Abstract
Compared to other muscle groups (e.g., elbow flexors), few studies (two) have investigated the role of sex on muscle fatigue of the elbow extensors (EE). The greater fatigability usually seen for males compared to females was not observed, so the EE warrant further study. PURPOSE: To investigate the effect of sex on peripheral, motoneuronal, and cortical excitability as well as central fatigue with a submaximal EE task. METHODS: To date, 13 participants (7 females) have performed a 15min sustained isometric EE contraction at the level of electromyographic activity (EMG) recorded at 15% of maximal torque, followed by recovery contractions over 5min. Pre- and post-fatigue as well as at the end of each minute, evoked potentials were recorded from triceps brachii in response to transcranial magnetic stimulation of the motor cortex (TMS; motor evoked potentials, MEPs), cervicomedullary stimulation (cervicomedullary motor evoked potentials, CMEPs) and brachial plexus stimulation (maximal M-wave; Mmax). MEPs and CMEPs were elicited 100ms after a conditioning TMS pulse. To assess central fatigue, voluntary activation (VA) was calculated pre- and post-fatigue using superimposed and resting tetani evoked via trains of 5 stimuli (100Hz) delivered over triceps brachii. RESULTS: During fatigue, Mmax area did not change. The CMEP (normalized to Mmax) was reduced to 46.9±15.6% and 50.1±36.9% of the control ratio in females and males, respectively. The MEP (normalized to CMEP) was facilitated to 153.8±95.8% in females and 260.9±195.1% in males. At 5min of recovery, the normalized CMEP remained depressed in females and males (57.1±31.8% and 47.6±34.0%, respectively), whereas the normalized MEP remained facilitated at 220.1±106.4% in females and 184.5±101.3% in males. VA decreased from 97.0±1.8% to 84.3±18.0% in females and 95.5±3.4% to 76.7±21.0% in males at task termination, and showed incomplete recovery at 5min (89.0±6.9% females and 84.6±13.0% males). CONCLUSIONS: Apart from a trend toward greater central fatigue in males, the preliminary results support published findings, which show negligible sex differences with isometric fatigue of the EE. Additional research is needed to ascertain why the EE differ from other muscle groups in this regard. Supported by NSERC, CFI, and BCKDF
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