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The purpose of the study was to determine the effect of motor-unit synchronization on the surface electromyogram (EMG) and isometric force using a computer model of muscle contraction. The EMG and force were simulated by generating muscle fiber action potentials, defining motor-unit mechanical characteristics and territories, estimating motor-unit action potentials, specifying motor-unit discharge times, and imposing various levels of motor-unit synchronization. The output (EMG and force) was simulated at 11 levels of excitation, ranging from 5 to 100% of maximum. To synchronize motor-unit activity, selected motor-unit discharge times were adjusted; however, the number of motor units recruited and the average discharge rate of each unit was constant across synchronization conditions for a given level of excitation. Two levels of synchronization were imposed on the discharge times: a moderate and a high level, which approximated the experimentally observed range of motor-unit synchronization. The moderate level of synchrony caused the average EMG to increase by approximately 65%, whereas the high level caused a 130% increase in the EMG with respect to the no-synchrony condition. Neither synchrony condition influenced the magnitude of the average force. However, motor-unit synchronization did increase the amplitude of the fluctuations in the simulated force, especially at intermediate levels of excitation. In conclusion, motor-unit synchronization increased the amplitude of the average rectified EMG and decreased the steadiness of the force exerted by the muscle in simulated contractions.
Yao et al. (Sat,) studied this question.