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1 Kinesiology and Applied Physiology, University of Colorado, Boulder, CO, USA
* To whom correspondence should be addressed. E-mail: roger.enoka{at}colorado.edu.
To identify the mechanisms responsible for the fluctuations in force that occur during voluntary contractions, experimental measurements were compared with simulated forces in the time and frequency domains at contraction intensities that ranged from 2 to 98% of the maximum voluntary contraction (MVC). The abduction force exerted by the index finger due to an isometric contraction of the first dorsal interosseus muscle was measured in ten young adults. Force was simulated with computer models of motor-unit recruitment and rate coding for a population of 120 motor units. The models varied recruitment and rate-coding properties of the motor units and the activation pattern of the motor-unit population. The main finding was that the experimental observations of a minimum in the coefficient of variation for force (1.7%) at approximately 30% MVC and a plateau at higher forces could not be replicated by any of the models. Nonetheless, the short-term synchrony model provided the closest fit to the experimentally observed relation between the coefficient of variation for force and the mean force. In addition, the results for the synchronization model extended previous modeling efforts to show that the effect of synchronization is independent from that of discharge rate variability. All of the models contained a significant amount of power below 5 Hz in the force spectra. Only a model that included a low-frequency oscillation in excitation, however, could approximate the experimental finding of peak power at a frequency below 2 Hz; 38% of total power at 0.99 Hz and 43% at 1.37 Hz, respectively. In contrast to the experimental power spectra, all model spectra included a second peak at a higher frequency. The secondary peak was less prominent in the synchronization model due to greater variability in discharge rate. These results indicate that the variation in force fluctuations across the operating range of the muscle cannot be explained by a single mechanism that influences the activation of the motor-unit population.
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