Motor unit action potential amplitude during low torque fatiguing contractions versus high torque non-fatiguing contractions: a multilevel analysis

Harmon, Kylie K.; Hamilton, Adam S.; Johnson, Brent D.; Bartek, Frank J.; Girts, Ryan M.; MacLennan, Rob J.; Hahs-Vaughn, Debbie L.; Stock, Matt S.

doi:10.1007/s00421-021-04606-7

Original Article
Published: 23 January 2021

Motor unit action potential amplitude during low torque fatiguing contractions versus high torque non-fatiguing contractions: a multilevel analysis

Kylie K. Harmon^1,2,
Adam S. Hamilton^1,2,
Brent D. Johnson^1,2,
Frank J. Bartek^1,2,
Ryan M. Girts^1,2,
Rob J. MacLennan³,
Debbie L. Hahs-Vaughn⁴ &
Matt S. Stock ORCID: orcid.org/0000-0003-1156-1084^1,2

European Journal of Applied Physiology volume 121, pages 1145–1157 (2021)Cite this article

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Abstract

Purpose

The ability to maintain an absolute, submaximal torque level during fatiguing contractions is controlled, in part, by the recruitment of larger motor units. These motor units are commonly identified based on greater action potential peak-to-peak amplitude values. It is unclear, however, if motor unit action potential (MUAP) amplitude values during low torque, fatiguing contractions reach similar levels as those observed during non-fatigued, high torque contractions. To establish a clearer understanding of motor unit control during fatigue, we compared MUAP amplitude during 50 and 80% maximum voluntary contraction (MVC) torque contractions and at the beginning, middle, and end of a 30% MVC fatigue protocol.

Methods

Eleven untrained men (mean age = 24 years) performed isometric contractions at 50 and 80% MVC, followed by repeated contractions at 30% MVC. Surface electromyographic (EMG) signals were detected from the vastus lateralis and decomposed to quantify the peak-to-peak amplitude of individual MUAPs. A two-level multilevel model was estimated, allowing examination of simultaneous measures of MUAP amplitude within participants and controlling for the dependence between measures within participants.

Results

Results from the multilevel analyses suggested that there were not statistically significant differences in MUAP amplitude between 80% MVC and end fatigue. Separate repeated-measures analyses of variance indicated that there were not statistically significant mean differences in greatest MUAP or surface EMG amplitude between 80% MVC and end fatigue.

Conclusions

MUAP and surface EMG amplitude values during a 30% MVC fatiguing protocol appear to be comparable to those observed during a non-fatigued 80% MVC condition.

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Data availability

All data analyzed during this study are included in supplementary information files.

Abbreviations

ANOVA:: Analysis of variance
EMG:: Electromyography
MUAP:: Motor unit action potential
MVC:: Maximum voluntary contraction

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Funding

No funding was provided for this study.

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Authors and Affiliations

Neuromuscular Plasticity Laboratory, Institute of Exercise Physiology and Rehabilitation Science, University of Central Florida, 12805 Pegasus Drive, HS 1, Room 258, Orlando, FL, 32816-2205, USA
Kylie K. Harmon, Adam S. Hamilton, Brent D. Johnson, Frank J. Bartek, Ryan M. Girts & Matt S. Stock
School of Kinesiology and Physical Therapy, University of Central Florida, Orlando, FL, 32816-2205, USA
Kylie K. Harmon, Adam S. Hamilton, Brent D. Johnson, Frank J. Bartek, Ryan M. Girts & Matt S. Stock
School of Kinesiology, Applied Health and Recreation, Oklahoma State University, Stillwater, OK, USA
Rob J. MacLennan
Department of Learning Sciences and Educational Research, University of Central Florida, Orlando, FL, 32816-2205, USA
Debbie L. Hahs-Vaughn

Authors

Kylie K. Harmon
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Adam S. Hamilton
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Brent D. Johnson
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Frank J. Bartek
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Ryan M. Girts
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Rob J. MacLennan
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Debbie L. Hahs-Vaughn
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Matt S. Stock
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Contributions

This study was conceived of and designed by MS. Data collection was performed by all authors, except for DHV. DHV conducted the statistical analyses and provided feedback concerning data interpretation. The first draft of the manuscript was written by KH. All authors provided edits and feedback in preparation for manuscript submission. All authors read and approved the final manuscript.

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Correspondence to Matt S. Stock.

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The authors declare that they have no conflict of interest.

Ethical approval

All procedures performed involving human participants were in accordance with the ethical standards of the Institutional Review Board and with the 1964 Helsinki Declaration and its later amendments. The study was approved by the Institutional Review Board of The University of Central Florida (BIO-17-12855).

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Informed consent was obtained from all individual participants included in the study.

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Communicated by William J. Kraemer.

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Harmon, K.K., Hamilton, A.S., Johnson, B.D. et al. Motor unit action potential amplitude during low torque fatiguing contractions versus high torque non-fatiguing contractions: a multilevel analysis. Eur J Appl Physiol 121, 1145–1157 (2021). https://doi.org/10.1007/s00421-021-04606-7

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Received: 12 July 2020
Accepted: 13 January 2021
Published: 23 January 2021
Issue Date: April 2021
DOI: https://doi.org/10.1007/s00421-021-04606-7

Keywords

Motor unit recruitment
Surface EMG
Fatigue
Motor unit size