons at hallmark events and phases revealed significantly different transverse pelvis and trunk kinematics between stride length conditions, influencing pelvic-trunk separation and proximal plyometrics. Peak pelvic-trunk separation near stride-foot contact mediated greater PPE ratios earlier in the pitching delivery that may regulate trunk angular velocity relative to the pelvis at ball release. Conversely, PTS concomitant with high PPE ratios near ball release may be compensatory for reduced drive leg propulsion observed during the generation phase, that could impact trunk-throwing arm momentum exchanges.