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 (VATMS) 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.
Highlights
In recent years, a prolonged (20–60 min) period of local vibration (LV) has been validated as an efficient training method to increase muscle strength in healthy young subjects for plantar flexor (Lapole and Pérot, 2010) and dorsiflexor muscles (Souron et al, 2017) as well as for knee extensor muscles in old subjects (Tankisheva et al, 2015)
Responses to electrical stimulation of the descending corticospinal tract at the level of the mastoids or thoracic spine provide the most direct assessment of the motoneuron pool’s responsiveness to synaptic input because: (i) a large proportion of the response is monosynaptic for the upper limb (Petersen et al, 2002) and probably the lower limb (Martin et al, 2008); and (ii) evidence exists that descending tracts are not influenced by presynaptic inhibition (Nielsen and Petersen, 1994)
Insufficient statistical power could have prevented detection of age-related differences, because there was no effect of the group on the magnitude of changes reported in this study, as demonstrated by the lack of significant group × condition × time interaction, data from young and old participants were pooled
Summary
A prolonged (20–60 min) period of local vibration (LV) has been validated as an efficient training method to increase muscle strength in healthy young subjects for plantar flexor (Lapole and Pérot, 2010) and dorsiflexor muscles (Souron et al, 2017) as well as for knee extensor muscles in old subjects (Tankisheva et al, 2015) Because this method does not require an active contribution and may be applied while the subject is relaxed (sitting, lying down), prolonged LV may be beneficial for specific populations; e.g., older subjects (Tankisheva et al, 2015). Despite the utility of corticospinal tract stimulation, to date, no study has examined the response of CMEPs or TMEPs to prolonged LV
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