This study was performed to examine the effects of movement velocity and maximal concentric and eccentric actions on the bilateral deficit. Eighteen female participants performed maximal unilateral and bilateral knee extensions concentrically and eccentrically across six movement velocities (30, 60, 90, 120, 150, and 180[degrees]/s). Repeated measures analyses of variance revealed significant differences (p less than 0.025) between bilateral and summed unilateral contractions both concentrically and eccentrically at each velocity tested. Post hoc analyses revealed that the degree of bilateral deficit increased as movement velocity increased for concentric actions (e.g., 17-33% deficit, for 30 and 180 [degrees]/s, respectively), and an increasing trend was seen for eccentric actions (e.g., 18-25% deficit, for 30 and 180 [degrees]/s, respectively). These findings suggest that with increased velocity, a decreased or incomplete activation of fast twitch muscle fibers may have occurred in bilateral actions when compared to unilateral actions. (Contains 4 figures.)