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 (CV) 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 CV for force decreased from 4.9 to 1.4% as force increased from 2 to 15% MVC ( P < 0.01) and 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 no significant difference 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.

Treatment-related side effects, lack of time and fatigue were key barriers to exercise for survivors of varied cancer types. Insufficient patient education may contribute to the belief that exercise is not helpful when experiencing side effects of treatment, including fatigue. Identifying barriers and facilitators leads to improved support and education from health professionals which is required to provide safe and effective exercise recommendations for survivors.

The discharge of single motor units (n = 34) in the first dorsal interosseus muscle and the fluctuations in force during steady contractions were measured across a range of index finger abduction forces in old adults (77.1 +/- 6.9 yr, n = 20). These results were compared with previously reported data on 38 motor units from young adults (25.7 +/- 5.7 yr). Both minimal and peak discharge rates increased with recruitment threshold, but the strength of these relations was notably weaker for the old adults. Minimal discharge rates were similar for young and old adults (P = 0.77), whereas peak discharge rates were lower for old adults (P < 0.01). Consequently, the range of rate coding for each motor unit was substantially less for the old adults (7.1 pps) compared with the young adults (12.1 pps, P < 0.01). However, the variability in motor-unit discharge was similar for young and old adults; the coefficient of variation of the interspike intervals was similar at recruitment (old: 25.4%, young: 27.1%, P = 0.39) and declined with an increase in discharge rate (old: 13.2%, young: 14.2%, P = 0.21). Furthermore, the fluctuations in force during steady isometric contractions (2-95% of maximal force) were similar for young and old adults, except that the relative variability at the lowest force was greater for the old adults. A computational model of motor-unit recruitment and rate coding incorporated the experimental observations and was able to match the measured and simulated values for force steadiness across the operating range of the muscle.

This brief review summarizes progress that has been made in the study of muscle fatigue since a review published 15 years ago (Enoka RM, Stuart DG. 1992. Neurobiology of muscle fatigue. J Appl Physiol 72:1631-48.). The present review first discusses progress on the four themes identified in the 1992 review and then describes a new approach that can be used to identify the functionally significant physiological adjustments that occur during fatiguing contractions. As described in the previous review, it is currently not possible to develop a comprehensive model of muscle fatigue because the prevailing mechanism that impairs performance varies with the characteristics of the task that is being performed. An alternative approach is to focus on the mechanisms that cause failure to complete the task. This task-failure approach involves comparing two performances and identifying the adjustments that limit the rate for the more difficult condition. With this approach, initial studies have demonstrated that the time to failure of a sustained contraction can be influenced by such variables as the type of load supported by the limb, the posture of the limb, and the group of muscles involved in the task. The challenge is to identify the mechanisms that enable these different variables influence the time to task failure.