Fig. 1. Isolated leg swinging was modeled as a simple pendulum. Leg angle was defined relative to vertical, and torque T due to muscle force was defined as positive in the same direction as . We assumed a relatively constant moment arm for muscle force. Torque and rate of work requirements increase with the square and cube, respectively, of swing frequency f (Hz) or (rad s^-1) above the natural frequency, f_n or _n respectively.