Dynamics of human ankle stiffness: variation with displacement amplitude.

The left foot of five normal human subjects was rotated in a fixed stochastic pattern about a constant ankle angle and the torques opposing these rotations 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 to about 40 dB/decade at high frequencies. The mean ankle torque was held constant and the peak-to-peak amplitude of the displacement was varied. The low frequency gain and resonant frequency decreased progressively with increases in the peak-to-peak amplitude of the displacement. The dynamic stiffness was well described by a linear, second-order transfer function having inertial, viscous and elastic terms. Estimates of the inertial parameter were independent of the displacement amplitude but the viscous and elastic parameters decreased with increases in displacement amplitude.