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



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.


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 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.


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



Analysis of variance




Motor unit action potential


Maximum voluntary contraction


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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.

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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).

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  • Motor unit recruitment
  • Surface EMG
  • Fatigue
  • Motor unit size