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1 Integrative Physiology, University of Colorado, Boulder, CO, USA
* To whom correspondence should be addressed. E-mail: ctmoritz{at}u.washington.edu.
The goal of this study was to improve the ability of a motor-unit model to predict experimentally measured force variability across a wide range of forces. Motor-unit discharge characteristics were obtained from 38 motor units of the first dorsal interosseus muscle. Motor unit discharges were recorded in separate isometric contractions that ranged from 4 to 85% of the maximal voluntary contraction (MVC) force above recruitment threshold. High-threshold motor units exhibited both greater minimal and peak discharge rates compared with low-threshold units (P < 0.01). Minimal discharge rate increased from 7 to 23 pps and peak discharge rate increased from 14 to 38 pps with an increase in recruitment threshold. Relative discharge rate variability (coefficient of variation) decreased exponentially for each motor unit from an average of 30% to 13% as index finger force increased above recruitment threshold. In separate experiments, force variability was assessed at eight force levels from 2 to 95% MVC. The coefficient of variation for force decreased from 4.9 to 1.4% as force increased from 2 to 15% MVC (P < 0.01), and then remained constant at higher forces (1.2 - 1.9%; P = 0.14). When the motor-unit model was revised using these experimental findings, discharge rate variability was the critical factor that resulted in a match between simulated and experimental force variability (P = 0.22) at all force levels. These results support the hypothesis that discharge rate variability is a major determinant of the trends in isometric force variability across the working range of a muscle.
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