Inter-muscle differences in modulation of motor evoked potentials and posterior root-muscle reflexes evoked from lower-limb muscles during agonist and antagonist muscle contractions

Akira Saito,Kimitaka Nakazawa,Kento Nakagawa,Yohei Masugi

doi:10.1007/s00221-020-05973-x

Abstract

Voluntary contraction facilitates corticospinal and spinal reflex circuit excitabilities of the contracted muscle and inhibits spinal reflex circuit excitability of the antagonist. It has been suggested that modulation of spinal reflex circuit excitability in agonist and antagonist muscles during voluntary contraction differs among lower-limb muscles. However, whether the effects of voluntary contraction on the excitabilities of corticospinal and spinal reflex circuits depend on the tested muscles remains unknown. The purpose of this study was to examine inter-muscle differences in modulation of the corticospinal and spinal reflex circuit excitabilities of multiple lower-limb muscles during voluntary contraction. Eleven young males performed isometric plantar-flexion, dorsi-flexion, knee extension, and flexion at low torque levels. Motor evoked potentials (MEPs) and posterior root-muscle reflexes from seven lower-leg and thigh muscles were evoked by transcranial magnetic stimulation and transcutaneous spinal cord stimulation, respectively, at rest and during weak voluntary contractions. MEP and posterior root-muscle reflex amplitudes of agonists were significantly increased as agonist torque level increased, except for the reflex of the tibialis anterior. MEP amplitudes of antagonists were significantly increased in relation to the agonist torque level, but those of the rectus femoris were slightly depressed during knee flexion. Regarding the posterior root-muscle reflex of the antagonists, the amplitudes of triceps surae and the hamstrings were significantly decreased, but those of the quadriceps femoris were significantly increased as the agonist torque level increased. These results demonstrate that modulation of corticospinal and spinal reflex circuit excitabilities during agonist and antagonist muscle contractions differed among lower-limb muscles.

Highlights

Voluntary muscle contraction facilitates the excitability of corticospinal tract and spinal reflex circuits of the contracted muscle
We hypothesized that modulation of both Motor evoked potentials (MEPs) and posterior root-muscle reflexes would differ among lower-limb muscles during agonist and antagonist voluntary muscle contractions
The second response amplitudes relative to the first responses were, SOL: 5.0 ± 4.2%, medial head of gastrocnemius (MG): 5.7 ± 5.2%, lateral head of gastrocnemius (LG): 5.2 ± 4.5%, tibialis anterior (TA): 9.8 ± 6.3%, rectus femoris (RF): 51.2 ± 32.9%, vastus lateralis (VL): 31.7 ± 20.3%, and biceps femoris (BF): 12.3 ± 10.8%

Summary

Introduction

Voluntary muscle contraction facilitates the excitability of corticospinal tract and spinal reflex circuits of the contracted muscle. Previous studies demonstrated that facilitation of corticospinal tract and spinal reflex circuit excitabilities during voluntary contraction differed between extensor and flexor muscles around the ankle joint (Nielsen et al 1993; Morita et al 2000; Geertsen et al 2010). Whether the effects of voluntary contraction on the spinal reflex circuit excitabilities of antagonist muscles differ among the lower-limb muscles is unclear. Significant effects of contraction intensity (p < 0.001) were observed in the level of background EMG activity of each agonist muscle. The background EMG activity of TA was significantly higher than that of SOL, MG, LG, RF, and VL muscles during their agonist contraction at each intensity (p < 0.05). The background EMG activity of TA was higher than that of SOL during their antagonist contraction at 5%, 10%, and 20%MVC levels (p < 0.05).

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