Postvibration depression of the H-reflex as a result of a dual mechanism: an experimental study in humans

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.

Tactile stimulation restores inhibited stretch reflex attributable to attenuation of Ia afferents during surprise landing

Arthrogenic muscle inhibition (AMI) is induced by pathological knee conditions. The present study aimed to investigate the effect of tactile stimulation on reflex changes induced by simulated AMI during unpredictable landing performances. Twenty participants performed six unilateral landing tasks: 15 cm normal landing (15NL), 30 cm normal landing (30NL), surprise landing (SL), 30 cm normal landing following vibration (30NLV), SL following vibration (SLV), and SL following vibration with Kinesiology tape (SLK). For SL, the solid landing platform (15 cm) was removed and replaced by a false floor. Since the false floor dislodged easily under load, participants unpredictably fell through the platform to the actual landing surface 15 cm below. After completing 15NL, 30NL, and SL, vibration was applied to participants' knees to induce neurological changes similar to AMI. After vibration, participants performed 30NLV, SLV, and SLK in a random order. EMG signals in the post-landing short latency (31-60 ms) and medium latency (61-90 ms) periods were examined. EMG signals from the vastus lateralis (VL), vastus medialis (VM), and biceps femoris (BF) were recorded and compared between tasks. EMG signals of all muscles in SL were significantly enhanced in the medium latency period as compared with 30NL. Enhanced EMG signals in SL were suppressed by vibration stimulation in the VL, but the suppressed EMG signals were restored after cutaneous stimulation with Kinesiology tape (p < 0.01). Our findings suggest that AMI could alter motor control patterns during unpredictable landing and that tactile stimulation could restore the altered motor control to a normal state.

Inhibitory Effects of Prolonged Focal Muscle Vibration on Maximal Grip Strength and Muscle Activity of Wrist and Extrinsic Finger Flexor Muscles

Objective

The objective of this study was to identify effective stimulus time by quantifying the inhibitory effects of focal muscle vibration (FMV) on maximal grip strength and muscle activities of the wrist and extrinsic finger flexors.

Methods

A randomized repeated-measures design was used in this study. A total of 22 healthy volunteers (mean age, 20.9 years) participated. An FMV of 86 Hz was applied to the anterior surface of the distal forearm under the following 3 conditions: no FMV (control), 5-minute FMV, and 10-minute FMV. Maximal grip strength was measured before and after FMV. The muscle activities of the flexor digitorum superficialis, flexor digitorum profundus (FDP), and flexor carpi ulnaris were simultaneously recorded using surface electromyography. Discomfort and complications following FMV were also assessed.

Results

Compared with the control group, a significant decrease in muscle activity was observed in both the flexor digitorum superficialis and flexor carpi ulnaris after 5 and 10 minutes of FMV. In contrast, there was no significant decrease in the maximal grip strength or FDP muscle activity after either FMV condition. The discomfort was significantly higher immediately after both FMV conditions than in the control group, but it decreased 15 minutes after FMV, indicating no significant difference among the 3 conditions. Redness and/or swelling were observed in 13.6% and 36.3% of the participants after 5 and 10 minutes of FMV, respectively.

Conclusion

Five-minute FMV to the distal forearm could be a useful therapeutic method with few complications. However, the FMV in this area alone was not sufficient to suppress the muscle activity of the FDP located in the deep layer.

Effectiveness and Electrophysiological Mechanisms of Focal Vibration on Upper Limb Motor Dysfunction in Patients with Subacute Stroke: A Randomized Controlled Trial

Upper limb motor dysfunction is a common complication after stroke, which has a negative impact on the daily life of patients. Focal vibration (FV) has been used to improve upper limb motor function in acute and chronic stroke patients, but its application in subacute stroke patients has not been extensively explored. Therefore, the purpose of this study was to explore the therapeutic effect of FV on upper limb motor function in subacute stroke patients and its underlying electrophysiological mechanism. Twenty-nine patients were enrolled and randomized into two groups: control group and vibration group. The control group were treated with conventional therapy including passive and active physical activity training, standing and sitting balance exercises, muscle strength training, hand extension and grasping exercises. The vibration group were given conventional rehabilitation and vibration therapy. A deep muscle stimulator (DMS) with a frequency of 60 Hz and an amplitude of 6 mm was used to provide vibration stimulation, which was sequentially applied along the biceps muscle to the flexor radialis of the affected limb for 10 minutes, once a day, and 6 times a week. Both groups received treatments for 4 consecutive weeks. In the vibration group, the motor evoked potential (MEP) latency and the somatosensory evoked potential (SEP) latency were significantly shortened (P<0.05) immediately after vibration and 30 minutes after vibration; the SEP amplitude and MEP amplitude were significantly increased (P<0.05) immediately after vibration and 30 minutes after vibration. The MEP latency (P=0.001) and SEP N20 latency (P=0.001) were shortened, and the MEP amplitude (P=0.011) and SEP N20 amplitude (P=0.017) were significantly increased after 4 weeks in the vibration group. After 4 consecutive weeks, the vibration group showed significant improvements in Modified Ashworth Scale (MAS) (P=0.037), Brunnstrom stage for upper extremity (BS-UE) (P=0.020), Fugl-Meyer assessment for upper extremity (FMA-UE) (P=0.029), Modified Barthel Index (MBI) (P=0.024), and SEP N20 (P=0.046) compared to the control group. The Brunnstrom stage for hand (BS-H) (P=0.451) did not show significant differences between the two groups. This study showed that FV was effective in improving upper limb motor function in subacute stroke patients. The underlying mechanism of FV may be that it enhances the efficacy of sensory pathways and induces plastic changes in the sensorimotor cortex.

Vibration decreases the responsiveness of Ia afferents and spinal motoneurons in humans

After vibration, Hoffmann reflex (H reflex) amplitude is depressed; however, the mechanisms underlying these phenomena remain unknown. This study investigated the influence of frequency and duration of vibration on the H reflex amplitude, heteronymous facilitation of the tendon jerk (T wave) mediated by group Ia afferents, and cervicomedullary motor evoked potential (CMEP) amplitude in 18 healthy human subjects. The H reflex of the flexor carpi radialis (FCR) was induced by median nerve stimulation at the elbow, and the conditioning FCR stimulation enhanced the T wave of the biceps brachii (BB). After vibration was applied to the FCR muscle belly, the amplitudes of the H reflex and heteronymous facilitation of the T wave were depressed; these influences persisted after the removal of vibration in all subjects. For the H reflex, there was no difference in the amount of depression among the frequencies of vibration used (57, 77, and 100 Hz). Higher frequencies of vibration were associated with longer recovery times of postvibration depression, and a longer duration of vibration was associated with a longer recovery time of the depression. Similar results were observed for heteronymous facilitation of the T wave, suggesting that the depression is caused by a decrease in postsynaptic potentials evoked by Ia afferents in spinal motoneurons; it was probably due to reduction in the number of Ia afferents recruited by the median nerve stimulation. Moreover, because the FCR CMEP amplitude was depressed after vibration, vibration should affect the responsiveness of spinal motoneurons. These mechanisms are considered to contribute to the H reflex depression after vibration.NEW & NOTEWORTHY Vibration decreased the responsiveness of Ia afferents from the muscle exposed to vibration, and the duration of depressive effect was modulated by the duration and frequency of the vibration: a longer duration and a higher frequency of vibration led to a longer recovery time of the depression. In addition to this presynaptic effect, it also depressed the responsiveness of spinal motoneurons, indicating postsynaptic inhibition through specific circuits triggered by Ia impulses.

Plasticity Induced in the Human Spinal Cord by Focal Muscle Vibration

The spinal cord spinal cord has in the past been considered a hardwired system which responds to inputs in a stereotyped way. A growing body of data have instead demonstrated its ability to retain information and modify its effector capabilities, showing activity-dependent plasticity. Whereas, plasticity in the spinal cord is well documented after different forms of physical exercise, whether exogenous stimulation can induce similar changes is still a matter of debate. This issue is both of scientific and clinical relevance, since at least one form of stimulation, i.e., focal muscle vibration (fMV), is currently used as a treatment for spasticity. The aim of the present study was to assess whether fMV can induce plasticity at the SC level when applied to different muscles of the upper limb. Changes in different electrophysiological measures, such as H-reflex testing homonymous and heteronymous pathways, reciprocal inhibition and somatosensory evoked potentials were used as outcomes. We found that fMV was able to induce long-term depression-like plasticity in specific spinal cord circuits depending on the muscle vibrated. These findings helped understand the basic mechanisms underlying the effects of fMV and might help to develop more advanced stimulation protocols.

Acute and chronic neuromuscular adaptations to local vibration training

Vibratory stimuli are thought to have the potential to promote neural and/or muscular (re)conditioning. This has been well described for whole-body vibration (WBV), which is commonly used as a training method to improve strength and/or functional abilities. Yet, this technique may present some limitations, especially in clinical settings where patients are unable to maintain an active position during the vibration exposure. Thus, a local vibration (LV) technique, which consists of applying portable vibrators directly over the tendon or muscle belly without active contribution from the participant, may present an alternative to WBV. The purpose of this narrative review is (1) to provide a comprehensive overview of the literature related to the acute and chronic neuromuscular changes associated with LV, and (2) to show that LV training may be an innovative and efficient alternative method to the 'classic' training programs, including in the context of muscle deconditioning prevention or rehabilitation. An acute LV application (one bout of 20-60 min) may be considered as a significant neuromuscular workload, as demonstrated by an impairment of force generating capacity and LV-induced neural changes. Accordingly, it has been reported that a training period of LV is efficient in improving muscular performance over a wide range of training (duration, number of session) and vibration (frequency, amplitude, site of application) parameters. The functional improvements are principally triggered by adaptations within the central nervous system. A model illustrating the current research on LV-induced adaptations is provided.

Neurophysiological characterization of transpinal evoked potentials in human leg muscles

The objectives of this study were to characterize the neurophysiological properties of the compound muscle action potentials (CMAPs) evoked by transcutaneous electric stimulation of the spine (tsESS), and the effects of tsESS on the soleus H-reflex in seated and standing healthy human subjects. In seated semi-prone subjects with the trunk semi-flexed, two re-usable self-adhering electrodes (cathode), connected to act as one electrode, were placed bilaterally on the iliac crests. A re-usable pregelled electrode (anode) was placed on the thoracolumbar region at thoracic 10-lumbar 1 and held under constant pressure throughout the experiment. CMAPs were recorded bilaterally from ankle muscles with subjects seated semi-prone at 1.0, 0.3, 0.2, 0.125, and 0.1 Hz following tsESS. The soleus H-reflex, evoked by posterior tibial nerve stimulation via conventional methods, was measured following tsESS at inter-stimulus intervals (ISIs) that ranged from -100 to 100 ms with the subjects seated semi-prone and during standing. The tsESS-induced CMAPs were not decreased at low stimulation frequencies, and the soleus H-reflex excitability was profoundly decreased by tsESS at ISIs that ranged from -5 to 20 ms with the subjects seated semi-prone and during standing. CMAPs induced by tsESS may be utilized to assess spinal-to-muscle conduction time while bypassing spinal motoneuron excitability and tsESS can be used as a modality to decrease spinal reflex hyper-excitability in neurological disorders.

Effects of mechanical vibration of the foot sole and ankle tendons on cutaneomuscular responses in man

The modulation of cutaneomuscular responses in response to mechanical vibration applied to the foot sole and to the ankle tendons was established in ten healthy subjects. The effects of mechanical vibration applied to the skin adjacent to the tibialis anterior (TA) and Achilles tendons were examined in two subjects. With the subjects seated, mechanical vibration applied to the TA and/or Achilles tendons significantly depressed the cutaneomuscular responses in all subjects, regardless of the frequency (50, 150, 250 Hz) of vibration. Mechanical vibration applied either to the foot sole or to the skin adjacent to the tendons induced no significant effects. The demonstration that mechanical vibration applied to muscle tendons exerts an inhibitory effect on cutaneomuscular responses supports the hypothesis that receptors that mediate body kinesthesia can be used as a vehicle to alter the spinal excitability state. The data suggests that tendon vibration could be utilized in neurological disorders to induce exogenous-mediated potentiation of presynaptic inhibition.

Neurophysiological characteristics of human leg muscle action potentials evoked by transcutaneous magnetic stimulation of the spine

The objectives of this study were to establish the neurophysiological properties of the compound muscle action potentials (CMAPs) evoked by transcutaneous magnetic stimulation of the spine (tsMSS) and the effects of tsMSS on the soleus H-reflex. In semi-prone seated subjects with trunk semi-flexed, the epicenter of a figure-of-eight magnetic coil was placed at Thoracic 10 with the handle on the midline of the vertebral column. The magnetic stimulator was triggered by monophasic single pulses of 1 ms, and the intensity ranged from 90% to 100% of the stimulator output across subjects. CMAPs were recorded bilaterally from ankle and knee muscles at the interstimulus intervals of 1, 3, 5, 8, and 10 s. The CMAPs evoked were also conditioned by posterior tibial and common peroneal nerve stimulation at a conditioning-test (C-T) interval of 50 ms. The soleus H-reflex was conditioned by tsMSS at the C-T intervals of 50, 20, -20, and -50 ms. The amplitude of the CMAPs was not decreased when evoked at low stimulation frequencies, excitation of group I afferents from mixed peripheral nerves in the leg affected the CMAPs in a non-somatotopical neural organization pattern, and tsMSS depressed soleus H-reflex excitability. These CMAPs are likely due to orthodromic excitation of nerve motor fibers and antidromic depolarization of different types of afferents. The latency of these CMAPs may be utilized to establish the spine-to-muscle conduction time in central and peripheral nervous system disorders in humans. tsMSS may constitute a non-invasive modality to decrease spinal reflex hyperexcitability and treat hypertonia in neurological disorders.

Prolonged Vibration of the Biceps Brachii Tendon Reduces Time to Failure When Maintaining Arm Position With a Submaximal Load

Vibration reduces the amplitudes of the tendon jerk response and the Hoffmann and stretch reflexes in the muscle exposed to the vibration, yet does not alter the time to task failure when the task involves exerting a submaximal force against a rigid restraint. Because the amplitude of the stretch reflex is greater when a limb acts against a compliant load than a rigid restraint, the purpose was to determine the influence of prolonged tendon vibration on the time to failure when maintaining limb position with the elbow flexor muscles. Twenty-five healthy men performed the fatiguing contraction by maintaining elbow angle at 1.57 rad until failure while supporting a load equal to 20% of maximal voluntary contraction (MVC) force. The fatiguing contraction was performed on 3 separate days with different levels of vibration applied to the biceps brachii tendon: no vibration, subthreshold for a tonic vibration reflex (TVR), and suprathreshold for a TVR. MVC force before the fatiguing contraction was similar across the three sessions (mean of 3 sessions: 313 ± 54 N, P = 0.83). Despite a similar decline in MVC force after the fatiguing contraction across conditions (–18.0 ± 8.0%, P > 0.05), the time to task failure was 3.7 ± 1.4 min for the suprathreshold TVR condition, 4.3 ± 2.1 min for the subthreshold TVR condition, and 5.0 ± 2.2 min for the no-vibration condition ( P < 0 0.001). The average EMG of the elbow flexor muscles was similar ( P = 0.22) during the fatiguing contractions. However, the fluctuations in limb acceleration at task onset were greater for the suprathreshold TVR condition ( P < 0.01), but were not different between the subthreshold TVR and no-vibration conditions ( P ≥ 0.22). Furthermore, the difference in the SD of limb acceleration between the no-vibration and vibration conditions was correlated with the difference in time to failure for the no-vibration and subthreshold TVR conditions ( P = 0.03; r2 = 0.22), but not for the no-vibration and suprathreshold TVR conditions ( P = 0.90; r2 = 0.001). These findings indicate that prolonged vibration reduced the time to failure of a sustained contraction when subjects maintained limb position, suggesting that peripheral inputs to the motor neuron pool play a significant role in sustaining a contraction during tasks that require active control of limb position.

Prolonged muscle vibration increases stretch reflex amplitude, motor unit discharge rate, and force fluctuations in a hand muscle

The purpose of this study was to compare the influence of prolonged vibration of a hand muscle on the amplitude of the stretch reflex, motor unit discharge rate, and force fluctuations during steady, submaximal contractions. Thirty-two young adults performed 10 isometric contractions at a constant force (5.0 +/- 2.3% of maximal force) with the first dorsal interosseus muscle. Each contraction was held steady for 10 s, and then stretch reflexes were evoked. Subsequently, 20 subjects had vibration applied to the relaxed muscle for 30 min, and 12 subjects received no vibration. The muscle vibration induced a tonic vibration reflex. The intervention (vibration or no vibration) was followed by 2 sets of 10 constant-force contractions with applied stretches (After and Recovery trials). The mean electromyogram amplitude of the short-latency component of the stretch reflex increased by 33% during the After trials (P < 0.01) and by 38% during the Recovery trials (P < 0.01). The standard deviation of force during the steady contractions increased by 21% during the After trials (P < 0.05) and by 28% during the Recovery trials (P < 0.01). The discharge rate of motor units increased from 10.3 +/- 2.7 pulses/s (pps) before vibration to 12.2 +/- 3.1 pps (P < 0.01) during the After trials and to 11.9 +/- 2.6 pps during the Recovery trials (P < 0.01). There was no change in force fluctuations or stretch reflex magnitude for the subjects in the Control group. The results indicate that prolonged vibration increased the short-latency component of the stretch reflex, the discharge rate of motor units, and the fluctuations in force during contractions by a hand muscle. These adjustments were necessary to achieve the target force due to the vibration-induced decrease in the force capacity of the muscle.