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

The effects of local vibration inducing a tonic vibration reflex or movement illusion on acute modulations of corticospinal excitability

Stimulation of muscle afferents by local vibration (LV) can lead to two distinct perceptual and motor responses: the tonic vibration reflex (TVR) or the movement illusion. This study aimed to evaluate the effect of TVR and movement illusion on corticospinal excitability. In two experiments, EMG activity of the vibrated flexor carpi radialis (FCR) muscle (80 Hz, 6 min) and the extensor carpi radialis (ECR) muscle were recorded. Illusion was assessed using questionnaires. LV conditions were adjusted to favour either TVR (visual attention focused on the vibrating wrist) or ILLUSION (hidden hand, visual attention focused on the EMG of the FCR muscle). Motor-evoked potential (MEP) and cervicomedullary motor-evoked potential (CMEP) were recorded at rest for both muscles before (10 and 0 min) and after (0 and 30 min) each LV condition. Only the TVR condition increased EMG of the FCR muscle (+490% compared to resting, P = 0.005), while movement illusion was greater in the ILLUSION condition (P < 0.001). Concerning the vibrated muscle at P0, TVR reduced the amplitude of CMEP (-13.8 ± 15.8%, P = 0.011) without altering MEP (0.3 ± 27.9%, P = 1), whereas the opposite occurred with movement illusion (i.e. CMEP: -4.5 ± 13.7%, P = 0.891; MEP: -25.1 ± 17.2%, P = 0.002). Cortical excitability (MEP/CMEP ratio) of the vibrated muscle was reduced by 24 ± 13.3% on average compared to values obtained before LV, only in the ILLUSION condition. In conclusion, this study highlights the relevance of measuring and reporting the perceptual and motor responses induced during LV, demonstrating that TVR and movement illusion partly determine the acute effects on the neural network. KEY POINTS: Tonic vibration reflex and movement illusion are rarely controlled and measured in studies investigating the effect of LV on corticospinal excitability. The application of LV with visual attention focused on the vibrated muscle promotes the presence of a tonic vibration reflex (TVR). The absence of visual feedback on the latter promotes the presence of an illusion of movement. The cortical excitability of the vibrated muscle is influenced differently according to the perceptual and motor responses induced during LV, with an opposite effect on the cortical excitability of the antagonist muscle. Improved control of LV application conditions, quantification of perceptual and motor responses, and reporting of results (e.g. EMG activity of the vibrated muscle or illusion of movement during the protocol) are required to enhance our understanding of the physiological mechanisms associated with LV use and, consequently, the effectiveness of LV as a therapeutic modality.

A single exposure to prolonged flexor carpi radialis muscle vibration increases sensorimotor cortical areas activity

Prolonged local vibration (LV) is thought to promote brain plasticity through repeated Ia afferents discharge. However, the underlying mechanisms remain unclear. This study therefore aimed at determining the acute after-effects of 30-min LV of the flexor carpi radialis muscle (FCR) on sensorimotor (S1, M1) and posterior parietal cortex (PPC) areas activity. Sixteen healthy participants were tested before and immediately after 30 min of FCR LV. Electroencephalographic signals were recorded during isometric submaximal wrist flexions. Time-frequency analyses were performed at source levels during contraction preparation, contraction initiation, force plateau, and relaxation. After LV, the results showed an increase in α and β desynchronizations in the source activity for the estimated M1, S1, and PPC during contraction preparation (P ≤ 0.05) and contraction initiation (P ≤ 0.05; except for PPC in the β band: P = 0.07), and a greater α desynchronization in M1, S1, and PPC (P < 0.01) during force plateau. No LV-induced changes were observed during relaxation. Prolonged LV on the upper limb could increase estimated cortical activity within M1, S1, and PPC areas during subsequent isometric contractions. This could be due to LV-induced Ia afferents inputs projecting onto cortical areas through proprioceptive pathways, and likely triggering brain use-dependent plasticity.NEW & NOTEWORTHY Prolonged local vibration (LV) is thought to promote brain plasticity, yet the underlying mechanisms remain unclear. In the present study, we used electroencephalography in healthy subjects and found increased activity in primary motor, primary somatosensory, and posterior parietal areas after a single exposure to LV. This may be due to LV-induced Ia afferents inputs projecting onto cortical areas through proprioceptive pathways, and likely triggering brain plasticity.

Acute effects of local vibration inducing tonic vibration reflex or illusion of movement on maximal wrist force production

Local vibration (LV) mainly stimulates primary afferents (Ia) and can induce a tonic vibration reflex (TVR) and an illusion of movement. This study aimed to evaluate the effect of these two phenomena on maximal voluntary isometric contraction (MVIC) capacity. LV (80 Hz) was applied to the wrist flexor muscles in two randomized experiments for 6 min. LV conditions were adjusted to promote either TVR (visual focus on the vibrated wrist) or ILLUSION [hand hidden, visual focus on electromyographic activity of the flexor carpi radialis muscle (FCR)]. Mechanical and electromyographic (EMG) responses of the FCR and extensor carpi radialis muscles were recorded during MVIC in flexion and extension and during electrically evoked contractions at supramaximal intensity. Measurements were performed before (10 min and just before) and after (0 and 30 min) LV protocol. An increase in FCR EMG was observed during LV in the TVR condition (+340%) compared with the illusion condition (P = 0.003). In contrast, the movement illusion was greater in the ILLUSION condition (assessed through subjective scales) (P = 0.004). MVIC was reduced in flexion only after the TVR condition ([Formula: see text], all P < 0.034). Moreover, the decrease in force was correlated with the amount of TVR recorded on the FCR muscle (r = -0.64, P = 0.005). Although potentiated doublets of each muscle did not evolve differently between conditions, a decrease was observed between the first and the last measure. In conclusion, when conducting research to assess maximal strength, it is necessary to have better control and reporting of the phenomena induced during LV.NEW & NOTEWORTHY The maximal force production of the vibrated muscle is reduced after 6 min of LV only in TVR condition. Furthermore, the amount of TVR is negatively correlated with this force decrease. When measuring the effects of LV on maximal force production, it is important to control and report any phenomena induced during vibration, such as TVR or movement illusion, which can be achieved by recording EMG activity of vibrated muscle and quantifying illusion.

Effects of prolonged vibration to the flexor carpi radialis muscle on intracortical excitability

Prolonged local vibration (LV) can induce neurophysiological adaptations thought to be related to long-term potentiation or depression. Yet, how changes in intracortical excitability may be involved remains to be further investigated as previous studies reported equivocal results. We therefore investigated the effects of 30 min of LV applied to the right flexor carpi radialis muscle (FCR) on both short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF). SICI and ICF were measured through transcranial magnetic stimulation before and immediately after 30 min of FCR LV (vibration condition) or 30 min of rest (control condition). Measurements were performed during a low-intensity contraction (n = 17) or at rest (n = 7). No significant SICI nor ICF modulations were observed, whether measured during isometric contractions or at rest (p = 0.2). Yet, we observed an increase in inter-individual variability for post measurements after LV. In conclusion, while intracortical excitability was not significantly modulated after LV, increased inter-variability observed after LV may suggest the possibility of divergent responses to prolonged LV exposure.

Local vibration induces changes in spinal and corticospinal excitability in vibrated and antagonist muscles

Local vibration (LV) applied over the muscle tendon constitutes a powerful stimulus to activate the muscle spindle primary (Ia) afferents that project to the spinal level and are conveyed to the cortical level. This study aimed to identify the neuromuscular changes induced by a 30-min LV-inducing illusions of hand extension on the vibrated flexor carpi radialis (FCR) and the antagonist extensor carpi radialis (ECR) muscles. We studied the change of the maximal voluntary isometric contraction (MVIC, experiment 1) for carpal flexion and extension, motor-evoked potentials (MEPs, experiment 2), cervicomedullary motor-evoked potentials (CMEPs, experiment 2), and Hoffmann's reflex (H-reflex, experiment 3) for both muscles at rest. Measurements were performed before (PRE) and at 0, 30, and 60 min after LV protocol. A lasting decrease in strength was only observed for the vibrated muscle. The reduction in CMEPs observed for both muscles seems to support a decrease in alpha motoneurons excitability. In contrast, a slight decrease in MEPs responses was observed only for the vibrated muscle. The MEP/CMEP ratio increase suggested greater cortical excitability after LV for both muscles. In addition, the H-reflex largely decreased for the vibrated and the antagonist muscles. The decrease in the H/CMEP ratio for the vibrated muscle supported both pre- and postsynaptic causes of the decrease in the H-reflex. Finally, LV-inducing illusions of movement reduced alpha motoneurons excitability for both muscles with a concomitant increase in cortical excitability.NEW & NOTEWORTHY Spinal disturbances confound the interpretation of excitability changes in motor areas and compromise the conclusions reached by previous studies using only a corticospinal marker for both vibrated and antagonist muscles. The time course recovery suggests that the H-reflex perturbations for the vibrated muscle do not only depend on changes in alpha motoneurons excitability. Local vibration induces neuromuscular changes in both vibrated and antagonist muscles at the spinal and cortical levels.

Can local vibration alter the contribution of persistent inward currents to human motoneuron firing?

The response of spinal motoneurons to synaptic input greatly depends on the activation of persistent inward currents (PICs), which in turn are enhanced by the neuromodulators serotonin and noradrenaline. Local vibration (LV) induces excitatory Ia input onto motoneurons and may alter neuromodulatory inputs. Therefore, we investigated whether LV influences the contribution of PICs to motoneuron firing. This was assessed in voluntary contractions with concurrent, ongoing LV, as well as after a bout of prolonged LV. High-density surface electromyograms (HD-EMG) of the tibialis anterior were recorded with a 64-electrode matrix. Twenty males performed isometric, triangular, dorsiflexion contractions to 20% and 50% of maximal torque at baseline, during LV of the tibialis anterior muscle, and after 30-min of LV. HD-EMG signals were decomposed, and motor units tracked across time points to estimate PICs through a paired motor unit analysis, which quantifies motor unit recruitment-derecruitment hysteresis (ΔF). During ongoing LV, ΔF was lower for both 20% and 50% ramps. Although significant changes in ΔF were not observed after prolonged LV, a differential effect across the motoneuron pool was observed. This study demonstrates that PICs can be non-pharmacologically modulated by LV. Given that LV leads to reflexive motor unit activation, it is postulated that lower PIC contribution to motoneuron firing during ongoing LV results from decreased neuromodulatory inputs associated with lower descending corticospinal drive. A differential effect in motoneurons of different recruitment thresholds after prolonged LV is provocative, challenging the interpretation of previous observations and motivating future investigations. KEY POINTS: Neuromodulatory inputs from the brainstem influence motoneuron intrinsic excitability through activation of persistent inward currents (PICs). PICs make motoneurons more responsive to excitatory input. We demonstrate that vibration applied on the muscle modulates the contribution of PICs to motoneuron firing, as observed through analysis of the firing of single motor units. The effects of PICs on motoneuron firing were lower when vibration was concurrently applied during voluntary ramp contractions, likely due to lower levels of neuromodulation. Additionally, prolonged exposure to vibration led to differential effects of lower- vs. higher-threshold motor units on PICs, with lower-threshold motor units tending to present an increased and higher-threshold motor units a decreased contribution of PICs to motoneuron firing. These results demonstrate that muscle vibration has the potential to influence the effects of neuromodulation on motoneuron firing. The potential of using vibration as a non-pharmacological neuromodulatory intervention should be further investigated.

Effects of prolonged local vibration superimposed to muscle contraction on motoneuronal and cortical excitability

Introduction: Acute effects of prolonged local vibration (LV) at the central nervous system level have been well investigated demonstrating an altered motoneuronal excitability with a concomitant increase in cortical excitability. While applying LV during isometric voluntary contraction is thought to optimize the effects of LV, this has never been addressed considering the acute changes in central nervous system excitability. Materials and Methods: In the present study, nineteen healthy participants were engaged in four randomized sessions. LV was applied for 30 min to the relaxed flexor carpi radialis muscle (VIBRELAXED) or during wrist flexions (i.e. intermittent contractions at 10% of the maximal voluntary contraction: 15 s ON and 15 s OFF; VIBCONTRACT). A control condition and a condition where participants only performed repeated low-contractions at 10% maximal force (CONTRACT) were also performed. For each condition, motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation and cervicomedullary evoked potentials (CMEPs) elicited by corticospinal tract electrical stimulation were measured before (PRE) and immediately after prolonged LV (POST) to investigate motoneuronal and corticospinal excitability, respectively. We further calculated the MEP/CMEP ratio as a proxy of cortical excitability. Results: No changes were observed in the control nor CONTRACT condition. At POST, CMEP decreased similarly in VIBRELAXED (-32% ± 42%, p < .001) and VIBCONTRACT (-41% ± 32%, p < .001). MEP/CMEP increased by 110% ± 140% (p = .01) for VIBRELAXED and by 120% ± 208% (p = .02) for VIBCONTRACT without differences between those conditions. Discussion: Our results suggest that LV to the flexor carpi radialis muscle, either relaxed or contracted, acutely decreases motoneuronal excitability and induces some priming of cortical excitability.

Acute effects of quadriceps muscle versus tendon prolonged local vibration on force production capacities and central nervous system excitability

PURPOSE

The present study aimed to directly compare the effects of 30 min muscle (VIB_muscle) vs. tendon (VIB_tendon) local vibration (LV) to the quadriceps on maximal voluntary isometric contraction (MVIC) and rate of torque development (RTD) as well as on central nervous system excitability (i.e. motoneuron and cortical excitability).

METHODS

Before (PRE) and immediately after (POST) LV applied to the quadriceps muscle or its tendon, we investigated MVIC and RTD (STUDY #1; n = 20) or vastus lateralis (VL), vastus medialis (VM) and rectus femoris (RF) electromyography responses to thoracic electrical stimulation (TMEPs; motoneuron excitability) and transcranial magnetic stimulation (MEPs; corticospinal excitability) (STUDY #2; n = 17). MEP/TMEP ratios were further calculated to quantify changes in cortical excitability.

RESULTS

MVIC decreased at POST (P = 0.017) without any difference between VIB_tendon and VIB_muscle, while RTD decreased for VIB_tendon (P = 0.013) but not VIB_muscle. TMEP amplitudes were significantly decreased for all muscles (P = 0.014, P < 0.001 and P = 0.004 for VL, VM and RF, respectively) for both LV sites. While no changes were observed for MEP amplitude, MEP/TMEP ratios increased at POST for VM and RF muscles (P = 0.009 and P = 0.013, respectively) for both VIB_tendon and VIB_muscle.

CONCLUSION

The present results suggest that prolonged muscle and tendon LV are similarly effective in modulating central nervous system excitability and decreasing maximal force. Yet, altered explosive performance after tendon but not muscle LV suggests greater neural alterations when tendons are vibrated.

Corticospinal modulation of vibration-induced H-reflex depression

The purpose of this study was to examine corticospinal modulation of spinal reflex excitability, by determining the effect of transcranial magnetic stimulation (TMS) on soleus H-reflexes while they were almost completely suppressed by lower extremity vibration. In 15 healthy adults, a novel method of single-limb vibration (0.6 g, 30 Hz, 0.33 mm displacement) was applied to the non-dominant leg. Soleus muscle responses were examined in six stimulation conditions: (1) H-reflex elicited by tibial nerve stimulation, (2) tibial nerve stimulation during vibration, (3) subthreshold TMS, (4) subthreshold TMS during vibration, (5) tibial nerve stimulation 10 ms after a subthreshold TMS pulse, and (6) tibial nerve stimulation 10 ms after a subthreshold TMS pulse, during vibration. With or without vibration, subthreshold TMS produced no motor evoked potentials and had no effect on soleus electromyography (p > 0.05). In the absence of vibration, H-reflex amplitudes were not affected by subthreshold TMS conditioning (median (md) 35, interquartile range (IQ) 18-56 vs. md 46, IQ 22-59% of the maximal M wave (Mmax), p > 0.05). During vibration, however, unconditioned H-reflexes were nearly abolished, and a TMS conditioning pulse increased the H-reflex more than fourfold (md 0.3, IQ 0.1-0.7 vs. md 2, IQ 0.9-5.0% of Mmax, p < 0.008). Limb vibration alone had no significant effect on corticospinal excitability. In the absence of vibration, a subthreshold TMS pulse did not influence the soleus H-reflex. During limb vibration, however, while the H-reflex was almost completely suppressed, a subthreshold TMS pulse partially restored the H-reflex. This disinhibition of the H-reflex by a corticospinal signal may represent a mechanism involved in the control of voluntary movement. Corticospinal signals that carry the descending motor command may also reduce presynaptic inhibition, temporarily increasing the impact of sensory inputs on motoneuron activation.

Local vibration training improves the recovery of quadriceps strength in early rehabilitation after anterior cruciate ligament reconstruction: A feasibility randomised controlled trial

BACKGROUND

After anterior cruciate ligament reconstruction (ACLR), quadriceps strength must be maximised as early as possible.

OBJECTIVES

We tested whether local vibration training (LVT) during the early post-ACLR period (i.e., ∼10 weeks) could improve strength recovery.

METHODS

This was a multicentric, open, parallel-group, randomised controlled trial. Thirty individuals attending ACLR were randomised by use of a dedicated Web application to 2 groups: vibration (standardised rehabilitation plus LVT, n=16) or control (standardised rehabilitation alone, n=14). Experimenters, physiotherapists and participants were not blinded. Both groups received 24 sessions of standardised rehabilitation over ∼10 weeks. In addition, the vibration group received 1 hour of vibration applied to the relaxed quadriceps of the injured leg at the end of each rehabilitation session. The primary outcome - maximal isometric strength of both injured and non-injured legs (i.e., allowing for limb asymmetry measurement) - was evaluated before ACLR (PRE) and after the 10-week rehabilitation (POST).

RESULTS

Seven participants were lost to follow-up, so data for 23 participants were used in the complete-case analysis. For the injured leg, the mean (SD) decrease in maximal strength from PRE to POST was significantly lower for the vibration than control group (n=11, -16% [10] vs. n=12, -30% [11]; P=0.0045, Cohen's d effect size=1.33). Mean PRE-POST change in limb symmetry was lower for the vibration than control group (-19% [11] vs. -29% [13]) but not significantly (P=0.051, Cohen's d effect size=0.85).

CONCLUSION

LVT improved strength recovery after ACLR. This feasibility study suggests that LVT applied to relaxed muscles is a promising modality of vibration therapy that could be implemented early in ACLR.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT02929004.

Comparison of the on-line effects of different motor simulation conditions on corticospinal excitability in healthy participants

In healthy participants, corticospinal excitability is known to increase during motor simulations such as motor imagery (MI), action observation (AO) and mirror therapy (MT), suggesting their interest to promote plasticity in neurorehabilitation. Further comparing these methods and investigating their combination may potentially provide clues to optimize their use in patients. To this end, we compared in 18 healthy participants abductor pollicis brevis (APB) corticospinal excitability during MI, AO or MT, as well as MI combined with either AO or MT. In each condition, 15 motor-evoked potentials (MEPs) and three maximal M-wave were elicited in the right APB. Compared to the control condition, mean normalized MEP amplitude (i.e. MEP/M) increased during MI (P = .003), MT (P < .001) and MT + MI (P < .001), without any difference between the three conditions. No MEP modulation was evidenced during AO or AO + MI. Because MI provided no additional influence when combined with AO or MT, our results may suggest that, in healthy subjects, visual feedback and unilateral movement with a mirror may provide the greatest effects among all the tested motor simulations.

Voluntary activation of knee extensor muscles with transcranial magnetic stimulation

We examined if transcranial magnetic stimulation (TMS) is a valid tool for assessment of voluntary activation of the knee extensors in healthy individuals. Maximal M-waves (M_max) of vastus lateralis (VL) were evoked with electrical stimulation of femoral nerve (FNS); M_max of medial hamstrings (HS) was evoked with electrical stimulation of sciatic nerve branches; motor evoked potentials (MEPs) of VL and HS were evoked with TMS; superimposed twitches (SIT) of knee extensors were evoked with FNS and TMS. In study 1, TMS intensity [69% output (SD: 5)] was optimized for MEP sizes, but guidelines for test validity could not be met. Agonist VL MEPs were too small [51.4% M_max (SD: 11.9); guideline ≥70% M_max] and antagonist HS MEPs were too big [16.5% M_max (SD: 10.3); guideline <10% M_max]. Consequently, the TMS estimated resting twitch [99.1 N (SD: 37.2)] and FNS resting twitch [142.4 N (SD: 41.8)] were different. In study 2, SITs at 90% maximal voluntary contraction (MVC) were similar between TMS [16.1 N (SD: 10.3)] and FNS [20.9 N (SD: 16.7)], when TMS intensity was optimized for this purpose, suggesting a procedure that combines TMS SITs with FNS resting twitches could be valid. In study 3, which tested the TMS intensity [56% output (SD: 18)] that evoked the largest SIT at 90% MVC, voluntary activation from TMS [87.3% (SD: 7.1)] and FNS [84.5% (SD: 7.6)] was different. In sum, the contemporary procedure for TMS-based voluntary activation of the knee extensors is invalid. A modified procedure improves validity but only in individuals who meet rigorous inclusion criteria for SITs and MEPs.NEW & NOTEWORTHY We discovered that the contemporary procedure for assessing voluntary activation of the knee extensor muscles with transcranial magnetic stimulation (TMS) is invalid. TMS activates too few agonist quadriceps motoneurons and too many antagonist hamstrings motoneurons to estimate the resting twitch accurately. A modified procedure, in which TMS-evoked superimposed twitches are considered together with the resting twitch from femoral nerve stimulation, is valid but only in select individuals who meet rigorous eligibility criteria.

Time course of the effects of vibration on quadriceps function in individuals with anterior cruciate ligament reconstruction

Quadriceps dysfunction is a common, chronic complication following anterior cruciate ligament reconstruction (ACLR) that contributes to aberrant gait biomechanics and poor joint health. Vibration enhances quadriceps function in individuals with ACLR, but the duration of these effects is unknown. This study evaluated the time course of the effects of whole body vibration (WBV) and local muscle vibration (LMV) on quadriceps function. Twenty-four volunteers with ACLR completed 3 testing sessions during which quadriceps isometric peak torque, rate of torque development, and EMG amplitude were assessed prior to and immediately, 10, 20, 30, 45, and 60 min following a WBV, LMV, or control intervention. WBV and LMV (30 Hz, 2g) were applied during six one-minute bouts. WBV increased peak torque 5-11% relative to baseline and control at all post-intervention time points. LMV increased peak torque 6% relative to baseline at 10 min post-intervention and 4-6% relative to control immediately, 10 min, and 20 min post-intervention. The interventions did not influence EMG amplitudes or rate of torque development. The sustained improvements in quadriceps following vibration, especially WBV, suggest that it could be applied at the beginning of rehabilitation sessions to "prime" the central nervous system, potentially improving the efficacy of ACLR rehabilitative exercise.

Acute effect of tendon vibration applied during isometric contraction at two knee angles on maximal knee extension force production

The aim of the current study was to investigate the effect of a single session of prolonged tendon vibration combined with low submaximal isometric contraction on maximal motor performance. Thirty-two young sedentary adults were assigned into two groups that differed based on the knee angle tested: 90° or 150° (180° = full knee extension). Participants performed two fatigue-inducing exercise protocols: one with three 10 min submaximal (10% of maximal voluntary contraction) knee extensor contractions and patellar tendon vibration (80 Hz) another with submaximal knee extensor contractions only. Before and after each fatigue protocol, maximal voluntary isometric contractions (MVC), voluntary activation level (assessed by the twitch interpolation technique), peak-to-peak amplitude of maximum compound action potentials of vastus medialis and vastus lateralis (assessed by electromyography with the use of electrical nerve stimulation), peak twitch amplitude and peak doublet force were measured. The knee extensor fatigue was significantly (P<0.05) greater in the 90° knee angle group (-20.6% MVC force, P<0.05) than the 150° knee angle group (-8.3% MVC force, P = 0.062). Both peripheral and central alterations could explain the reduction in MVC force at 90° knee angle. However, tendon vibration added to isometric contraction did not exacerbate the reduction in MVC force. These results clearly demonstrate that acute infrapatellar tendon vibration using a commercial apparatus operating at optimal conditions (i.e. contracted and stretched muscle) does not appear to induce knee extensor neuromuscular fatigue in young sedentary subjects.

Age-Related Differences in the Effect of Prolonged Vibration on Maximal and Rapid Force Production and Balance Ability

We tested the hypothesis that older adults would not likely experience deficits in maximal and explosive plantar flexion strength and standing balance performance induced by prolonged Achilles tendon vibration compared with young adults. Fifteen older men (OM, 73 ± 5 years) and 15 young men (YM, 24 ± 4 years) participated in two interventions on different days: lying in a quiet supine position for 30 min with or without prolonged vibration to the Achilles tendon. Before and after the interventions, maximal voluntary contraction (MVC) torque during plantar flexion, rate of torque development (RTD), and center of pressure (COP) speed during single-leg standing were measured. The root mean square of the electromyogram (RMS-EMG) during performance and V-wave and voluntary activation during MVC were assessed. The MVC torque (7 ± 7%) and RTD (16 ± 15%) of YM but not OM significantly decreased after vibration. In addition, the relative changes observed in YM positively correlated with changes in RMS-EMG of the medial gastrocnemius (MG) (MVC torque and RTD) and in MG V-wave and voluntary activation (MVC torque). COP speed significantly increased (16 ± 20%) in YM only after vibration and was accompanied by increased activation of the lateral gastrocnemius. This is the first study to show that the effects of prolonged Achilles tendon vibration on strength and balance performances were apparent in young adults only. The differences between the age groups may be related to the attenuated gastrocnemius neuromuscular function in older adults.

Single bout of vibration-induced hamstrings fatigue reduces quadriceps inhibition and coactivation of knee muscles after anterior cruciate ligament (ACL) reconstruction

Persistent quadriceps strength deficits in individuals with anterior cruciate ligament reconstruction (ACLr) have been attributed to arthrogenic muscle inhibition (AMI). The purpose of the present study was to investigate the effect of vibration-induced hamstrings fatigue on AMI in patients with ACLr. Eight participants with unilateral ACLr (post-surgery time: M = 46.5, SD = 23.5 months; age: M = 21.4, SD = 1.4 years) and eight individuals with no previous history of knee injury (age: M = 22.5, SD = 2.5 years) were recruited. A fatigue protocol, consisting of 10 min of prolonged local hamstrings vibration, was applied to both the ACLr and control groups. The central activation ratio (CAR) of the quadriceps was measured with a superimposed burst of electrical stimulation, and hamstrings/quadriceps coactivation was assessed using electromyography (EMG) during isometric knee extension exercises, both before and after prolonged local vibration. For the ACLr group, the hamstrings strength, measured by a load cell on a purpose-built chair, was significantly (P = 0.016) reduced about 14.5%, indicating fatigue was actually induced in the hamstrings. At baseline, the ACLr group showed a trend (P = 0.051) toward a lower quadriceps CAR (M = 93.2%, SD = 6.2% versus M = 98.1%, SD = 1.1%) and significantly (P = 0.001) higher hamstrings/quadriceps coactivation (M = 15.1%, SD = 6.2% versus M = 7.5%, SD = 4.0%) during knee extension compared to the control group. The fatigue protocol significantly (P = 0.001) increased quadriceps CAR (from M = 93.2%, SD = 6.2% to M = 97.9%, SD = 2.8%) and significantly (P = 0.006) decreased hamstrings/quadriceps coactivation during knee extension (from M = 15.1%, SD = 6.2% to M = 9.5%, SD = 4.5%) in the ACLr group. In conclusion, vibration-induced hamstrings fatigue can alleviate AMI of the quadriceps in patients with ACLr. This finding has clinical implications in the management of recovery for ACLr patients with quadriceps strength deficits and dysfunction.

Age-related Deficits in Voluntary Activation: A Systematic Review and Meta-analysis

Whether there are age-related differences in neural drive during maximal effort contractions is not clear. This review determined the effect of age on voluntary activation during maximal voluntary isometric contractions. The literature was systematically reviewed for studies reporting voluntary activation quantified with the interpolated twitch technique (ITT) or central activation ratio (CAR) during isometric contractions in young (18-35 yr) and old adults (>60 yr; mean, ≥65 yr). Of the 2697 articles identified, 54 were eligible for inclusion in the meta-analysis. Voluntary activation was assessed with electrical stimulation and transcranial magnetic stimulation on five different muscle groups. Random-effects meta-analysis revealed lower activation in old compared with young adults (d = -0.45; 95% confidence interval, -0.62 to -0.29; P < 0.001), with moderate heterogeneity (52.4%). To uncover the sources of heterogeneity, subgroup analyses were conducted for muscle group, calculation method (ITT or CAR), and stimulation type (electrical stimulation or transcranial magnetic stimulation) and number (single, paired, or train stimulations). The age-related reduction in voluntary activation occurred for all muscle groups investigated except the ankle dorsiflexors. Both ITT and CAR demonstrated an age-related reduction in voluntary activation of the elbow flexors, knee extensors, and plantar flexors. ITT performed with paired and train stimulations showed lower activation for old than young adults, with no age difference for the single electrical stimulation. Together, the meta-analysis revealed that healthy older adults have a reduced capacity to activate some upper and lower limb muscles during maximal voluntary isometric contractions; however, the effect was modest and best assessed with at least paired stimulations to detect the difference.

Improving the measurement of TMS-assessed voluntary activation in the knee extensors

PURPOSE

To test the accuracy, validity, reliability and sensitivity of an alternative method for the measure of TMS-assessed voluntary activation (VATMS) in the knee extensors.

METHODS

Ten healthy males (24 ± 5 years) completed a neuromuscular assessment protocol before and after a fatiguing isometric exercise: two sets of five contractions (50%, 62.5%, 75%, 87.5%, 100% Maximal Voluntary Contraction; MVC) with superimposed TMS-evoked twitches for calculation of VATMS using either the first 5 stimulations (1x5C) or all 10 (2x5C). This was performed on two separate occasions (between-day reliability). Accuracy and validity were compared with a routinely used protocol [i.e. 50%, 75%, and 100% of MVC (1x3C) performed three times (3x3C)].

RESULTS

95% confidence interval for estimated resting twitch, a key determinant of VATMS, was similar between 1x5C, 2x5C, and 3x3C but improved by six-fold when compared to 1x3C (P<0.05). In a fresh state, potentiated twitch force was unchanged following 1x5C but decreased following 2x5C (P<0.05). A recovery was found post-exercise but was smaller for 1x5C compared to 2x5C (P<0.05), with no difference between the latter two (P>0.05). Absolute reliability was strong enough for both 1x5C and 2x5C to depict a true detectable change in the sample's VATMS following the fatiguing exercise (TEM < 3% at rest, <9% post-exercise) but 2x5C was marginally more sensitive to individual's changes from baseline.

CONCLUSION

Both 1x5C and 2x5C provide reliable measures of VATMS. However, 1x5C may hold stronger internal validity. Both protocols allow detection of 'true' changes in sample's means but not individual scores following a fatiguing isometric exercise.

Targeted Subcutaneous Vibration With Single-Neuron Electrophysiology As a Novel Method for Understanding the Central Effects of Peripheral Vibrational Therapy in a Rodent Model

Very little is known about the effects of whole body vibration on the supraspinal central nervous system. Though much clinical outcome data and mechanistic data about peripheral neural and musculoskeletal mechanisms have been explored, the lack of central understanding is a barrier to evidence-based, best practice guidelines in the use of vibrational therapy. Disparate methods of administration render study to study comparisons difficult. To address this lack of uniformity, we present the use of targeted subcutaneous vibration combined with simultaneous in vivo electrophysiological recordings as a method of exploring the central effects of peripheral vibration therapy. We used implanted motors driven by both Grass stimulators and programmed microcontrollers to vary frequency and location of stimulation in an anesthetized in vivo rat model while simultaneously recording firing rate from gamma-aminobutyric acid (GABA) neurons in the ventral tegmental area. We show that peripheral vibration can alter GABA neuron firing rate in a location- and frequency-dependent manner. We include detailed schematics and code to aid others in the replication of this technique. This method allows for control of previous weaknesses in the literature including variability in body position, vibrational intensity, node and anti-node interactions with areas of differing mechanoreceptor densities, and prefrontal cortex influence.

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

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 (VIBMU), and tendon vibration (VIBTD). Three muscle lengths were tested for each condition, i.e., short/intermediate/long KE muscle length. Both VIBMU and VIBTD significantly increase MEPs compared to CON. Higher increases (P < 0.001) were found for VIBTD compared to VIBMU 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 VIBTD and VIBMU: +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.

Active versus local vibration warm-up effects on knee extensors stiffness and neuromuscular performance of healthy young males

OBJECTIVES

To compare the effects of local-vibration and active warm-up on knee extensors muscle stiffness and neuromuscular performance.

DESIGN

Experimental crossover study.

METHODS

Thirteen participants performed three 15-min warm-up protocols of control (CON), active (ACT) and local-vibration (LV) in separate testing session. Passive stiffness of vastus lateralis (VL) and vastus medialis (VM) by shear wave elastography and neuromuscular performance were assessed before and 2-min after each condition.

RESULTS

A decrease in muscle stiffness was reported after ACT for VL (-16.0±6.6%; p<0.001) and VM (-10.2±8.7%; p=0.03) while no changes were reported after CON (p=0.46 and p=0.34 for VL and VM, respectively) and LV (p=0.07 and p=0.46 for VL and VM, respectively). Maximal jump performances increased during squat (+8.5±6.6%; p<0.001) and countermovement jump (+5.2±5.8%; p<0.001) after ACT while no changes were reported after CON and LV during squat (p=0.16 and p=0.81, respectively) and countermovement jump (p=0.18 and p=0.31, respectively). We further report that each condition was ineffective to inducing changes in maximal voluntary isometric contraction force (p=0.18), rate of force development (p=0.92), twitch parameters (p>0.05) as well as central modulations as reported by the unchanged voluntary activation level (p=0.24) and maximal electromyography (EMG) recorded from the VL (p=0.44).

CONCLUSIONS

The active warm-up acutely reduced muscle stiffness and increased muscle performance during maximal dynamic tasks. With regard to LV, further studies are required to determine optimal parameters (frequency, amplitude, duration) to significantly increase muscle performance.

Neural adaptations in quadriceps muscle after 4 weeks of local vibration training in young versus older subjects

This study investigated the effects of a 4-week local vibration training (LVT) on the function of the knee extensors and corticospinal properties in healthy young and older subjects. Seventeen subjects (9 young and 8 older) performed 3 testing sessions: before (PRE₁) and after (PRE₂) a 4-week resting period to control the repeatability of the data as well as after the LVT (POST). Jump performance, maximal voluntary contraction (MVC) and electromyographic (EMG) activity on vastus lateralis and rectus femoris muscles were assessed. Single-pulse transcranial magnetic stimulation (TMS) allowed evaluation of cortical voluntary activation (VA_TMS), motor evoked potential (MEP) area, and silent period (SP) duration. All training adaptations were similar between young and older subjects (p > 0.05) and the following results reflect the pooled sample of subjects. MVC (+11.9% ± 8.0%, p < 0.001) and VA_TMS (+3.6% ± 5.2%, p = 0.004) were significantly increased at POST compared with PRE₂. Maximal vastus lateralis EMG was significantly increased at POST (+21.9% ± 33.7%, p = 0.03). No changes were reported for MEPs on both muscles (p > 0.05). SPs recorded during maximal and submaximal contractions decreased in both muscles at POST (p < 0.05). Vertical jump performance was increased at POST (p < 0.05). LVT seems as effective in young as in older subjects to improve maximal functional capacities through neural modulations occurring at least partly at the supra-spinal level. Local vibration may be used as an efficient alternative training method to improve muscular performance in both healthy young and older subjects.