The Motor Unit Firing Rate and the Power Spectrum of EMG in Humans

The EMG power spectrum is influenced by many factors such as the conduction velocity of the muscle fiber, the action potential of the motor unit, the number of motor units firing near the electrode, and the recording conditions. Model studies of the relation between motor unit firing rate and power spectrum of EMG have produced conflicting results. To examine this relation in vivo the brachial biceps muscle was examined in 14 controls at a force of 10% of maximum. The motor unit firing intervals were obtained from 164 motor units, sampled with a single fiber electrode. The EMG was sampled at 10 sites in each muscle with a concentric electrode and the power spectrum was obtained using fast Fourier transformation. The mean power frequency of the interference pattern as well as the relative power at 1400 Hz both decreased with increasing motor unit firing intervals between subjects. The study thus indicates that the amount of high frequencies in the power spectrum is greater in a subject with a high firing rate of the motor units than in a subject with a low firing rate.