Dynamics of human ankle stiffness: variation with mean ankle torque

The left foot of five human subjects was rotated in a fixed stochastic pattern about a constant ankle angle and the forces opposing these perturbations were measured. The dynamic stiffness transfer functions relating ankle angular position to ankle torque were calculated. Stiffness gain was flat at low frequencies, had a resonant valley at intermediate frequencies and rose at about 40 dB/decade at high frequencies. The low frequency gain and resonant frequency increased progressively with increases in tonic muscular activity. The dynamic stiffness of the ankle was well described by a linear, under-damped, second-order transfer function having inertial, viscous and elastic terms. Estimates of the inertial parameter were independent of the level of muscle activity whereas the viscous and elastic parameters increased with increases in mean torque level.