The Mechanisms On Central Electroneurophysiology During Fatigue Induced By Submaximal Voluntary Contractions Of Flexor Digitorum Muscle

ObjectiveMuscle fatigue lowers capacity of working and increases the injury rate, thus itis very important to understand the mechanisms on the fatigue. However, themechanisms on exercise-induced muscle fatigue are very complex, in which any sitefrom the cerebral cortex to muscle contraction may induce the muscle fatigue. Themechanism on the central nervous system is called central mechanism and the on theperipheral muscle is called peripheral mechanism. At present, the central mechanismwas the research highlight, but due to the limitation of technology, the centralmechanism on the muscle fatigue was not yet comprehensive. Previous researches onthe central mechanism of muscle fatigue mainly focused on analysis of blood sampleto explore the biochemistry and neurobiology, less was done onneuroelectrophysiological aspects. Surface electromyography (sEMG) andelectroencephalography (EEG), as non-invasive approaches, can reflect function ofcentral nervous system in real time. Therefore, sEMG and EEG were combined toexplore the neuroelectrophysiological mechanisms during muscle fatigue, to providethe references in exploring the human’s brain function.MethodsA total of28young volunteers participated in this study and were assigned tofatigue (15subjects) and non-fatigue (13subjects) groups.15subjects from thefatigue group performed200intermittent handgrip contractions of the right arm atapproximately30%MVC in a single session. Each contraction lasted approximate6s,followed by about4s inter-trial rest. Each subject controlled the force level of30%MVC by observing the real time visual force feedback presented on a computerscreen. The200contractions were divided into4blocks with each block involving50 trials, with a brief rest less than1min, when the rating of perception of effort (RPE)and maximal voluntary contraction (MVC) were measured.13subjects from thenon-fatigue group were also engaged in200handgrip contractions, but a longerinter-trial rest with approximate8s at the same force level of30%MVC. The EEGsignal, sEMG of flexor digitorum, force, and RPE were recorded during themovement. The readiness potential before movement and potential at movementexecution from movement-related cortical potentials (MRCPs) in sensorimotor areas,potential in prefrontal cortex, RMS of EMG,%MVC, and RPE were selected as theindexes.The first part in the experiment: to examine the changes of MRCPs, EMG, force,and perceived effort, in addition the mechanisms on the perceived effort duringmuscle fatigue.The second part in the experiment: Based on the first part, the potential at themovement execution from MRCPs was selected to observe the changes of brainsources during the muscle fatigue. In this part, the potential at the movementexecution from MRCPs of10subjects from fatigue group were selected to beanalyzed by the standard low resolution brain electromagnetic tomography(sLORETA). And the standard current density and voxels were selected asobservable indexes and statistical non-parameter maps method was used to presentthe differences of brain sources between initial blocks and late blocks during musclefatigue.ResultsThe first part：1. The output force could be maintained at30%MVC during the whole movementtask, regardless of subjects in the fatigue or non-fatigue groups. However, at thelate blocks the standard deviation of force plateau was significantly higher (P <0.001) in the fatigue group compared to the non-fatigue group. The experimentaldata showed significant decreases in both maximal voluntary contraction andRMS of sEMG and a significant increase in the average RMS of sEMG across4 blocks in the fatigue group. And the RPE scores reported by subjects in thefatigue group significantly increased (P <0.001). However, as for subjects in thenon-fatigue group, the other indexes showed no significant changes except for theincrease in RPE.2. There was no significant difference in the readiness potential (RP) between thefatigue and the non-fatigue groups at early stages, and even at late stages,significant differences were observed only at the Fp1and FC1sites. Motorpotential amplitudes were significantly higher in the fatigue group than in thenon-fatigue group irrespective of block or electrode positions. Positivewaveforms were observed in the prefrontal cortex in states without muscle fatigue,whereas a negative waveform pattern was observed with muscle fatigue.3. In term of EEG data, the MRCPs distribution seemed to shift toward anterior andipsilateral sites in brain with the development of muscle fatigue.4. Within-subjects correlation coefficients showed that there were significantlycorrelations between RPE and MP amplitude at C1site (r=-0.609, P <0.001, ηp2=0.37), between MP amplitude at C1site and RMS EMG (r=-0.439, P <0.05,ηp2=0.11), and between RPE and RMS EMG (r=0.541, P <0.001, ηp2=0.29).The second part:1. The sLORETA indicated that the activation of cortical cortex involved superiorfrontal gyrus, medial frontal gyrus, middle frontal gyrus, and inferior frontal gyrusin frontal cortex, and parietal lobe cortex and cingulate gyrus cortex during theexecution of submaximal voluntary isometric contractions in flexor digitorummuscle.2. With the development of muscle fatigue, standard current density and the numberof activated voxels in these cortex areas significantly increased.The sLORETA data demonstrated that with the development of muscle fatigue thebrain source of activation during the execution of movement shifted toward theipsilateral to movement, anterior and inferior. Conclusions1. The exercise protocol, in which subjects in the fatigue group performing200intermittent30%MVC isometric contractions by flexor digitorum musclesignificantly induced muscle fatigue but in the fatigue group the subjects showedno muscle fatigue, demonstrates the model of exercise-induced fatigue is set upsuccessfully.2. It seems that the motor potential (MP) during the movement execution maycorrelate with muscle fatigue while the readiness potential (RP) before movementmay reflect the cognitive demands during prolonged repeated exercise.3. Both MRCPs and sLORETA data indicated that with the development of musclefatigue the activation of movement-related cortex areas increases and the activatedmovement-related cortex areas enlarges. And the activated brain source during theexecution of movement presents the characteristics toward the ipsilateral, anteriorand inferior. These central changes may be the employed central strategy tocompensate for loss of force when muscle fatigued.4. Our results demonstrate the perception of effort correlates with central motorcommand during movement execution rather than movement preparation. Inaddition, our results provide additional evidence for a link between central motorcommand during movement execution and motor unit recruitment.