Conduction Velocity of Muscle Action Potential of Knee Extensor Muscle During Evoked and Voluntary Contractions After Exhaustive Leg Pedaling Exercise.

Kohei Watanabe,Keiko Iemitsu,Masakata Uchida,Masao Kanamori,Taiki Sakai,Motoyuki Iemitsu,Naoki Horii,Shosaku Kato,Katsunori Tsuji,Maki Yoshikawa,Natsuki Hasegawa,Natsuka Hashizume

doi:10.3389/fphys.2020.00546

Abstract

PurposeMuscle fiber conduction velocity (CV) has been developed to estimate neuromuscular fatigue and measured during voluntary (VC) and electrically evoked (EC) contractions. Since CV during VC and EC reflect different physiological phenomena, the two parameters would show inconsistent changes under the conditions of neuromuscular fatigue. We investigated the time-course changes of CV during EC and VC after fatiguing exercise.MethodsIn 14 young males, maximal voluntary contraction (MVC) of knee extensor muscles, CV during electrical stimulation (CV-EC) and MVC (CV-VC) were measured before and immediately, 30 min, 60 min, 120 min, and 24 h after exhaustive leg pedaling exercise.ResultsCV-EC significantly increased immediately after the fatiguing exercise (p < 0.05) and had a significant negative correlation with MVC in merged data from all time-periods (r = −0.511, p < 0.001). CV-VC significantly decreased 30, 60, and 120 min after the fatiguing exercise (p < 0.05) and did not show any correlations with MVC (p > 0.05).ConclusionThese results suggest that CV during EC and VC exhibits different time-course changes, and that CV during EC may be appropriate to estimate the degree of neuromuscular fatigue after fatiguing pedaling exercise.

Highlights

Muscle fiber conduction velocity (CV) recorded by multi-channel surface electromyography (EMG) has been used to assess muscular fatigue during and/or after exercise (Merletti et al, 1990)
Decreases in CV during and/or after fatiguing exercise were noted in previous studies, and this phenomenon has been explained by fatigue-induced physiological responses of peripheral muscle, Conduction Velocity After Exhaustive Pedaling such as the accumulation of metabolites or a decrease in the membrane potential (Mortimer et al, 1970; Moxham et al, 1982; Milner-Brown and Miller, 1986)
Surface EMG signals measured during voluntary contraction (VC) include information from both central and peripheral factors, since muscle action potentials during VC originate from rate coding and the recruitment of motor units regulated by the central nervous system (CNS)

Summary

Introduction

Muscle fiber conduction velocity (CV) recorded by multi-channel surface electromyography (EMG) has been used to assess muscular fatigue during and/or after exercise (Merletti et al, 1990). On the other hand, Schmitz et al (2012) reported that no decrease in CV was detected during exhaustive bicycling (Schmitz et al, 2012) This difference in results may be partly explained by task-dependent muscular fatigue and the use of surface EMG during VC to calculate CV. Piitulainen et al (2011) showed a decrease in CV during electrically evoked contraction (EC) 30 and 120 min after repeated eccentric contractions during elbow flexion (Piitulainen et al, 2011) They reported that decreases in CV were not detected after repeated concentric contractions during elbow flexion, meaning that CV could reflect taskdependent muscle fatigue. This study measured CV during MVC and the increase in CV immediately after repeated concentric contractions This means that different patterns are observed in CV calculated from surface EMG during VC and EC. While CV detection during EC could be useful to assess peripheral muscular fatigue, applications of this method to the multiple joint dynamic exercises generally used in physical training such as running and/or pedaling have yet to be investigated

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