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The Journal of Neurophysiology Vol. 88 No. 1 July 2002, pp. 265-276
Copyright ©2002 by the American Physiological Society
Department of Kinesiology and Applied Physiology, University of Colorado, Boulder, Colorado 80309-0354
Taylor, Anna M.,
Julie W. Steege, and
Roger M. Enoka.
Motor-Unit Synchronization Alters Spike-Triggered Average
Force in Simulated Contractions. J. Neurophysiol. 88: 265-276, 2002. The purpose of the study was to quantify the
effect of motor-unit synchronization on the spike-triggered average
forces of a population of motor units. Muscle force was simulated by
defining mechanical and activation characteristics of the motor units, specifying motor neuron discharge times, and imposing various levels of
motor-unit synchronization. The model comprised 120 motor units.
Simulations were performed for motor units 5-120 to compare
the spike-triggered average responses in the presence and absence of
motor-unit synchronization with the motor-unit twitch characteristics
defined in the model. To synchronize motor-unit activity, selected
motor-unit discharge times were adjusted; this kept the number of
action potentials constant across the three levels of synchrony for
each motor unit. Because there was some overlap of motor-unit twitches
even at minimal discharge rates, the simulations indicated that
spike-triggered averaging underestimates the twitch force of all motor
units and the contraction time of motor units with contraction times
longer than 49 ms. Although motor-unit synchronization increased the
estimated twitch force and decreased the estimated contraction time of
all motor units, spike-triggered average force changed systematically
with the level of synchrony in motor units 59-120 (upper
90% of the range of twitch forces). However, the reduction in
contraction time was similar for moderate and high synchrony. In
conclusion, spike-triggered averaging appears to provide a biased
estimate of the distribution of twitch properties for a population of
motor units because twitch fusion causes an underestimation of twitch
force for slow units and motor-unit synchronization causes an
overestimation of force for fast motor units.
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