The Time Course of Changes in Neuromuscular Responses during the Performance of Leg Extension Repetitions to Failure below and above Critical Resistance in Women

Taylor K. Dinyer, M. Travis Byrd, Pasquale J. Succi, Haley C. Bergstrom

Research output: Contribution to journalArticlepeer-review

3 Scopus citations


Dinyer, TK, Byrd, MT, Succi, PJ, and Bergstrom, HC. The time course of changes in neuromuscular responses during the performance of leg extension repetitions to failure below and above critical resistance in women. J Strength Cond Res 36(3): 608-614, 2022 - Critical resistance (CR) is the highest sustainable resistance that can be completed for an extended number of repetitions. Exercise performed below (CR-15%) and above (CR+15%) CR may represent 2 distinct intensities that demonstrate separate mechanisms of fatigue. Electromyography (EMG) and mechanomyography (MMG) have been used to examine the mechanism of fatigue during resistance exercise. Therefore, the purposes of this study were to (a) compare the patterns of responses and time course of changes in neuromuscular parameters (EMG and MMG amplitude [AMP] and mean power frequency [MPF]) during the performance of repetitions to failure at CR-15%and CR+15%and (b) identify the motor unit activation strategy that best describes the fatigue-induced changes in the EMG and MMG signals at CR-15%and CR+15%. Ten women completed one repetition maximum (1RM) testing and repetitions to failure at 50, 60, 70, and 80% 1RM (to determine CR), and at CR-15%and CR+15%on the leg extension. During all visits, EMG and MMG signals were measured from the vastus lateralis. There were similar patterns of responses in the neuromuscular parameters, and time-dependent changes in EMG AMP and EMG MPF, but not MMG AMP or MMG MPF, during resistance exercise performed at CR-15%and CR+15%(p < 0.05). The onset of fatigue occurred earlier for EMG AMP, but later for EMG MPF, during repetitions performed at CR+15%compared with those performed at CR-15%. Thus, resistance exercise performed below and above CR represented 2 distinct intensities that were defined by different neuromuscular fatigue mechanisms but followed similar motor unit activation strategies.

Original languageEnglish
Pages (from-to)608-614
Number of pages7
JournalJournal of Strength and Conditioning Research
Issue number3
StatePublished - Mar 1 2022

Bibliographical note

Funding Information:
The University of Kentucky Center for Clinical and Translational Sciences through the National Institute of Health grant UL1TR001998 provided recruitment materials for this study.

Publisher Copyright:
© 2022 NSCA National Strength and Conditioning Association. All rights reserved.


  • electromyography
  • mechanomyography
  • motor unit activation strategy
  • resistance exercise

ASJC Scopus subject areas

  • Orthopedics and Sports Medicine
  • Physical Therapy, Sports Therapy and Rehabilitation


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