Variability of electrically evoked muscle contractions with special regard to closed-loop controlled orthosis

Abstract

One of the essential deficiencies of functional electrical stimulation (FES) as a method of rehabilitation of hemiplegic patients is the great variability of responses. This paper treats quantitatively the variability of isometric responses. The parameters chosen for the evaluation of variability were the dynamics of isometric moment and the static value of the moment during maintained stimulation. Experiments were performed to evaluate both surface and implanted stimulation, as both of these methods have reached the stage of clinical application. It was established that the variability is quite large, relative dispersion being ν≐0.35. Using a closed-loop controller of FES of muscles it was shown that such an actuator is too variable to be used as an efficient orthosis. No significant difference was found between surface and implanted FES. The comparison of data obtained without electrode shifting and with readjustment of the electrodes to the same premarked position has convinced us that one of the main causes for the variability is the unpredictable displacement of different tissues between the electrodes and the stimulated nerve. The influence of reflex action and supraspinal centers was studied on a patient with compression of the peroneal nerve. It was found that this influence is strongly subjective. In some hemiplegics this influence was detectable, though on the average less important than the technological-anatomical properties of the contact between the electrodes and the nerve. Better reproducibility is expected with the introduction of improved electrode technology.