The Effects of Sex and Motoneuron Pool on Central Fatigue

AbstractPurposeIt is uncertain if sex influences central fatigue as the reduction in voluntary activation (VA) was not different between the sexes for elbow flexors (EFs) but greater in males compared to females for knee extensors. This disparity could result from the facilitatory and inhibitory effects of group III/IV muscle afferent effects on flexor vs. extensor motoneurons, respectively. The purpose of this study was to examine central fatigue and motoneuron responsiveness of EFs and elbow extensors (EEs) in males and females.MethodsTwenty-two participants (11 females) performed a 2-min isometric maximal voluntary contraction (MVC) of EFs and EEs (on separate days) followed by 2-min of recovery. Electromyographic potentials were recorded from biceps or triceps brachii in response to stimulation of the brachial plexus (Mmax), corticospinal tract (cervicomedullary motor evoked potential; CMEP), and motor cortex (motor evoked potential; MEP). Superimposed and resting doublets (for determining VA) were evoked via muscle belly stimulation of biceps or triceps brachii. Only CMEPs and superimposed doublets were recorded during fatigue.ResultsThere was no effect of sex on CMEP area for either muscle group during fatigue or recovery. During the 2-min after EE fatigue, mean normalized CMEP and MEP area were ~85% and ~141% of control, indicating inhibition and facilitation of the motoneurons and motor cortex, respectively. VA during recovery was significantly reduced in males but not females for the EFs, and unchanged in either sex for the EEs.ConclusionThe findings do not support the concept that equivocal findings regarding sex differences in central fatigue are related to augmented effects of group III/IV afferent feedback in males compared to females. Purpose It is uncertain if sex influences central fatigue as the reduction in voluntary activation (VA) was not different between the sexes for elbow flexors (EFs) but greater in males compared to females for knee extensors. This disparity could result from the facilitatory and inhibitory effects of group III/IV muscle afferent effects on flexor vs. extensor motoneurons, respectively. The purpose of this study was to examine central fatigue and motoneuron responsiveness of EFs and elbow extensors (EEs) in males and females. Methods Twenty-two participants (11 females) performed a 2-min isometric maximal voluntary contraction (MVC) of EFs and EEs (on separate days) followed by 2-min of recovery. Electromyographic potentials were recorded from biceps or triceps brachii in response to stimulation of the brachial plexus (Mmax), corticospinal tract (cervicomedullary motor evoked potential; CMEP), and motor cortex (motor evoked potential; MEP). Superimposed and resting doublets (for determining VA) were evoked via muscle belly stimulation of biceps or triceps brachii. Only CMEPs and superimposed doublets were recorded during fatigue. Results There was no effect of sex on CMEP area for either muscle group during fatigue or recovery. During the 2-min after EE fatigue, mean normalized CMEP and MEP area were ~85% and ~141% of control, indicating inhibition and facilitation of the motoneurons and motor cortex, respectively. VA during recovery was significantly reduced in males but not females for the EFs, and unchanged in either sex for the EEs. Conclusion The findings do not support the concept that equivocal findings regarding sex differences in central fatigue are related to augmented effects of group III/IV afferent feedback in males compared to females. Correspondence: Dr. Chris J. McNeil, School of Health and Exercise Sciences, Faculty of Health and Social Development, The University of British Columbia – Okanagan Campus, 1147 Research Road, Kelowna, British Columbia, Canada, V1V 1V7, E-mail: chris.mcneil@ubc.ca This study was supported by the Natural Sciences and Engineering Research Council of Canada and the Canadian Foundation for Innovation/British Columbia Knowledge Development Fund. The authors declare they have no conflict of interest. The results of this study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation and do not constitute endorsement by the American College of Sports Medicine. Accepted for Publication: 19 December 2017 © 2017 American College of Sports Medicine

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