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The Journal of Neurophysiology Vol. 83 No. 1 January 2000, pp. 441-452
Copyright ©2000 by the American Physiological Society
1Department of Kinesiology and Applied Physiology, University of Colorado, Boulder, Colorado 80309-0354; and 2Department of Physiology, University of Arizona, Tucson, Arizona 85721
Yao, Wanxiang,
Rew J. Fuglevand, and
Roger M. Enoka.
Motor-Unit Synchronization Increases EMG Amplitude and Decreases
Force Steadiness of Simulated Contractions. J. Neurophysiol. 83: 441-452, 2000. 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 ~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.
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