DISTRIBUTED RANDOM ELECTRICAL NEUROMUSCULAR STIMULATION - EFFECTS OF THE INTER-STIMULUS INTERVAL STATISTICS ON THE EMG SPECTRUM AND FREQUENCY PARAMETERS

An electrophysiological approach was used to study a distributed rando m electrical neuromuscular stimulation (ENMS) scheme in which a probab ility density is assigned to the inter-stimulus intervals (ISI) of the stimuli. One of the objectives of using ENMS techniques in the study of skeletal muscles is to obtain information about the electrical, phy siological, and mechanical properties of muscles in a near-physiologic al situation under a well-controlled experimental design in which prob lems related to the uncertainty of firing patterns of the central nerv ous system and physiological interference are avoided. In particular, ISI with a Gaussian density were varied in mean rate, standard deviati on (SD), and coefficient of variation. The influence of varying ISI, a nd the interaction of the ISI statistics with compound motor unit acti on potentials (CMUAP) on EMG power spectra and their frequency paramet ers, was assessed theoretically using a mathematical model which is si milar to that of EMG signal generation in the electrophysiological cas e. In order to quantify the effects of ISI statistics on the EMG spect rum, the median frequency was calculated as a function of stimulation rate using analytical expressions for various Values of the coefficien ts of a Gaussian ISI variation. The results obtained suggest that 1) t he interaction between ISI statistics and the shape of the CMUAP plays a major role in determining the EMG spectrum; 2) the median frequenci es (MF) determined from EMG spectra tend to increase with increasing m ean rates of stimulation for a given CMUAP. The rate of increase of th e MF depends on the coefficient of the ISI variation; 3) the EMG spect ra of random electrically stimulated muscle show peaks at the mean rat e of stimulation, and multiples of it, when the coefficient of variati on of ISI is small. These peaks decrease in magnitude with increasing coefficients of variation of ISI; and, 4) a variation in the ISI shoul d be introduced in the ENMS, when a reproduction of normal' EMG spectr a is needed. These results are consistent with those reported for volu ntary contraction of skeletal muscles.