Following lower extremity surgery, athletes demonstrate altered running mechanics particularly with regard to lower extremity joint moment impulses. The effect of speed and sex on joint impulses, even among healthy individuals, has not been investigated and describing these effects may facilitate more appropriate comparisons between injured and healthy athletes. PURPOSE: To determine the influence of sex and speed on hip extensor (HEXT), knee extensor (KEXT), and ankle plantarflexor (APF) moment impulses during running. METHODS: Whole body kinematics and ground reaction forces were collected for 99 NCAA Division I collegiate athletes (52 males) during treadmill running at 2.68, 3.35, and 4.47 m/s. Athletes were healthy at time of testing and had no history of lower extremity surgery. HEXT, KEXT, and APF were calculated during each stance phase and averaged across strides. Joint moment impulses for the right limb were compared between sex and speed using 2-way repeated measures ANOVAs. RESULTS: A significant sex by speed interaction (p < .01) for KEXT was observed. Females exhibited greater KEXT than males at all speeds (mean difference range, 6.8 to 15.2%). Among females, KEXT decreased significantly at each speed (-.21 ± .05 Nms/kg, -.21 ± .04 Nms/kg, -.19 ± .03 Nms/kg at 2.68, 3.35, and 4.47m/s, respectively; p ≤ .04), while there were no significant speed effects among males (p ≥ .19). No significant interactions (p ≥ .08) were present for HEXT or APF, though there were significant sex and speed main effects. Females demonstrated smaller HEXT and APF than males (mean difference, 20.2% and 14.6% for HEXT and APF, respectively, p < .01). Across sexes, HEXT increased significantly with speed (p < .01). APF at 4.47 m/s was significantly lower than all other speeds (-.39 ± .05 Nms/kg, -.39 ± .05 Nms/kg, -.38 ± .05 Nms/kg at 2.68, 3.35, and 4.47m/s, respectively; p < .01). CONCLUSIONS: Both sex and speed must be considered when evaluating the relative contribution of the hip, knee, and ankle during running. Females demonstrate greater KEXT than men at the same running speed but smaller HEXT and APF, indicating an increased reliance on the knee joint. As running speed increases, males increase demand at the hip while maintaining a constant demand at the knee, while females shift demands away from the knee and primarily toward the hip.