A motor unit-based model of muscle fatigue

Abstract

Muscle fatigue is a temporary decline in the force and power capacity of skeletal muscle resulting from muscle activity. Because control of muscle is realized at the level of the motor unit (MU), it seems important to consider the physiological properties of motor units when attempting to understand and predict muscle fatigue. Therefore, we developed a phenomenological model of motor unit fatigue as a tractable means to predict muscle fatigue for a variety of tasks and to illustrate the individual contractile responses of MUs whose collective action determines the trajectory of changes in muscle force capacity during prolonged activity. An existing MU population model was used to simulate MU firing rates and isometric muscle forces and, to that model, we added fatigue-related changes in MU force, contraction time, and firing rate associated with sustained voluntary contractions. The model accurately estimated endurance times for sustained isometric contractions across a wide range of target levels. In addition, simulations were run for situations that have little experimental precedent to demonstrate the potential utility of the model to predict motor unit fatigue for more complicated, real-world applications. Moreover, the model provided insight into the complex orchestration of MU force contributions during fatigue, that would be unattainable with current experimental approaches.

Altered anabolic signalling and reduced stimulation of myofibrillar protein synthesis after feeding and resistance exercise in people with obesity

“…Based on this, we provided (or what we considered to be) a potent resistance exercise prescription to maximize fed‐state myofibrillar protein synthesis rates. Specifically, the volunteers performed resistance exercise at four sets × 10–12 repetitions, with each set performed to volitional failure, which generally serves as a proxy indicator for a strong stimulus being provided to maximize muscle fibre recruitment patterns in both the NW and OB groups (Potvin & Fuglevand, ). Moreover, both the NW and OB groups performed similar amounts of external total work [repetitions × load (kg)] in both absolute terms and when expressed relative to maximal strength (Table ), which resulted in similar blood lactate concentrations immediately after resistance exercise.…”

Effort and duration matched ‘High Intensity Interval Training’ using cycle ergometry compared to leg press resistance training

“…Both Steele et al (2018) and Kuznetsov et al (2011) have reported findings suggesting that motor unit recruitment may be similar between modalities and indeed effort has been argued to arise from the central motor command required to drive the musculature to perform the task being attempted (Marcora, 2009;Pageaux et al, 2016). Thus, it is thought that effort, both that required and perceived, is likely intrinsically linked to motor command and motor unit recruitment (de Morree et al, 2012;Guo et al, 2017) and possibly due to Henneman's size principle (Potvin and Fuglevand, 2017).…”

Effects of fatiguing, submaximal high- versus low-torque isometric exercise on motor unit recruitment and firing behavior

“…Recently, a model was developed by Potvin and Fuglevand (2017a) that predicted that all available MUs would be recruited during low-force isometric contractions to fatigue. These investigators applied this model to compare MU behavior during simulated fatiguing low-(i.e., 20% MVIC) versus high-force contractions (i.e., 80% MVIC), and predicted that low-force contractions sustained to volitional fatigue would result in the eventual recruitment of the entire MU pool (Potvin and Fuglevand 2017b). However, it has been previously shown that, although muscle activation (i.e., EMG amplitude) increases throughout fatiguing submaximal isometric exercise, it may not reach maximal levels (Petrofsky et al 1982;Fuglevand et al 1993).…”