Fig. 7. Simulation results of walking as walking speed increases from 60 to 120% PTS for the soleus (SOL), medial gastrocnemius (GAS) and tibialis anterior (TA). (A) Muscle activation as a percent of maximum (Activation) and musculotendon force (MT force), and (B) normalized muscle fiber length and velocity. The fiber length was normalized to its optimal length and the velocity was normalized to its maximum contraction velocity, which was estimated as ten times the muscle fiber resting length per second (Zajac, 1989). Negative velocity values indicate muscle shortening. The gray filled regions indicate general area of muscle activity.