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

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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 (POSTABS) and relative to post-treatment MVCs (POSTREL). 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-EMGRMS) 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 POSTREL and POSTABS while N-EMGRMS increased for POSTABS with no differences between PRE and POSTREL. Type I %MHC isoform was correlated with the slope for the MFR vs. RT relationship during POSTABS and POSTREL 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 (POSTABS) and the firing frequency–excitation relationships (POSTREL). Evidence is provided that MHC isoforms influence the firing rate scheme of the muscle following short-term training.

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Data availability

Data in this study are available upon request. Please contact the corresponding author for access.

Abbreviations

CON:

Control treatment group

CYC:

Cycling treatment group

MU:

Motor unit

MVC:

Maximal voluntary contraction

EMGRMS :

Electromyographic amplitude

POSTABS :

Post-treatment absolute torque level

POSTREL :

Post-treatment relative torque level

PRE:

Baseline testing

VL:

Vastus lateralis

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Acknowledgements

We would like to thank P.R. Maier and J.D. Lippman who aided in data collection and analysis, as well as each subject for their selfless participation.

Funding

This publication was made possible by a National Strength and Conditioning Association Foundation (NSCAF) Graduate Research Doctoral Grant FND0074499 and a University of Kansas Doctoral Student Research Fund (DSRF) grant.

Author information

Authors and Affiliations

  1. Applied Neuromuscular Physiology Laboratory, Department of Health and Human Performance, Oklahoma State University, Stillwater, OK, 74078, USA

    Michael A. Trevino

  2. Faculty of Kinesiology, University of Calgary, Calgary, AB, T2N 1N4, Canada

    Hannah L. Dimmick

  3. School of Exercise and Sport Science, Mayborn College of Health Sciences, University of Mary Hardin-Baylor, Belton, TX, 76513, USA

    Mandy E. Parra

  4. Neuromuscular Research Laboratory, Department of Sports Medicine and Nutrition, University of Pittsburg, Pittsburgh, PA, 15203, USA

    Adam J. Sterczala

  5. Department of Chemical and Petroleum Engineering, The University of Kansas, Lawrence, KS, 66045, USA

    Jonathan D. Miller

  6. Gonzaga University, Spokane, WA, 99258, USA

    Jake A. Deckert

  7. Applied Physiology Laboratory, Department of Health, Sport, and Exercise Sciences, University of Kansas, Lawrence, KS, 66045, USA

    Philip M. Gallagher & Andrew C. Fry

  8. Neuromechanics Laboratory, Department of Health, Sport and Exercise Sciences, University of Kansas, 1301 Sunnyside Ave., Room 101BE, Lawrence, KS, 66045, USA

    Joseph P. Weir & Trent J. Herda

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  1. Michael A. Trevino
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Contributions

MAT and TJH designed the experiments, MAT, AJS, JDM, MEW, HLD, JAD, JPW, ACF, and PMG conducted the experiments, MAT, AJS, JDM, MEW, HLD, JAD, JPW, ACF, PMG, and TJH analyzed the data. MAT wrote the manuscript. MAT, TJH, ACF, and PMG edited and revised the manuscript. All authors approved the final version of the manuscript submitted for publication and agree to be accountable for all aspects of the work. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed.

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Correspondence to Trent J. Herda.

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The authors declare they have no competing interests.

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University of Kansas-Lawrence Institutional Review Board (# STUDY00002953).

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Communicated by Bill J Yates.

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Trevino, M.A., Dimmick, H.L., Parra, M.E. et al. 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 240, 825–839 (2022). https://doi.org/10.1007/s00221-021-06278-3

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