Compensatory adjustments in motor unit behavior during fatigue differ for younger versus older men
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ORIGINAL ARTICLE
Compensatory adjustments in motor unit behavior during fatigue differ for younger versus older men Jacob A. Mota1 · Dennis P. Kwon2 · Mary Kennedy2 · Eric J. Sobolewski3 · Youngdeok Kim4 · Joaquin U. Gonzales5 · Matt S. Stock2 Received: 5 August 2019 / Accepted: 3 December 2019 © Springer Nature Switzerland AG 2020
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
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