Directional Mechanical Impedance of the Human Ankle During Standing with Active Muscles

The directional mechanical impedance of the human ankle was identified from subjects in a standing posture with varying levels of muscle activity. The impedance modeled the different torque responses to angle perturbations about different axes of rotation. This work proposed a novel impedance model that incorporated the coupling between multiple degrees of freedom of the ankle and was validated theoretically and experimentally. The reconstructed torque had an average variance accounted above 94% across twelve subjects. In addition, the impedance varied between and within trials and this variation was explained by changes in the ankle states, i.e., the ankle angle, torque, and muscle activities. These results have implications in the design of new prostheses controllers and the understanding of the human ankle function.