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The purpose of this study was to quantify the horizontal and vertical velocities of the body mass center produced by the lower limb muscles during vertical and forward jumps. Vertical and forward jumps were simulated using a model of the musculoskeletal system consisting of four rigid segments actuated by six leg muscles. It was found that most of the vertical velocity of the body mass center was produced by the soleus and gastrocnemius in both jump conditions. The horizontal velocity at take-off was larger for the forward jump, caused by a larger forward velocity produced by the hamstrings, soleus, and gastrocnemius and a smaller backward velocity produced by the vasti and rectus femoris. The force development patterns were different only in the hamstrings and rectus femoris, indicating that these bi-articular muscles may play an important role in the control of jumping direction.