Motor-unit synchronization alters spike-triggered average force in simulated contractions.
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.
Taylor
Anna M
AM
Department of Kinesiology and Applied Physiology, University of Colorado, Boulder, Colorado 80309-0354, USA.
Steege
Julie W
JW
Enoka
Roger M
RM
D000200:Action Potentials / Q000502:physiology; D003198:Computer Simulation; D006225:Hand; D006801:Humans; D008954:Models, Biological; D009046:Motor Neurons / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology*; D011930:Reaction Time