The neurobiology of muscle fatigue: 15 years later

Abstract

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.

Mechanisms of Improved Exercise Performance under Hyperoxia

“…With higher exercise intensity, any further increase in O 2 supply to the muscles is limited; accumulation of H + and inorganic phosphates at the neuromuscular junction and in muscle cells reduce contractility and induce fatigue [3,4]. A reduced O 2 supply leads to recruitment of type II muscle fibers, which are less suitable for endurance than type I fibers and thus more prone to fatigue [5].…”

sEMG Techniques to Detect and Predict Localised Muscle Fatigue

“…Chaffin (1973) introduced the term 'localised muscle fatigue' as an example of physiological fatigue, which refers to the inability of a given muscle to maintain a desired force and is associated with localised pain. Studies on localised muscle fatigue have focused mainly on the decline in the force of a muscle contraction during a sustained activity (Barry & Enoka, 2007), which results in a definition of fatigue as the inability of a muscle to continue exerting force or power. Barry & Enoka argue that this definition indicates that fatigue occurs quickly after the onset of a sustained period of exercise, although the subject may be able to sustain the activity.…”

Assessment of Muscular Fatigue with Electromyography on Lower Back and Leg Muscles during Continuous Uphill and Downhill Load Carriage Task

“…Different mechanisms of fatigue often overlap in their definitions because of the difficulty in isolating one event 3 . Research has mainly focused on localised muscle fatigue as the process of a decline in the force during a sustained activity, which gives a definition of physiological fatigue as the inability to exert any more force or power 4 . Merletti and Parker 5 argue that muscle fatigue can be defined with an engineering approach to fatigue, where fatigue develops over time and is progressive, which defines muscle fatigue as all the physiological changes that occur in the muscle before reaching the inability to exert force.…”