Effects Of Isometric Exercise-induced Hypoalgesia

Background:Exercise can regulate and change the human body's perception of pain,which makes the Exercise-Induced Hypoalgesia(EIH),and can effectively produce analgesic effects.Isometric exercise is a common exercise pattern that can participate in pain adjustment patterns from a multidimensional perspective,producing the EIH effect.However,at present,there are few studies focusing on the neural center mechanism of EIH.Thus,this study used randomized controlled isometric exercise interventions to conduct this study and analyzed the impacts of exercise on physical pressure pain and heat pain.In addition,the study recorded scalp Electroencephalography(EEG)signals for further revealing the central mechanism of exercise analgesia.Method:Forty-eight healthy subjects were recruited and randomly divided into three groups:two exercise groups+one control group.The exercise groups used elbow flexion to exercise and were separated by Maximal Voluntary Contraction(MVC)of elbow joint.20%MVC and 40%MVC were subjected to intermittent isometric exercise training with90°elbow flexion in the sitting position.Participants all underwent three stages of Pretest-Exercise-Posttest.Before and after the training,the Pressure Pain Thresholds(PPT)at the the back of the hand and the biceps brachii of ipsilateral side of exercising elbow,Pressure Pain Rating(PPR)in the middle segment of the index finger of the contralateral side of exercising elbow and Heat Pain Thresholds(HPT)on the outside of the back of the hand and biceps brachii on the ipsilateral side of the exercise were measured.In addition,the areas of 2-5cm below the horizontal stripes of the elbow on the ipsilateral and contralateral sides of the exercise were stimulated by heat in order to record the EEG signals.Finally,One-Way ANOVA was used for the differences(Post-minus Pre-)of PPT,PPR,and HPT.And Two-Way Repeated Measure ANOVA was performed for the differences(Post-minus Pre-)of the EEG results.In the end,Spearman Correlation Analysis was used to correlate the behavioral and EEG signal results.Result:(1)In terms of PPT,there were statistical differences between three groups on the back of the hand(F(2,45)=16.002,P<0.001),and were also statistically different on the biceps brachii(F(2,45)=20.038,P<0.001).In terms of PPR,there were statistical differences between three groups(F(2,37)=4.030,P=0.026).In terms of HPT,there were statistical differences between three groups at the back of the hand(F(2,43)=13.015,P<0.001),and were also statistically different at the biceps brachii(F(2,43)=3.273,P=0.048).(2)In terms of EEG signals,the latency of N2 in the Heat Evoked Potentials(HEP)were statistically different between groups at ipsilateral side of exercising(F(2,30)=4.813,P=0.015,?~2=0.243),and the amplitude were also statistically different(F(2,30)=6.635,P=0.004,?~2=0.307).There were no statistical differences both amplitude and amplitude at the contralateral side of exercising(P>0.05).There were no statistical differences in the latency and amplitude of P2 both on the ipsilateral and contralateral groups(P>0.05).And the sum of the peak-to-peak amplitudes of N2-P2 had statistical differences at the ipsilateral sites(F(2,30)=7.938,P=0.002,?~2=0.346),whereas no statistical difference at the contralateral side(P>0.05).In the time-frequency distributions of the Event Related Potentials(ERP),there were statistical differences on the ipsilateral groups(F(2,30)=3.381,P=0.047,?~2=0.184),but no statistical difference on the contralateral groups(P>0.05).And the time-frequency distributions in the Event Related Desynchronization(ERD)were no statistical differences on the ipsilateral and contralateral sides(P>0.05).In the traceability analysis of N2 signal sources,the areas with the most obvious signal difference in the 20%MVC group(pre-and post-)were in the insular lobe(t=4.703,P=0.017)and the 40%MVC group(pre-and post-)had statistical differences in the cingulate cortex area(t=7.877,P<0.001).In the localization of P2 signal sources,with the increasing exercise intensity,the sensory cortex area of the 40%MVC group showed a statistical difference(t=4.353,P=0.047)before and after the exercise.In terms of correlation,both the behavioral PPT and HPT results had a certain degree of correlation between the sum of amplitudes of peak-to-peak of N2-P2 on the ipsilateral side of the EEG electrical signal(P<0.1).Among them,PPT on the back of the hand(?=-0.323,P=0.012),PPT on the biceps brachii(?=-0.365,P=0.005),PPR in the middle segment of the index finger(?=0.267,P=0.048)and HPT on the back of the hand(?=-0.271,P=0.034)statistically correleted with the sum of amplitudes of peak-to-peak of N2-P2.In addition,there was a significant correlation between PPR on the dorsal side of the middle of the index finger(?=0.504,P<0.001),the outside of the dorsal hand(?=-0.263,P=0.039)and the time-frequency energy of the ipsilateral side ERP.Conclusion:In healthy adults,intermittent isometric exercise could produce diffuse EIH effect on pressure pain.The analgesic effect produced by relatively high intensity was higher than the analgesic effect produced by low intensity.EIH of isometric exercise was still effective for contact heat pain.In terms of nerve center,exercise could suppress pain information,and this inhibition was particularly significant in areas of the pain network such as insular lobe,cingulate cortex,and sensory cortex.The intensity of exercise and the location of the stimulation were the main factors that affected the central inhibition of pain signals.The high-intensity exercise had a more obvious suppression of pain caused by pain stimulation near the exercise site.In addition,the behavioral performances of EIH for pressure and heat pain were related to the suppression of nerve central pain on electrical signals,indicating that exercise achieves the goal of analgesia by suppressing central pain signals.