Intermediate Muscle Length and Tendon Vibration Optimize Corticospinal Excitability During Knee Extensors Local Vibration

Abstract

While local vibration (LV) has been recently proposed as a potential modality for neuromuscular conditioning, no practical recommendations to optimize its effects have been published. Because changes in corticospinal excitability may reflect at which degree the neuromuscular function is modulated during LV exposure, this study investigated the effects of muscle length and vibration site on LV-induced on motor evoked potentials (MEPs) changes. Twenty-one subjects participated in a single session in which MEPs were evoked on the relaxed knee extensors (KE) during three conditions, i.e., no vibration (CON), muscle (VIB_MU), and tendon vibration (VIB_TD). Three muscle lengths were tested for each condition, i.e., short/intermediate/long KE muscle length. Both VIB_MU and VIB_TD significantly increase MEPs compared to CON. Higher increases (P < 0.001) were found for VIB_TD compared to VIB_MU for vastus lateralis (mean increases of the three angles: +241% vs.+ 148%), vastus medialis (+273% vs. + 180%) and rectus femoris muscles (+191% vs. +141%). The increase in MEPs amplitude was higher (p < 0.001) at an intermediate (mean pooled increase for VIB_TD and VIB_MU: +265%, +290%, and +212% for VL, VM, and RF, respectively) compared to short (+136%, + 144%, and + 127%) or long (+ 184%, + 246% and + 160%) muscle lengths. These results suggest that LV should be applied to the tendon at an intermediate muscle length to optimize the acute effects of LV on the KE neuromuscular function.

Keywords: knee extensors; local vibration; motor-evoked potentials; muscle length; transcranial magnetic stimulation; vibration site.

FIGURE 1

Overview of the experimental protocol (A). Series of ten transcranial magnetic (white arrows) and two peripheral nerves stimulations (black arrows) were performed at three different muscle lengths (i.e., corresponding to knee angles of 60, 120, and 180°) for the three conditions, i.e., no vibration (CON), muscle vibration (VIB_MU), and tendon vibration (VIB_TD). Illustration of the position of the vibratory devices on the rectus femoris muscle and the infrapatellar tendon at a knee angle of 120° (B).

FIGURE 2

Representative trace of MEPs recorded for vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF) at each muscle length (i.e., knee angle of 60, 120, and 180°) when vibration was locally applied on the muscle (solid black line) or the tendon (dotted black line) or when vibration was turned off, i.e., control condition (solid gray line).

FIGURE 3

Effect of muscle (white column) and tendon (gray column) vibration on MEP amplitudes of vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF) muscles at muscle lengths corresponding to knee angles of 60, 120, and 180°. MEP amplitudes are expressed as a percentage of the mean control MEP (CON) elicited at rest without vibration. Mean control MEP (i.e., 100% on the figure) is represented by the dahsed line. Condition × angle interaction effect: ^∗p < 0.05; ^∗∗∗p < 0.001. Main angle effect: $p < 0.05; $$p < 0.01; $$$p < 0.001.