A dynamic model of the ankle joint under functional electrical stimulation in free movement and isometric conditions

Abstract

This paper describes a dynamic model of the ankle joint in which ago- and antagonistic muscle groups are under functional electrical stimulation. To the first approximation these muscles perform the dorsal and plantar flexion of the ankle. The model describes the behaviour of the joint in isometric and in free movement conditions. Considered are the delays between excitation and response, and three non-linearities, namely; the stimulating voltage threshold, the Coulomb friction in the joint, and different muscle behaviour during contraction and relaxation. The majority of model parameters were directly measured on from 3 to 10 normal subjects. The rest of parameters were determined by the brute-force method of parameter identification on one normal subject. The real system responses to different stimulating functions are compared to the respective responses of the model simulated on a digital computer. The response agreement is investigated in the frequency and time domains.