A Methodological Framework to Capture Neuromuscular Fatigue Mechanisms Under Stress

Abstract

Neuromuscular fatigue is exacerbated under stress and is characterized by shorter endurance time, greater perceived effort, lower force steadiness, and higher electromyographic activity. However, the underlying mechanisms of fatigue under stress are not well-understood. This review investigated existing methods of identifying central mechanisms of neuromuscular fatigue and the potential mechanisms of the influence of stress on neuromuscular fatigue. We found that the influence of stress on the activity of the prefrontal cortex, which are also involved in exercise regulation, may contribute to exacerbated fatigue under stress. We also found that the traditional methods involve the synchronized use of transcranial magnetic stimulation, peripheral nerve stimulation, and electromyography to identify the contribution of supraspinal fatigue, through measures such as voluntary activation, motor evoked potential, and silent period. However, these popular techniques are unable to provide information about neural alterations upstream of the descending drive that may contribute to supraspinal fatigue development. To address this gap, we propose that functional brain imaging techniques, which provide insights on activation and information flow between brain regions, need to be combined with the traditional measures of measuring central fatigue to fully understand the mechanisms behind the influence of stress on fatigue.

Keywords: EMG; TMS; central fatigue; cognition; fNIRS (functional near infrared spectroscopy); fatigue; neuroimaging.

Figure 1

Mechanisms of neuromuscular fatigue and methods identifying these mechanisms. (A) Different mechanisms of fatigue acting along the neuromuscular pathway and location of bio instruments used for detecting mechanisms of fatigue involved. Activation and functional connectivity of the superficial cortical regions involved in neuromuscular performance are measured using fNIRS. (B) EMG and force profile of muscle during stimulation via TMS or peripheral nerve stimulation. (C) Brain imaging using fNIRS. Hemodynamic activity in a region is recorded by emitting infrared light of a given intensity in the region and detecting the intensity of light emitted back. The change in intensity is converted to concentration of oxygenated/deoxygenated blood using the Beer–lambert law.

Figure 2

Rhee and Mehta (2018), functional connectivity maps for males and females during submaximal (30% MVC) intermittent handgrip fatiguing protocol. The baseline period involves no physical exercise, the early period is from the early stages of exercise before the onset of fatigue and the late period is after the onset of fatigue during exercise. Color of each node depicts the strength of connectivity between regions. Nodes with solid lines indicate intra-hemispheric connectivity, and nodes with dotted lines indicate inter-hemispheric connectivity. Middle column shows the nodes where connectivity was significantly different between the sexes. The connectivity maps show that the strength of functional connectivity decreases with fatigue.