Motor unit synchronization and the power spectrum of the electromyogram

Spectral analysis of the electromyogram, i.e., computation of its power spectrum, has proved useful in studies of muscle fatigue and in studies of the relative contribution of firing rate modulation to force production by muscles (Weytjens & Van Steenberghe, 1984a & b). The models on which the interpretation has been based either do not include realistic assumptions as to motor unit synchronization (Lago & Jones, 1977), or do not make use of the classical definition of the power spectrum of a stationary stochastic process as the Fourier transform of the autocorrelation function (Blinowska et ai, 1980). This paper presents a realistic extension of the Lago and Jones model and then lists equations for the power spectra and the cross-power spectra of two synchronized motor unit action potential trains (Muapts) derived using their relations with the auto- and cross-correlation, respectively (Weytjens & Van Steenberghe, 1984c), from which the effects of motor unit synchronization can easily be understood.