ANNALS OF BIOMEDICAL ENGINEERING 2, 226-238 (1974) Variability of Electrically Evoked Muscle Contractions with Special Regard to Closed-Loop Controlled Orthosis A. TRNKOCZY Institute J. Stefan and Faculty of Electrical Engineering, University of Ljubljana, Yugoslavia Received February 2, 1974 One of the essential deficiencies of functional electrical stimulation (FES) as a method of rehabilitation of hemiplegic patients is the great variability of response. This paper treats quantitatively the variability of isometric responses. The param- eters chosen for the evaluation of variability were the dynamics of isometric moment and the static value of the moment during maintained stimulation. Ex- periments were performed to evaluate both surface and implanted stimulation, as both of these methods have reached the stage of clinical application. It was es- tablished that the variability is quite large, relative dispersion being u--0.35. Using a closed-loop controller of FES of muscles it was shown that such an ac- tuator 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 detect- able, though on the average less important than the technological-anatomical prop- erties of the contact between the electrodes and the nerve. Better reproducibility is expected with the introduction of improved electrode technology. INTRODUCTION Functional electrical stimulation (FES) is doubtlessly the most advanced trend in the rehabilitation of plegic and paretic patients (Vodovnik, 1971). With this method functional movements are evoked by electrical excitation of the affected extremities. It is evident that FES has a number of advantages, such as high power gain, and good therapeutic, psychological and cosmetic effects, but still some unresolved problems remain. The major ones are the rapid decline and great variability of responses. The amplitude of the resulting muscle force as a response to electrical stimulation is mostly determined by the stimulation pulse amplitude and width. But experience with FES of normal subjects as well as hemiplegic and paraplegic patients has proved that the muscle responses to con- stant FES parameters vary in time and that these variations are unpredictable. It is clear that the accuracy and dynamics which can be obtained by an orthosis 226 Copyright O 1974 by Academic Press, Inc. All rights of reproduction in any form reserved.