Fig. 8. Internal rotation of the tibiofibula at the starting jump position enhances the vertical component of the ground reaction force (GRF). Left column, model 3; right column, model 2; bottom panels, position of models at the start (0 ms) of the jumping simulation; top panels, position of models and orientation of the GRF (red arrow) 30 ms into the simulation. Model 3 had an extra degree of freedom about the knee compared with model 2, wherein the tibiofibula (the bone colored pink on the right side of model 3) was internally rotated about its long axis. By bringing the foot under the frog's body, this internal rotation increased the vertical component of the GRF relative to the horizontal component during the early portion of the jumping simulation. The GRF shown for both models was calculated in response to the same extensor torque pattern applied about the hip, knee and ankle joints.