Attempts to optimise functional electrical stimulation of antagonistic muscles by mathematical modelling

The mathematical relationship between the kinetic data of joint motion and the functional electrical stimulation (FES) voltage of the corresponding antagonistic pair of muscles is given on the basis of a dynamic ankle joint model. The mathematical model is solved with the aid of state variables, while the resulting electrical stimulation voltage is found as a solution of the Volterra integral equation. The calculated stimulation voltage was applied to the plantar and dorsiflexors of the ankle joint of a hemiplegic patient. The measured ground reaction forces and goniograms during walking with and without electrical stimulation showed a significant improvement of the patient's gait. The problems of low saturation muscle force during FES, the need for individual determination of model parameters, nonlinearities of the system and the variability of gait are discussed.