THE STOCHASTIC METHOD CAN DETERMINE SYNCHRONIZATION IN A MOTOR-UNIT POPULATION IN SIMULATED MUSCLE CONTRACTIONS

Abstract

Motor unit synchronization refers to temporal associations among action potentials of the active motor units in a muscle contraction. A method developed by Milner-Brown et al. (1973; 1975) has been widely cited for assessing short-term synchronization in a population of motor units. A major limitation of this method, however, is its inability to accurately distinguish synchronization from methodological artifact related to signal rectification (Yue et al., 1995). The purpose of this study was to examine the effectiveness of an alternative assessment, a Stochastic Method (Xiong et al., 2000), for quantifying synchrony in a population of motor units. Surface EMG was simulated at 4 levels of excitation: 10%, 30%, 50%, and 70% of the maximum level. It was simulated by generating muscle fiber action potentials; acquiring motor unit action potentials; and specifying motor unit discharge times. The level of synchrony among the simulated motor units was determined by adjusting the timing of the occurrence of the selected motor unit action potentials. The synchronization index (SI) was calculated by the stochastic method. To test the efficacy of the stochastic method in assessing motor unit synchronization, the SI determined by the stochastic method was compared to a recently published SI (Yao et al., 2000). The method by Yao et al. requires to dividing the entire simulation period into separated 1-ms bins and enumerating the action potentials of the active motor units in each bin. Thus, this method is highly accurate in determining synchronization in a population of motor units from simulated EMG. By comparing the SI of the stochastic method to that of Yao et al., the accuracy of the stochastic method SE can be determined. Pearson Correlation Coefficient calculated by using the indexes determined by the two different methods was 0.8149, and p < .001. This result suggests that the stochastic method is a fairly accurate method in determining synchronization in a population of motor units. Unlike the method of Yao et al., which cannot be used for synchronization measurements based on real EMG data, the stochastic method is fully applicable for synchronization measurements using real EMG data because all parameters needed for the synchronization calculation are directly measurable. This advantage enables evaluations of motor unit synchronization by the stochastic method in a variety of experimental and clinical situations, in which determining synchrony with accuracy among a motor unit population was not previously possible.