Simulation of electromyograms showing interference patterns

Abstract

The model of the EMG showing an interference pattern in which motor units are discharged independently (which is typical for relatively weak contractions) gives an adequate picture of this type of activity, agreeing with the actual EMG in appearance as well as in some important parameters (the number of maxima, frequency spectrum and autocorrelation function). Some of the interference pattern parameters (mean duration of fluctuation, the possition of the first zero of the autocorrelation function, frequency of the main spectral maximum) are in simple relation with the duration of motor unit action potentials composing this interference pattern. The study of dependence of integrated electrical activity of the asynchronous model on the number of active motor units N, revealed that after a certain level of activity is reached, the model disagrees with the results of physiological experimentation. The latter shows a more rapid increase in electrical activity with increased number of motor units (force of contraction). If synchronization increasing with increased N is put into the model, it becomes closer to the actual EMG in the range of activity of a great number of motor units. Simulation has shown that changes in the mean duration of interference pattern fluctuation and the frequency spectrum, observed during synchronization in physiological experiments, are not caused by synchronization but by the fact that this synchronization is not complete in the muscle. Small shifts in the impulses of motor units discharging synchronously account for an increase in mean duration of the interference pattern fluctuation and also for the displacement of the main spectral maximum to the left.