Maximal Effort Repetitive Concentric Elbow Extensions Reduced Cortical And Motoneuronal Excitability In Females And Males

Females are typically less fatigable than males when performing isometric contractions, but it is less clear if there are sex-related differences for fatigue during dynamic tasks. Further, little is known regarding fatigue-related alterations of cortical and motoneuronal excitability in response to a bout of repetitive, concentric contractions. PURPOSE: The purpose of this study was to determine how triceps brachii cortical and motoneuronal excitability is altered by a series of isokinetic, concentric elbow extensions in females and males. METHODS: Twenty-two healthy participants (12 females: 28 ± 5y; 10 males: 28 ± 5y) performed 60 maximal effort elbow extensions at an isokinetic velocity of 40°·s-1. Peak power was assessed to characterize performance fatigability. Motor evoked potentials (MEPs) from transcranial magnetic stimulation (TMS), cervicomedullary motor evoked potentials (CMEPs) from transmastoid electrical stimulation, and maximal compound muscle action potentials (Mmax) from brachial plexus electrical stimulation were evoked during a TMS-induced silent period. To isolate cortical and motoneuronal excitability, the MEP and CMEP were normalized to CMEP and Mmax area, respectively. Peripheral excitability was evaluated using Mmax area. Neural excitability was assessed during the first six (START) and last six contractions of the fatigue task. RESULTS: Peak power (START: 40.34 ± 12.08 W) decreased by 38.0 ± 12.2% (p < 0.01) with no differences between females and males (p = 0.38). Compared to START (MEP/CMEP area: 1.10 ± 0.39, CMEP/Mmax area: 0.21 ± 0.06, Mmax area: 0.14 ± 0.03 mV·s), MEP/CMEP and CMEP/Mmax decreased by 17.2 ± 87.7% (p = 0.02) and 28.8 ± 36.0% (p < 0.01), respectively, whereas Mmaxarea was unaffected by the fatigue task (p = 0.10). None of fatigue-related alterations in neural excitability were influenced by sex (p ≥ 0.27). CONCLUSIONS: Irrespective of sex, fatigue-induced reductions in peak power during repetitive, isokinetic elbow extensions is, in part, likely owing to decreases in cortical and motoneuronal excitability.