An Acute Exposure to Muscle Vibration Decreases Knee Extensors Force Production and Modulates Associated Central Nervous System Excitability

Abstract

Local vibration (LV) has been recently validated as an efficient training method to improve muscle strength. Understanding the acute effects may help elucidate the mechanism(s). This study aimed to investigate the effects of a single bout of prolonged LV on knee extensor force production and corticospinal responsiveness of vastus lateralis (VL) and rectus femoris (RF) muscles in healthy young and old adults. Across two visits, 23 adult subjects (20-75 years old) performed pre- and post-test measurements, separated by 30-min of either rest (control; CON) or LV. Maximal voluntary contraction (MVC) force was assessed and transcranial magnetic stimulation (TMS) was used to evaluate cortical voluntary activation (VA_TMS) as well as the motor evoked potential (MEP) and silent period (SP). In 11 young adults, thoracic electrical stimulation was used to assess the thoracic motor evoked potential (TMEP). Although MVC decreased after both CON (-6.3 ± 4.4%, p = 0.01) and LV (-12.9 ± 7.7%, p < 0.001), the MVC loss was greater after LV (p = 0.001). Normalized maximal electromyographic (EMG) activity decreased after LV for both VL (-25.1 ± 10.7%) and RF (-20.9 ± 16.5%; p < 0.001), while it was unchanged after CON (p = 0.32). For RF, the TMEP and MEP/TMEP ratio decreased (p = 0.01) and increased (p = 0.01) after LV, respectively. Both measures were unchanged for VL (p = 0.27 and p = 0.15, respectively). No changes were reported for TMS-related parameters. These results confirm our hypothesis that modulations within the central nervous system would accompany the significant reduction of maximal voluntary force. A reduced motoneuron excitability seems to explain the decreased MVC after prolonged LV, as suggested by reductions in maximal EMG (all subjects) and TMEP area (data from 11 young subjects). A concomitant increased cortical excitability seems to compensate for lower excitability at the spinal level.

Keywords: cortical voluntary activation; corticospinal excitability and inhibition; local vibration; thoracic electrical stimulation; transcranial magnetic stimulation.

Figure 1

Overview of the neuromuscular testing protocol prior to the interventions (A). Following the interventions, the warm-up and stimulation configurations were not completed. Peripheral nerve stimulation (PNS), transcranial magnetic stimulation (TMS) and electrical stimulation over the thoracic spine are represented by dotted black, gray and white arrows, respectively. The position of the vibratory device on the leg during the 30-min vibration period is illustrated in (B).

Figure 2

Stimulus-response curves of motor evoked potentials (MEPs) recorded from agonist and antagonist muscles in a single, representative subject. The optimal stimulation intensity was that which showed a plateau of the vastus lateralis (VL; white circles) and rectus femoris (RF; black circles) MEPs, without a marked increase of the MEP for the antagonist biceps femoris (BF; gray triangles). For this subject, 70% of the maximal stimulator output was chosen as the optimal intensity.

Figure 3

Representative trace of a MEP, thoracic motor evoked potential (TMEP) and M-wave (M_{max_50}) recorded for local vibration (LV) session at PRE (solid line) and POST (dotted line) during a 50% maximal voluntary contraction (MVC) submaximal voluntary contraction on VL and RF muscles (A). Raw traces from a single subject of the TMS superimposed twitches (SIT) evoked during maximal (MVC) and submaximal voluntary contractions at 75 and 50% MVC are displayed in (B). Black arrows represent the time of magnetic or electrical stimulations.

Figure 4

Knee extensor MVC force (MVC, A) and associated maximal electromyography (EMG) root-mean-square (RMS) normalized to M_{max_MVC} for VL and RF muscles (B) recorded before (PRE) and after (POST) a 30-min resting (CON) or LV period. The displayed data are mean values. Error bars denote the 95% confidence interval. Data from individual young (dashed lines) and old (solid lines) subjects are displayed for both testing sessions at PRE and POST measurements. *Significantly different from PRE; † significantly different from POST CON (p < 0.05).

Figure 5

TMEP normalized to M_{max_50} (A) and MEP/TMEP ratios (B) recorded in 11 healthy young subjects for VL and RF muscles before (PRE) and after (POST) a 30-min resting (CON) or LV period. The displayed data are mean values. Error bars denote the 95% confidence interval. Data from the individual subjects are displayed (dashed lines) for both testing sessions at PRE and POST measurements. *Significantly different from PRE (p < 0.05).