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Compensatory adjustments in motor unit behavior during fatigue differ for younger versus older men

Abstract

Background

The ability to maintain a submaximal force as a muscle fatigues is supplemented by compensatory adjustments in the nervous system’s control of motor units.

Aim

We sought to compare vastus lateralis motor unit recruitment and firing rate data for younger versus older men during isometric fatigue.

Methods

Twelve younger (age = 25 ± 3 years) and 12 older (75 ± 8 years) men performed contractions of the knee extensors at 50% of maximal voluntary contraction force until exhaustion. Surface electromyographic (sEMG) signals were detected from the vastus lateralis. A sEMG signal decomposition algorithm was used to quantify the motor unit action potential (MUAP) amplitude, mean firing rates, and recruitment threshold of each motor unit. For the latter two variables, our analyses only included motor units that featured similar action potential amplitude throughout the protocol.

Results

There was no group difference for time to task failure (p = 0.362, d = 0.381). Both groups showed increases in MUAP amplitude [younger and older slopes = 0.0174 ± 0.0123 and 0.0073 ± 0.0123 mV/contraction, respectively (p = 0.082, d = 0.710)], but the change was more linear for the younger men (mean r2 values = 0.565 and 0.455). Mean firing rates increased over time for the younger (p < 0.001), but not the older (p = 0.579), men. Similarly, recruitment thresholds decreased for younger men (p = 0.001).

Conclusion

We propose that aging results in neuromuscular impairments that hinder older adults’ ability to make compensatory adjustments in motor unit control during fatigue.

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Funding

This study was not funded by an external agency.

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Correspondence to Matt S. Stock.

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All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional research committee (Texas Tech University Human Research Protection Program [#505518]) and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

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Mota, J.A., Kwon, D.P., Kennedy, M. et al. Compensatory adjustments in motor unit behavior during fatigue differ for younger versus older men. Aging Clin Exp Res 32, 2259–2269 (2020). https://doi.org/10.1007/s40520-019-01438-6

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  • DOI: https://doi.org/10.1007/s40520-019-01438-6

Keywords

  • Force
  • Knee extensors
  • Motor unit
  • Neuromuscular