Sex‐related differences in motor unit behavior are influenced by myosin heavy chain during high‐ but not moderate‐intensity contractions

Abstract

AimsMotor unit recruitment and firing rate patterns of the vastus lateralis (VL) have not been compared between sexes during moderate‐ and high‐intensity contraction intensities. Additionally, the influence of fiber composition on potential sex‐related differences remains unquantified.MethodsEleven males and 11 females performed 40% and 70% maximal voluntary contractions (MVCs). Surface electromyographic (EMG) signals recorded from the VL were decomposed. Recruitment thresholds (RTs), MU action potential amplitudes (MUAPAMP), initial firing rates (IFRs), mean firing rates (MFRs), and normalized EMG amplitude (N‐EMGRMS) at steady torque were analyzed. Y‐intercepts and slopes were calculated for MUAPAMP, IFR, and MFR versus RT relationships. Type I myosin heavy chain isoform (MHC) was determined with muscle biopsies.ResultsThere were no sex‐related differences in MU characteristics at 40% MVC. At 70% MVC, males exhibited greater slopes (p = 0.002) for the MUAPAMP, whereas females displayed greater slopes (p = 0.001–0.007) for the IFR and MFR versus RT relationships. N‐EMGRMS at 70% MVC was greater for females (p < 0.001). Type I %MHC was greater for females (p = 0.006), and was correlated (p = 0.018–0.031) with the slopes for the MUAPAMP, IFR, and MFR versus RT relationships at 70% MVC (r = −0.599–0.585).ConclusionBoth sexes exhibited an inverse relationship between MU firing rates and recruitment thresholds. However, the sex‐related differences in MU recruitment and firing rate patterns and N‐EMGRMS at 70% MVC were likely due to greater type I% MHC and smaller twitch forces of the higher threshold MUs for the females. Evidence is provided that muscle fiber composition may explain divergent MU behavior between sexes.

Characterising sex‐related differences in lower‐ and higher‐threshold motor unit behaviour through high‐density surface electromyography

“…According to these findings, the present study underscores that female motor units manifest distinct firing strategies and similar neural drive considering motor unit size during low‐intensity contractions, a pattern that inversely aligns with the findings at higher intensities, suggesting a greater neural drive among male HTMUs in biceps brachii. As reported previously (Olmos et al., 2023 ), the differences in fibre composition may affect motor unit properties since myosin heavy chain expression in VL muscle is different comparing males and females (Haizlip et al., 2015 ).…”

“…Recent evidence has shed light on neuromuscular system complexity by studying motor units (MUs), representing the functional units and the actual link between the neural and the musculoskeletal systems (Del Vecchio, Negro, et al., 2019 ; Germer et al., 2021 ; Holobar et al., 2009 ; Nuccio et al., 2021 ). However, a dimension of their complexity that has gained prominence only recently relates to sex‐dependent differences (Guo, Jones, et al., 2022 ; Handelsman et al., 2018 ; Harrison et al., 2023 ; Inglis & Gabriel, 2020 ; Jenz et al., 2023 ; Lulic‐Kuryllo & Inglis, 2022 ; Olmos et al., 2023 ).…”

“…The recognition of distinct motor control strategies in individuals of different sexes has brought forth a new era of inquiry in the fields of physiology, kinesiology and sports science (Jo et al., 2022 ; Mcdougall et al., 2023 ; Piasecki et al., 2021 ; Taylor et al., 2022 ). While numerous studies have documented disparities in skeletal muscle mass, strength and endurance capacities between males and females, the underlying neuromuscular mechanisms governing these differences remain a subject of active investigation (Lulic‐Kuryllo & Inglis, 2022 ; Olmos et al., 2023 ). Previous evidence highlighted differences in surface electromyography (sEMG) parameters, reporting greater root mean square values in male participants (Pincivero et al., 2000 ) accompanied by a lower firing variability (Ahamed et al., 2015 ).…”

“…Previous evidence highlighted differences in surface electromyography (sEMG) parameters, reporting greater root mean square values in male participants (Pincivero et al., 2000 ) accompanied by a lower firing variability (Ahamed et al., 2015 ). To understand the physiological mechanisms explaining these distinctions, researchers have redirected their attention towards investigating the inherent differences in motor unit properties (Guo, Jones, et al., 2022 ; Jenz et al., 2023 ; Olmos et al., 2023 ; Piasecki et al., 2021 ; Taylor et al., 2022 ), including recruitment thresholds, firing rates and contractile characteristics, between males and females. Nevertheless, a definitive determination of whether reported disparities can be attributed to motoneuronal and musculoskeletal factors rather than variations in hormonal fluctuations remains elusive (Handelsman et al., 2018 ; Lulic‐Kuryllo & Inglis, 2022 ; Piasecki et al., 2023 , 2024 ; Smith & Woolley, 2004 ; Velders & Diel, 2013 ; Wierman, 2007 ).…”

Muscle physiology and its relations to the whole body in health and disease

“…Another moderate‐ and high‐intensity contraction study supports the idea of sex‐related differences in neuromuscular patterns. They observed divergent motor unit behavior between sexes during high‐intensity contraction 20 …”