Effects of continuous cycling training on motor unit firing rates, input excitation, and myosin heavy chain of the vastus lateralis in sedentary females

Michael A Trevino; Hannah L Dimmick; Mandy E Parra; Adam J Sterczala; Jonathan D Miller; Jake A Deckert; Philip M Gallagher; Andrew C Fry; Joseph P Weir; Trent J Herda

doi:10.1007/s00221-021-06278-3

Effects of continuous cycling training on motor unit firing rates, input excitation, and myosin heavy chain of the vastus lateralis in sedentary females

Exp Brain Res. 2022 Mar;240(3):825-839. doi: 10.1007/s00221-021-06278-3. Epub 2022 Jan 20.

Authors

Affiliations

¹ Applied Neuromuscular Physiology Laboratory, Department of Health and Human Performance, Oklahoma State University, Stillwater, OK, 74078, USA.
² Faculty of Kinesiology, University of Calgary, Calgary, AB, T2N 1N4, Canada.
³ School of Exercise and Sport Science, Mayborn College of Health Sciences, University of Mary Hardin-Baylor, Belton, TX, 76513, USA.
⁴ Neuromuscular Research Laboratory, Department of Sports Medicine and Nutrition, University of Pittsburg, Pittsburgh, PA, 15203, USA.
⁵ Department of Chemical and Petroleum Engineering, The University of Kansas, Lawrence, KS, 66045, USA.
⁶ Gonzaga University, Spokane, WA, 99258, USA.
⁷ Applied Physiology Laboratory, Department of Health, Sport, and Exercise Sciences, University of Kansas, Lawrence, KS, 66045, USA.
⁸ Neuromechanics Laboratory, Department of Health, Sport and Exercise Sciences, University of Kansas, 1301 Sunnyside Ave., Room 101BE, Lawrence, KS, 66045, USA.
⁹ Neuromechanics Laboratory, Department of Health, Sport and Exercise Sciences, University of Kansas, 1301 Sunnyside Ave., Room 101BE, Lawrence, KS, 66045, USA. t.herda@ku.edu.

PMID: 35048160
DOI: 10.1007/s00221-021-06278-3

Abstract

This study examined the effects of continuous endurance training on motor unit (MU) mean firing rates (MFR), percent myosin heavy chain (%MHC) isoforms, and muscle cross-sectional area (mCSA) of the vastus lateralis (VL). Twelve females completed 5-weeks of continuous cycling-training (CYC), while 8 females were controls (CON). Participants performed maximal voluntary contractions (MVCs) and 40% MVCs of the knee extensors before (PRE) and after the 5-week treatment period at the same absolute pre-treatment submaximal torque (POST_ABS) and relative to post-treatment MVCs (POST_REL). Surface electromyographic (EMG) signals were decomposed with the Precision Decomposition III algorithm. MU firing times and waveforms were validated with reconstruct-and-test and spike trigger average procedures. MFRs at steady torque, recruitment thresholds (RT), and normalized EMG amplitude (N-EMG_RMS) were analyzed. Y-intercepts and slopes were calculated for the MFR vs. RT relationships. MHC isoforms and mCSA were determined with muscle biopsies and ultrasonography. CYC decreased MVCs and type IIX %MHC isoform without changes in mCSA. The slopes for the MFR vs. RT relationships decreased for CYC during POST_REL and POST_ABS while N-EMG_RMS increased for POST_ABS with no differences between PRE and POST_REL. Type I %MHC isoform was correlated with the slope for the MFR vs. RT relationship during POST_ABS and POST_REL for CYC. This study provides evidence that decreases in the MFRs of higher threshold MUs post-CYC is likely a function of changes in input excitation (POST_ABS) and the firing frequency-excitation relationships (POST_REL). Evidence is provided that MHC isoforms influence the firing rate scheme of the muscle following short-term training.

Keywords: Endurance training; Motor unit; Myosin heavy chain; Vastus lateralis.

MeSH terms

Action Potentials / physiology
Electromyography
Female
Humans
Isometric Contraction / physiology
Muscle, Skeletal / physiology
Myosin Heavy Chains*
Quadriceps Muscle* / diagnostic imaging
Quadriceps Muscle* / physiology
Recruitment, Neurophysiological / physiology
Torque

Substances

Myosin Heavy Chains

Grants and funding

FND0074499/National Strength and Conditioning Association Foundation