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J Biomech. 2019 Mar 27;86:251-255. doi: 10.1016/j.jbiomech.2019.01.050. Epub 2019 Feb 14.

Muscular strength and power are correlated with motor unit action potential amplitudes, but not myosin heavy chain isoforms in sedentary males and females.

Author information

1
Neuromechanics Laboratory, Department of Health, Sport and Exercise Sciences, University of Kansas, Lawrence, KS, USA. Electronic address: t.herda@KU.edu.
2
Applied Neuromuscular Physiology Laboratory, Department of Health and Human Performance, Oklahoma State University, Stillwater, OK, USA.
3
Neuromuscular Research Laboratory, Department of Sports Medicine and Nutrition, University of Pittsburgh, Pittsburgh, PA, USA.
4
Neuromechanics Laboratory, Department of Health, Sport and Exercise Sciences, University of Kansas, Lawrence, KS, USA.
5
Applied Physiology Laboratory, Department of Health, Sport and Exercise Sciences, University of Kansas, Lawrence, KS, USA.
6
Jayhawk Athletic Performance Laboratory, Department of Health, Sport and Exercise Sciences, University of Kansas, Lawrence, KS, USA.

Abstract

It remains unclear if the sizes of higher-threshold motor units (MU) are associated with muscular strength and power. Therefore, the purpose of this study was to examine sex-related differences in muscle cross-sectional area (mCSA), percent myosin heavy chain (%MHC) isoform expression, and the MU action potential amplitudes (MUAPAMPS)-recruitment threshold (RT) relationships of the vastus lateralis and isometric peak torque, isokinetic peak torque and mean power at 1.05 rad·s-1 of the leg extensors. Surface electromyographic decomposition techniques were used to quantify MUAPAMPS recorded during isometric muscle actions at 70% of maximal voluntary contractions and regressed against RTs with the slopes calculated. Ultrasound images were used to measure mCSA. Males had greater slopes from the MUAPAMP-RT relationship than the females (P < 0.05). The greater slopes likely reflected larger higher-threshold MUs for the males. The mCSAs and slopes from the relationships were strongly correlated with isometric and isokinetic peak torque and isokinetic mean power (r = 0.78-0.82), however, type I %MHC isoform was only moderately correlated with isometric peak torque (r = -0.54). The results indicated that sex-related differences in muscular strength and power were associated more so with the sizes of the higher-threshold MUs (slopes) and mCSA than MHC isoforms. The amount of cross-bridge activity within muscle fibers that comprise higher-threshold MUs may be the primary contributor to muscular strength and power rather than the contractile properties of the muscle.

KEYWORDS:

EMG decomposition; Isokinetic; Motor unit action potential amplitude; Muscle cross-sectional area; Myosin heavy chain; Ultrasound

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