Movement-related cortical potentials during muscle fatigue induced by upper limb submaximal isometric contractions

EEG-Based Spectral Analysis Showing Brainwave Changes Related to Modulating Progressive Fatigue During a Prolonged Intermittent Motor Task

Repeatedly performing a submaximal motor task for a prolonged period of time leads to muscle fatigue comprising a central and peripheral component, which demands a gradually increasing effort. However, the brain contribution to the enhancement of effort to cope with progressing fatigue lacks a complete understanding. The intermittent motor tasks (IMTs) closely resemble many activities of daily living (ADL), thus remaining physiologically relevant to study fatigue. The scope of this study is therefore to investigate the EEG-based brain activation patterns in healthy subjects performing IMT until self-perceived exhaustion. Fourteen participants (median age 51.5 years; age range 26−72 years; 6 males) repeated elbow flexion contractions at 40% maximum voluntary contraction by following visual cues displayed on an oscilloscope screen until subjective exhaustion. Each contraction lasted ≈5 s with a 2-s rest between trials. The force, EEG, and surface EMG (from elbow joint muscles) data were simultaneously collected. After preprocessing, we selected a subset of trials at the beginning, middle, and end of the study session representing brain activities germane to mild, moderate, and severe fatigue conditions, respectively, to compare and contrast the changes in the EEG time-frequency (TF) characteristics across the conditions. The outcome of channel- and source-level TF analyses reveals that the theta, alpha, and beta power spectral densities vary in proportion to fatigue levels in cortical motor areas. We observed a statistically significant change in the band-specific spectral power in relation to the graded fatigue from both the steady- and post-contraction EEG data. The findings would enhance our understanding on the etiology and physiology of voluntary motor-action-related fatigue and provide pointers to counteract the perception of muscle weakness and lack of motor endurance associated with ADL. The study outcome would help rationalize why certain patients experience exacerbated fatigue while carrying out mundane tasks, evaluate how clinical conditions such as neurological disorders and cancer treatment alter neural mechanisms underlying fatigue in future studies, and develop therapeutic strategies for restoring the patients' ability to participate in ADL by mitigating the central and muscle fatigue.

Brain source imaging based on movement-related cortical potentials induced by fatigue during self-paced handgrip contractions

PURPOSE

By using standard low resolution electromagnetic tomography (sLORETA), we sought to explore the changes in brain source localization when performing right handgrip contractions in the condition of muscular fatigue.

METHODS

Ten healthy adults volunteered for this study, and were asked to perform repeated and intermittent self-paced right handgrip contractions at 30% maximal voluntary contraction based on visual feedback leading to fatigue of right flexor digitorum profundus. Motor potentials from the movement-related cortical potentials were extracted from the electroencephalogram and were further analyzed by sLORETA.

RESULTS

The activated cortical regions were mainly the Brodmann area 6 on the superior frontal and medial frontal gyri, and the BA 10 on the frontal and medial frontal gyri. With the development of muscular fatigue, current density of the motor potential significantly increased and the activated cortical areas markedly enlarged.

CONCLUSION

In an attempt to maintain a target level of force during upper limb muscle fatigue induced by low intensity repetitive activation, the brain enhances the activation of sensorimotor cortex and enlarges the sensorimotor cortex area, especially in the ipsilateral hemisphere.

Perceived exertion during muscle fatigue as reflected in movement-related cortical potentials: an event-related potential study

The aim of this study was to explore the mechanism on perceived exertion during muscle fatigue. A total of 15 individuals in the fatigue group and 13 individuals in the nonfatigue group were recruited into this study, performing 200 intermittent handgrip contractions with 30% maximal voluntary contraction. The force, surface electromyography (sEMG), movement-related cortical potentials (MRCPs), and rating perception of effort (RPE) were combined to evaluate the perceived exertion during muscle fatigue. The maximal handgrip force significantly decreased (P<0.01), the root mean square of sEMG over each block significantly increased (P<0.01), and SD of force at plateau increased (P<0.01) during muscle fatigue. The RPE scores reported by the individuals and the motor potential amplitude of MRCPs in the fatigue group significantly increased (P<0.001). However, as for the individuals in the nonfatigue group, the other indexes showed no significant changes except for a little increase in the RPE. The within-subject correlation coefficients showed that there were significant correlations between RPE and motor potential amplitude of MRCPs at the C1 site (r=-0.609, P<0.001) and between RPE and root mean square of sEMG (r=0.541, P<0.001). Our results substantiate that the perceived exertion correlates with the central motor command during movement execution rather than the preparatory process. The perceived exertion not only reflects central fatigue but could also reflect the peripheral local muscle fatigue.