| Background and purpose:Exercise-induced fatigue refers to a series of complex physiological changes in the motor system(muscles,bones,ligaments and joints)caused by prolonged muscular exercise.It can be divided into central fatigue and peripheral fatigue according to the occurrence position.Among them,central fatigue is a reduction in the ability of muscles to activate autonomously as a result of muscular exercise.It can be further differentiated into spinal fatigue and supraspinal fatigue.As a complex multifactorial mechanism controlled by the central nervous system(CNS),a decline in the level of muscle autonomic activation is often considered a fundamental feature of exercise-induced fatigue.Peripheral or muscle fatigue is the result of the altered generation or conduction of action potentials in muscle fibers.Although exerciseinduced fatigue can cause peripheral neuromuscular changes,its effects on spinal fatigue and supraspinal fatigue are unclear due to differences in exercise patterns(general muscular exercise and local muscular exercise).Especially,the underlying mechanism of the characteristic changes in the cerebral cortex is still insufficiently understood based on previous findings,which not only affects the in-depth understanding of the characteristic changes in "brain-muscle" control after exerciseinduced fatigue,but also fails to highlight the regulatory role of the brain in the state of fatigue.However,if increasing cortical excitability and strengthening functional connectivity of the brain network before exercise can help to delay fatigue and maintain a good exercise state?As a "neuromodulation technique",transcranial direct current stimulation(tDCS)is expected to play a positive role in delaying fatigue.Previous relevant studies have not explored the mechanism of periodic intervention and tDCS on different exercise patterns.Therefore,the objectives of this study were:1)to compare the characteristic changes of exercise-induced fatigue on the central nervous system and peripheral neuromuscular system in different exercise patterns;2)to further investigate the potential mechanism of transcranial direct current stimulation in exercise-induced fatigue and clarify its effects.Methods:In Study Ⅰ,twenty-four male participants were recruited to randomly perform a running fatigue test(general muscular exercise)at personalized constant velocity(corresponding to the first ventilation threshold)and an isokinetic maximum plantar dorsiflexion(local muscular exercise,60 degrees per second)separated by at least five days.The important experimental instruments and parameters used before and after exercise-induced fatigue included:the torques of maximal voluntary isometric contraction(MVIC)of the plantar flexors and dorsiflexors were collected by CONTREX isokinetic muscle strength testing system;the tibial nerve evoked potential instrument was used to obtain M-wave and H-reflex under resting condition,and Mwave during maximum contraction of plantar flexors,respectively;resting-state electroencephalogram(EEG);surface electromyography(sEMG)and EEG were recorded simultaneously during the task of isokinetic maximum plantar dorsiflexion(used to calculate the cortico-muscular coherence,CMC);the changes of root mean square(RMS)amplitude of sEMG the task of isokinetic maximum plantar dorsiflexion;blood lactic acid;and the heart rate(HR)and Borg rating of perceived exertion(RPE)were monitored in real-time during exercise.Two-way repeatedmeasures analysis of variance(ANOVA)models was used primarily to analyze the effects of fatigue induction mode on central and peripheral characteristic changes in this section.In Study Ⅱ,after five consecutive days of the high-definition tDCS(HD-tDCS)(intensity:2 mA;duration:20 min;target:Cz),the participants took part in personalized constant speed running exercise(anodal tDCS:12,sham tDCS:12,a total of 24;running time was the same as that in study 1)or the isokinetic maximum plantar dorsiflexion(a-tDCS:10,s-tDCS:10,a total of 20;the time was the same as that in the first time-note,because the participants were re-recruited,a fatigue induction was required before the intervention as a baseline of exercise intensity).Similar to Study I,muscle mechanical properties,neuromuscular properties,cerebral cortex properties,α motor neuron conduction properties,blood lactic acid were also evaluated before and after exercise-induced fatigue in Study Ⅱ,and HR and RPE were also evaluated during exercise.Two-way repeated-measures analysis of variance(ANOVA)models was used to analyze the independent variables(2 stimulation protocols × 2 times)before and after exercise.Results:Study Ⅰ:1.In terms of characteristic changes of peripheral fatigue,1)the dorsiflexion torque significantly decreased in both exercise patterns(running fatigue:p<0.001;plantar dorsiflexion fatigue:p<0.001),and the isokinetic maximal plantar dorsiflexion significantly decreased after fatigue than that of running fatigue(p<0.001);2)the RMS amplitude of plantar flexors decreased after plantar dorsiflexion fatigue,while the RMS amplitude of tibialis anterior decreased after running fatigue;3)blood lactic acid significantly increased after both exercise patterns(running fatigue:p<0.001;plantar dorsiflexion fatigue:p<0.001),and the increase was more obvious after running fatigue(p<0.001);4)HR and RPE gradually increased with the occurrence and development of fatigue,and most of the pairwise differences were significant(p<0.001);5)the HR during running fatigue was significantly higher than that during isokinetic plantar dorsiflexion fatigue in the early,middle and late stages(p<0.001);6)the RPE of isokinetic plantar dorsiflexion fatigue was significantly higher than that of running fatigue in the early and middle stages(p<0.001).2.In terms of "CMC",1)the coherences of soleus,gastrocnemius with FC1(p=0.009;p=0.011;p=0.002)and FCz(p=0.005;p=0.040;p=0.011)after personalized constant speed running were significantly higher than those after isokinetic plantar dorsiflexion fatigue;similarly,2)the coherences of plantar flexors(i.e.,soleus and lateral head of gastrocnemius)with Cz(p=0.001;p=0.048)after running fatigue were also higher than those after isokinetic plantar dorsiflexion fatigue;the CMC increased after running fatigue,while decreased after isokinetic plantar dorsiflexion fatigue.3.In terms of brain functional network connectivity,1)in the Beta and Gamma bands,the clustering coefficient(Beta:p=0.005;Gamma:p=0.006),global efficiency(Beta:p=0.003;Gamma:p=0.001)and local efficiency(Beta:p=0.004;Gamma:p=0.012)after running fatigue were significantly higher than those after isokinetic plantar dorsiflexion fatigue,while the path length was significantly decreased(Beta:p=0.003;Gamma:p=0.007);2)compared to those before running fatigue,the clustering coefficient(p<0.001),global efficiency(p<0.001)and local efficiency(p<0.001)of the Beta band after fatigue were significantly increased,while the path length was significantly reduced(p<0.001);2)the clustering coefficient(p=0.013),global efficiency(p=0.012)and local efficiency(p=0.018)of the Gamma band significantly decreased after fatigue compared to those before isokinetic plantar dorsiflexion fatigue,while the path length was significantly increased(p=0.022).4.In terms of the characteristic changes of the spinal segment,compared to isokinetic plantar dorsiflexion fatigue,the Hmax and Hmax/Mmax decreased and the presynaptic inhibition(PSI)level increased after running fatigue.Study Ⅱ:1.The effect of HD-tDCS intervention on running fatigue:1)Mmax and dorsiflexion torque decreased,while blood lactic acid increased in the s-tDCS group after exercise;2)compared to the s-tDCS group,the coherences of plantar flexors(e.g.,soleus and gastrocnemius)with C1(p=0.009;p=0.012;p=0.017)and FC1(p=0.016;p=0.010;p=0.005)after exercise were significantly greater in the a-tDCS group,and the coherences of the above muscles with C1(p=0.011;p=0.033;p=0.021),FC1(p=0.002;p=0.007;p=0.037)and FCz(p=0.019;p=0.017;p=0.015)in the a-tDCS group were also significantly higher than those before exercise(p<0.05);3)compared to the s-tDCS group,the clustering coefficient,global efficiency and local efficiency of the Beta band in a-tDCS group increased after the exercise,while the path length decreased;4)the values of Hmax and Hmax/Mmax in the s-tDCS group were decreased compared to those in the a-tDCS group after exercise,while the PSI level was increased.2.The effect of HD-tDCS intervention on the isokinetic maximum plantar dorsiflexion:1)Mmax(a-tDCS:p=0.002;s-tDCS:p=0.001),plantar flexion torque(a-tDCS:p<0.001;s-tDCS:p<0.001)and dorsiflexion torque(a-tDCS:p<0.001;stDCS:p<0.001)were significantly decreased after exercise in both groups,but compared to the a-tDCS group,the dorsiflexion torque of the s-tDCS group was significantly decreased after exercise(p=0.009);2)compared to the s-tDCS group,the coherence between the lateral head of gastrocnemius and FC1 after exercise was significantly greater in the a-tDCS group(p=0.003),while the coherence in the atDCS group was significantly higher after exercise than that before exercise(p=0.027);3)compared to the a-tDCS group,the clustering coefficient,global efficiency and local efficiency of the Gamma band after the exercise were significantly decreased,while the path length was significantly increased;4)both groups significantly decreased the level of voluntary activation(a-tDCS:p=0.004;s-tDCS:p<0.001)and Hmax(a-tDCS:p<0.001;s-tDCS:p<0.001)after exercise,but compared to the a-tDCS group,the level of voluntary activation in the s-tDCS group was significantly decreased after exercise(p=0.019).Conclusion:1.Exercise-induced fatigue by different exercise patterns has different effects on cortical-muscle coherence:exercise-induced fatigue produced by running exercise can improve the synchronization between contralateral cortexes and peripheral muscles in the Beta band,while exercise-induced fatigue produced by isokinetic maximum plantar dorsiflexion can weaken them.2.Exercise-induced fatigue in the Beta band can strengthen the clustering characteristics of the brain functional network and improve the connectivity between brain regions,thereby promoting local information processing and whole-brain information integration,and increasing the transmission of descending pathway information.However,the fatigue triggered the defensive strategy of the body,the level of PSI was increased and the activity of α motor neurons was decreased in the spinal segment.In addition,the information transmission of the central nervous system was blocked,and the balance of the inner environment was out of balance.As a result,the strength of the exercise unit was reduced,and the exercise task failed finally.3.The intervention of a-tDCS for five consecutive days improved the "brainmuscle" coupling and mainly focused on three brain regions(i.e.,C1,FC1 and FCZ)in running fatigue.In addition,a-tDCS can increase the local connectivity in the Beta band and the global connectivity in the Gamma band,which would help achieve the optimized allocation of resources and effective regulation of the peripheral neuromuscular;the intervention of a-tDCS for five consecutive days in plantar dorsiflexion fatigue alleviated the magnitude of the reduction in spinal segmental αmotor neuron activity.The decline in information transmission capacity at peripheral"nerve muscle" junctions was suppressed to some extent. |
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