RELATION BETWEEN THE SIZE OF MOTOR UNITS AND THE SPECTRAL CHARACTERISTICS OF THEIR ACTION-POTENTIALS

Objective: The purpose of this study was to clarify the influence of t he size of an individual motor unit on the spectral characteristics of its motor unit action potentials (MUAPs). Methods: In 4 human subject s, we first averaged the isometric force and the surface EMC signals t riggered by the intramuscularly recorded action potentials of each mot or unit during voluntary isometric contraction. Then, we obtained aver aged twitch contraction curves, averaged MUAPs derived from surface EM G signals (S-MUAPs), and the power spectrum of the S-MUAPs. Finally, w e tested for correlations among these results in each subject. Results : There was a positive correlation among the mean power frequency of t he S-MUAPs, the maximal amplitude of the S-MUAPs, and the maximal twit ch tension of their motor unit in each subject (r = 0.58-0.85; P < 0.0 5). Conclusions: Since the twitch tension and the amplitude of the S-M UAPs are well-known predictors of the size of motor units, we conclude that the spectral characteristics of the S-MUAPs reflect the size of the motor unit from which they originate. (C) 1998 Elsevier Science Ir eland Ltd. All rights reserved.