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

Alex A Olmos; Adam J Sterczala; Mandy E Parra; Hannah L Dimmick; Jonathan D Miller; Jake A Deckert; Stephanie A Sontag; Philip M Gallagher; Andrew C Fry; Trent J Herda; Michael A Trevino

doi:10.1111/apha.14024

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

Acta Physiol (Oxf). 2023 Sep;239(1):e14024. doi: 10.1111/apha.14024. Epub 2023 Aug 8.

Authors

Affiliations

¹ Applied Neuromuscular Physiology Lab, Department of Kinesiology, Applied Health, and Recreation, Oklahoma State University, Stillwater, Oklahoma, USA.
² Neuromuscular Research Laboratory, Department of Sports Medicine and Nutrition, University of Pittsburgh, Pittsburgh, Pennsylvania, USA.
³ School of Exercise and Sport Science, University of Mary Hardin-Baylor, Belton, Texas, USA.
⁴ Faculty of Kinesiology, University of Calgary, Calgary, Alberta, Canada.
⁵ Department of Chemical and Petroleum Engineering, University of Kansas, Lawrence, Kansas, USA.
⁶ Department of Human Physiology, Gonzaga University, Spokane, Washington, USA.
⁷ Applied Physiology Laboratory, Department of Health, Sport, and Exercise Sciences, University of Kansas, Lawrence, Kansas, USA.
⁸ Jayhawk Athletic Performance Laboratory - Wu Tsai Human Performance Alliance, University of Kansas, Lawrence, Kansas, USA.
⁹ Neuromechanics Laboratory, Department of Health, Sport, and Exercise Sciences, University of Kansas, Lawrence, Kansas, USA.

PMID: 37551144
DOI: 10.1111/apha.14024

Abstract

Aims: Motor 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.

Methods: Eleven 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 (MUAP_AMP ), initial firing rates (IFRs), mean firing rates (MFRs), and normalized EMG amplitude (N-EMG_RMS ) at steady torque were analyzed. Y-intercepts and slopes were calculated for MUAP_AMP , IFR, and MFR versus RT relationships. Type I myosin heavy chain isoform (MHC) was determined with muscle biopsies.

Results: There were no sex-related differences in MU characteristics at 40% MVC. At 70% MVC, males exhibited greater slopes (p = 0.002) for the MUAP_AMP , whereas females displayed greater slopes (p = 0.001-0.007) for the IFR and MFR versus RT relationships. N-EMG_RMS 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 MUAP_AMP , IFR, and MFR versus RT relationships at 70% MVC (r = -0.599-0.585).

Conclusion: Both 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-EMG_RMS 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.

Keywords: motor unit; myosin heavy chain; sex differences; vastus lateralis.

Publication types

Research Support, Non-U.S. Gov't

MeSH terms

Action Potentials / physiology
Electromyography
Female
Humans
Isometric Contraction / physiology
Male
Muscle Fibers, Skeletal
Muscle, Skeletal* / physiology
Myosin Heavy Chains*
Quadriceps Muscle / physiology
Recruitment, Neurophysiological / physiology

Substances

Myosin Heavy Chains