Achilles Tendon Vibration Research Articles

Reduction and recovery of self-sustained muscle activity after fatiguing plantar flexor contractions.

Persistent inward calcium and sodium currents (PICs) are crucial for initiation and maintenance of motoneuron firing, and thus muscular force. However, there is a lack of data describing the effects of fatiguing exercise on PIC activity in humans. We simultaneously applied tendon vibration and neuromuscular electrical stimulation (VibStim) before and after fatiguing exercise. VibStim induces self-sustained muscle activity that is proposed to result from PIC activation. Twelve men performed 5-s maximal isometric plantar flexor contractions (MVC) with 5-s rests until joint torque was reduced to 70%MVC. VibStim trials consisted of five 2-s trains of neuromuscular electrical stimulation (20 Hz, evoking 10% MVC) of triceps surae with simultaneous Achilles tendon vibration (115 Hz) without voluntary muscle activation. VibStim was applied before (PRE), immediately (POST), 5-min (POST-5), and 10-min (POST-10) after exercise completion. Sustained torque (Tsust) and soleus electromyogram amplitudes (EMG) measured 3 s after VibStim were reduced (Tsust: -59.0%, p<0.001; soleus EMG: -38.4%, p<0.001) but largely recovered by POST-5, and changes in MVC and Tsust were correlated across the four time points (r=0.69; p<0.001). After normalisation to values obtained at the end of the vibration phase to control for changes in fibre-specific force and EMG signal characteristics, decreases in Tsust (-42.9%) and soleus EMG (-22.6%) remained significant and were each correlated with loss and recovery of MVC (r=0.41 and 0.46, respectively). The parallel changes observed in evoked self-sustained muscle activity and force generation capacity provide motivation for future examinations on the potential influence of fatigue-induced PIC changes on motoneuron output.

Do soleus responses to transcutaneous spinal cord stimulation show similar changes to H-reflex in response to Achilles tendon vibration?

Recently, the use of transcutaneous spinal cord stimulation (TSCS) has been proposed as a viable alternative to the H-reflex. The aim of the current study was to investigate to what extent the two modes of spinal cord excitability investigation would be similarly sensitive to the well-known vibration-induced depression. Fourteen healthy participants (8 men and 6 women; age: 26.7±4.8years) were engaged in the study. The right soleus H-reflex and TSCS responses were recorded at baseline (PRE), during right Achilles tendon vibration (VIB) and following 20min of vibration exposure (POST-VIB). Care was taken to match H-reflex and TSCS responses amplitude at PRE and to maintain effective stimulus intensities constant throughout time points. The statistical analysis showed a significant effect of time for the H-reflex, with VIB (13±5% of maximal M-wave (Mmax) and POST-VIB (36±4% of Mmax) values being lower than PRE-values (48±6% of Mmax). Similarly, TSCS responses changed over time, VIB (9±5% of Mmax) and POST-VIB (27±5% of Mmax) values being lower than PRE-values (46±6% of Mmax). Pearson correlation analyses revealed positive correlation between H-reflex and TSCS responses PRE-to-VIB changes, but not for PRE- to POST-VIB changes. While the sensitivity of TSCS seems to be similar to the gold standard H-reflex to highlight the vibratory paradox, both responses showed different sensitivity to the effects of prolonged vibration, suggesting slightly different pathways may actually contribute to evoked responses of both stimulation modalities.

Effects of jaw clenching and mental stress on persistent inward currents estimated by two different methods.

Spinal motoneuron firing depends greatly on persistent inward currents (PICs), which in turn are facilitated by the neuromodulators serotonin and noradrenaline. The aim of this study was to determine whether jaw clenching (JC) and mental stress (MS), which may increase neuromodulator release, facilitate PICs in human motoneurons. The paired motor unit (MU) technique was used to estimate PIC contribution to motoneuron firing. Surface electromyograms were collected using a 32-channel matrix on gastrocnemius medialis (GM) during voluntary, ramp, plantar flexor contractions. MU discharges were identified, and delta frequency (ΔF), a measure of recruitment-derecruitment hysteresis, was calculated. Additionally, another technique was used (VibStim) that evokes involuntary contractions that persist after cessation of combined Achilles tendon vibration and triceps surae neuromuscular electrical stimulation. VibStim measures of plantar flexor torque and soleus activity may reflect PIC activation. ΔF was not significantly altered by JC (p = .679, n = 18, 9 females) or MS (p = .147, n = 14, 5 females). However, all VibStim variables quantifying involuntary torque and muscle activity during and after vibration cessation were significantly increased in JC (p < .011, n = 20, 10 females) and some, but not all, increased in MS (p = .017-.05, n = 19, 10 females). JC and MS significantly increased the magnitude of involuntary contractions (VibStim) but had no effect on GM ΔF during voluntary contractions. Effects of increased neuromodulator release on PIC contribution to motoneuron firing might differ between synergists or be context dependent. Based on these data, the background level of voluntary contraction and, hence, both neuromodulation and ionotropic inputs could influence neuromodulatory PIC enhancement.

Postural Responses to Achilles Tendon Vibration Depend on Feet Positioning.

Mechanical vibration of the Achilles tendon is widely used to analyze the role of proprioception in postural control. The response to this tendon vibration (TV) has been analyzed in the upright posture, but the feet positions have varied in past research. Moreover, investigators have addressed only temporal parameters of the center of pressure (CoP). We investigated the effect of TV on both temporal and spectral characteristics of the CoP motion. Eighteen healthy young adults, stood barefoot, with one foot on each side of a dual platform, wearing glasses with opaque lenses. We applied 20seconds of Achilles TV (bilaterally with inertial vibrators at a frequency of 80Hz and an amplitude of .2-.5mm). We analyzed CoP signals pre-vibration (PRE,4-seconds), during vibration (VIB,20 seconds), and after vibration cessation (REC,20 seconds). We repeated this protocol in natural and standardized positions (15° feet angular opening). For determining CoP amplitude and velocity, we divided the 20seconds into five phases of fourseconds each and calculated spectral parameters for the whole 20-second signals. There was an adaptation process in the speed of the CoP mediolateral (p < .01) and anteroposterior (p < .01) and in the displacement of the CoP anteroposterior (p < .01), with higher values in the VIB condition. Velocity and displacement decreased progressively in the REC condition. Median and peak frequencies were higher in the VIB condition when compared to the REC condition, but only in the mediolateral direction (p = .01). The standardized foot position led to increased speed in CoP mediolateral, anteroposterior, and mediolateral displacement (p < .01). CoP spectral characteristics were not affected by foot positioning. We concluded that adaptation of CoP motion in the presence of TV and after its cessation are observable both in time and frequency domains. Feet positioning influenced CoP motion in the presence of TV and after its cessation but it did not affect its spectral characteristics.

Temporal modulation of H-reflex in young and older people: Acute effects during Achilles tendon vibration while standing.

Examining how timely is sensory input processed and regulated after a perturbation while standing, is a key element to understand postural control, especially in people with balance deficits, such as older adults. In this study, we investigated the age-related temporal modulations in spinal excitability, by measuring the soleus H-reflex within the first 250ms after switching on (V-ON) and off (V-OFF) a pair of vibrators, placed over the Achilles tendons. Twenty young (25.1±5.1yrs) and 16 older (71.8±6.1yrs) adults stood blindfolded, while 12-15s periods of Achilles tendon vibration (ATV) were applied with 20-24s intervals. Anterior/posterior center of pressure and electromyographic (EMG) responses [soleus (SOL) and tibialis anterior (TA)] were assessed whereas SOL H-reflex normalized to maximum M-wave (H/Mmax) and SOL EMG (H/SOL) were evaluated before ATV (Pre) and at 50, 100, 150, 200 and 250ms after the V-ON and V-OFF. Only in young adults the TA/SOL EMG ratio decreased 100 (p=0.032) and 150ms (p<0.001) after V-ON and the H/Mmax and H/SOL decreased 150-250ms after V-ON (p<0.001). At 50-250ms after V-OFF, H/Mmax was reduced compared to Pre values (p<0.001), with no differences between the age groups (p>0.05). H/SOL was decreased for the young adults (p<0.001) and remained reduced at least for the first 250ms after V-OFF. At 150ms after V-OFF, SOL/Mmax was decreased only for the older adults (p<0.001), whereas TA/SOL EMG gradually increased for both groups (p<0.001). When ATV is introduced while standing, the spinal excitability of older people is reduced later and to a lesser extent compared to young adults. Their limited capacity to down-regulate the "noisy" sensory input generated by ATV gives further evidence of a possible mechanism for their inefficient postural control.

Effects of reciprocal inhibition and whole-body relaxation on persistent inward currents estimated by two different methods.

Persistent inward currents (PICs) are crucial for initiation, acceleration, and maintenance of motoneuron firing. As PICs are highly sensitive to synaptic inhibition and facilitated by serotonin and noradrenaline, we hypothesised that both reciprocal inhibition (RI) induced by antagonist nerve stimulation and whole‐body relaxation (WBR) would reduce PICs in humans. To test this, we estimated PICs using the well‐established paired motor unit (MU) technique. High‐density surface electromyograms were recorded from gastrocnemius medialis during voluntary, isometric 20‐s ramp, plantarflexor contractions and decomposed into MU discharges to calculate delta frequency (ΔF). Moreover, another technique (VibStim), which evokes involuntary contractions proposed to result from PIC activation, was used. Plantarflexion torque and soleus activity were recorded during 33‐s Achilles tendon vibration and simultaneous 20‐Hz bouts of neuromuscular electrical stimulation (NMES) of triceps surae. ΔF was decreased by RI (n = 15, 5 females) and WBR (n = 15, 7 females). In VibStim, torque during vibration at the end of NMES and sustained post‐vibration torque were reduced by WBR (n = 19, 10 females), while other variables remained unchanged. All VibStim variables remained unaltered in RI (n = 20, 10 females). Analysis of multiple human MUs in this study demonstrates the ability of local, focused inhibition to attenuate the effects of PICs on motoneuron output during voluntary motor control. Moreover, it shows the potential to reduce PICs through non‐pharmacological, neuromodulatory interventions such as WBR. The absence of a consistent effect in VibStim might be explained by a floor effect resulting from low‐magnitude involuntary torque combined with the negative effects of the interventions.Key points Spinal motoneurons transmit signals to skeletal muscles to regulate their contraction. Motoneuron firing partly depends on their intrinsic properties such as the strength of persistent (long‐lasting) inward currents (PICs) that make motoneurons more responsive to excitatory input.In this study, we demonstrate that both reciprocal inhibition onto motoneurons and whole‐body relaxation reduce the contribution of PICs to human motoneuron firing. This was observed through analysis of the firing of single motor units during voluntary contractions.However, an alternative technique that involves tendon vibration and neuromuscular electrical stimulation to evoke involuntary contractions showed less effect. Thus, it remains unclear whether this alternative technique can be used to estimate PICs under all physiological conditions.These results improve our understanding of the mechanisms of PIC depression in human motoneurons. Potentially, non‐pharmacological interventions such as electrical stimulation or relaxation could attenuate unwanted PIC‐induced muscle contractions in conditions characterised by motoneuron hyperexcitability.

Electromyographic responses to unexpected Achilles tendon vibration-induced perturbations during standing in young and older people.

This study aimed to investigate age-related differences in electromyographic (EMG) responses to unexpected Achilles tendon vibration (ATV) perturbations while standing blindfold. ATV with variable and random duration (12-15s) and rest periods (20-24s) was applied on 18 young and 16 older volunteers. The anterior/posterior center of pressure (CoP) and the soleus (SOL) and tibialis anterior (TA) EMG were analyzed for 1s before and 8s after the ATV onset and offset. ATV induced a posterior shift of CoP in both groups, with more pronounced shift in the older group. During ATV onset, the older group demonstrated less SOL and more TA EMG increase compared to the young group. During the first 0.5s of ATV offset, SOL EMG was decreased in both age groups, while TA showed a burst of EMG activity that was greater in the older group. No difference in the latencies of EMG peaks or valleys was observed between the groups. It is concluded that ATV induces greater posterior CoP shift in older adults, and they adopt a recovery strategy, characterized by a decreased SOL activation and an increased TA activation. These differences are possibly attributed to the increased fear of falling, decreased limits of stability and reduced capacity of older people to reweight their sensory inflow when proprioception is distorted.

Concurrent Achilles tendon vibration and tibial nerve stimulation to estimate persistent inward current strength in motoneurons.

Vibratory (Tvib) and sustained (Tsust) torque responses to concurrent Achilles tendon vibration and neuromuscular electrical stimulation applied over the muscle belly (vib+stim) are used as indicators of motoneuron facilitation and, theoretically, persistent inward current strength. However, neuromuscular electrical stimulation (NMES) applied to the nerve trunk may potentiate motoneuronal excitability more than muscle belly NMES, yet it remains unclear whether NMES applied over the nerve evokes robust Tvib and Tsust responses when used during the vib+stim protocol. This study tested whether a nerve-targeted vib+stim protocol elicits Tvib and Tsust responses in the ankle plantar flexors with acceptable intra- and inter-session reliability. Fifteen men performed the vib+stim protocol with NMES applied over the tibial nerve three times across two sessions; twice in a single session (5-min apart) to test intrasession reliability and then again after 48 h to test intersession reliability. Intraclass correlation coefficients (ICC3,1), within-participant coefficients of variation (CV) and pairwise comparisons were used to verify relative and absolute reliability as well as systematic bias. Thirteen men presented Tvib and Tsust responses (response rate of 87%). Intrasession Tvib and Tsust ICCs were >0.73 but inter-session ICCs were <0.5. Although no systematic bias was detected (p>0.05), both intra- and inter-session CVs were large (>10%) for Tvib and Tsust. The Vib+stim protocol with NMES applied over the nerve evoked Tvib and Tsust in almost all participants, but presented a large intra- and inter-session variability. The method does not appear to be effective for assessing motoneuron facilitation in the plantar flexors.

Involuntary sustained firing of plantar flexor motor neurones: effect of electrical stimulation parameters during tendon vibration

PurposeSimultaneous application of tendon vibration and neuromuscular electrical stimulation (NMES) induces an involuntary sustained torque. We examined the effect of different NMES parameters (intensity, pattern of stimulation and pulse width) on the magnitude of the evoked involuntary torque.MethodsPlantar flexor torque was recorded during 33-s Achilles tendon vibration with simultaneous 20-Hz NMES bouts on triceps surae (n = 20; 13 women). Intensity was set to elicit 10, 20 or 30% of maximal voluntary contraction torque (MVC), pulse width was narrow (0.2 ms) or wide (1 ms), and the stimulus pattern varied (5 × 2-s or 10 × 1-s). Up to 12 different trials were performed in a randomized order, and then repeated in those who produced a sustained involuntary torque after the cessation of vibration.ResultsSix of 7 men and 5 of 13 women produced a post-vibration sustained torque. Eight of 20 participants did not complete the 30% trials, as they were perceived as painful. Torque during vibration at the end of NMES and the increase in torque throughout the trial were significantly higher in 20 than 10% trials (n = 11; 9.7 ± 9.0 vs 7.1 ± 6.1% MVC and 4.3 ± 4.5 vs 3.6 ± 3.5% MVC, respectively). Post-vibration sustained torque was higher in wide pulse-width trials (5.4 ± 5.9 vs 4.1 ± 4.3% MVC). Measures of involuntary torque were not different between 20 and 30% trials (n = 8).ConclusionBouts of 5 × 2-s NMES with wide pulse width eliciting 20% MVC provides the most robust responses and could be used to maximise the production of involuntary torque in triceps surae.

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.

Influence of age on the frequency characteristics of the soleus muscle response to Achilles tendon vibration during standing.

Proprioceptive sensory information from the ankle joint is critical for the control of upright posture and balance. We examined the influence of age (n=54 healthy adults, 20-82 years old) on lower limb muscle responses to proprioceptive perturbations evoked by Achilles tendon vibration during standing. The frequency bandwidth of the muscle response became narrower, and the gain (the muscle response relative to the stimulus) and scaling (increases in response amplitude with increases in stimulus amplitude) decreased with age. Mechanics of the muscle-tendon unit (mechanical admittance) did not differ with age during standing, and thus probably did not mediate the age-related changes observed in soleus muscle responses to vibration. These findings add to our understanding of how altered proprioceptive responses may contribute to impaired mobility and falls with ageing. Proprioceptive information from the ankle joint plays an important role in the control of upright posture and balance. Ageing influences many components of the sensorimotor system, which leads to poor mobility and falls. However, little is known about the influence of age on the characteristics of short latency muscle responses to proprioceptive stimuli during standing across frequencies that are encoded by muscle spindles. We examined the frequency characteristics of the soleus muscle response to noisy (10-115Hz) Achilles tendon vibration during standing in 54 healthy adults across a broad age range (20-82 years). The results showed the frequency bandwidth of the soleus response (vibration-electromyography coherence) became progressively narrower with ageing. Coherence was significantly lower in middle-aged relative to young adults between ∼7-11 and 28-62Hz, lower in older relative to middle-aged adults between ∼30-50Hz and lower in older relative to young adults between ∼7-64Hz. Muscle response gain was similar between age groups at low frequencies, although gain was lower in older relative to young adults between ∼28-54Hz. Across the age range, the response amplitude (peak-to-peak cross-covariance) and the scaling of the response with stimulus amplitude were both negatively correlated with age. Muscle-tendon mechanics (admittance) did not differ with age, suggesting this did not mediate differences in soleus responses. Our findings suggest there is a progressive change in the soleus response to proprioceptive stimuli with ageing during standing, which could contribute to poorer mobility and falls.

Frequency characteristics of heteronymous responses evoked by Achilles tendon vibration during quiet stance

Primary (Ia) sensory afferents that innervate muscle spindles provide strong synaptic input to homonymous motoneurons and are thought to play a role in balance control. In addition, Ia afferents have broad heteronymous connections; i.e., projections to motoneurons that innervate other muscles that act at the same joint as well as at different joints. The purpose of the current study was to investigate heteronymous Ia afferent connections from the triceps surae muscles to lower limb and back muscles during quiet standing in humans. We applied supra-threshold noisy vibration (10−115 Hz) to the right Achilles tendon of 12 participants maintaining quiet stance and recorded EMG activity bilaterally from homonymous (Soleus) and heteronymous muscles (Semitendinosus, Vastus Lateralis, Erector Spinae). We estimated coherence, phase, and gain between the tendon probe acceleration and rectified EMG from each muscle. We found significant coherence between the probe acceleration and EMG in ipsilateral Soleus (5−100 Hz), Semitendinosus (10−75 Hz), Vastus Lateralis (5-70 Hz), and bilateral Erector Spinae muscles (10−70 Hz). These results provide evidence that triceps surae muscle spindle afferents can influence the activity of muscles proximal to the ankle joint across a broad frequency band during quiet standing.

Vibration attenuates spasm-like activity in humans with spinal cord injury.

Cutaneous reflexes were tested to examine the neuronal mechanisms contributing to muscle spasms in humans with chronic spinal cord injury (SCI). Specifically, we tested the effect of Achilles and tibialis anterior tendon vibration on the early and late components of the cutaneous reflex and reciprocal Ia inhibition in the soleus and tibialis anterior muscles in humans with chronic SCI. We found that tendon vibration reduced the amplitude of later but not earlier cutaneous reflex in the antagonist but not in the agonist muscle relative to the location of the vibration. In addition, reciprocal Ia inhibition between antagonist ankle muscles increased with tendon vibration and participants with a larger suppression of the later component of the cutaneous reflex had stronger reciprocal Ia inhibition from the antagonistic muscle. Our study is the first to provide evidence that tendon vibration attenuates late cutaneous spasm-like reflex activity, likely via reciprocal inhibitory mechanisms, and may represent a method, when properly targeted, for controlling spasms in humans with SCI. The neuronal mechanisms contributing to the generation of involuntary muscle contractions (spasms) in humans with spinal cord injury (SCI) remain poorly understood. To address this question, we examined the effect of Achilles and tibialis anterior tendon vibration at 20, 40, 80 and 120Hz on the amplitude of the long-polysynaptic (LPR, from reflex onset to 500ms) and long-lasting (LLR, from 500ms to reflex offset) cutaneous reflex evoked by medial plantar nerve stimulation in the soleus and tibialis anterior, and reciprocal Ia inhibition between these muscles, in 25 individuals with chronic SCI. We found that Achilles tendon vibration at 40 and 80Hz, but not other frequencies, reduced the amplitude of the LLR in the tibialis anterior, but not the soleus muscle, without affecting the amplitude of the LPR. Vibratory effects were stronger at 80 than 40Hz. Similar results were found in the soleus muscle when the tibialis anterior tendon was vibrated. Notably, tendon vibration at 80Hz increased reciprocal Ia inhibition between antagonistic ankle muscles and vibratory-induced increases in reciprocal Ia inhibition were correlated with decreases in the LLR, suggesting that participants with a larger suppression of later cutaneous reflex activity had stronger reciprocal Ia inhibition from the antagonistic muscle. Our study is the first to provide evidence that tendon vibration suppresses late spasm-like activity in antagonist but not agonist muscles, likely via reciprocal inhibitory mechanisms, in humans with chronic SCI. We argue that targeted vibration of antagonistic tendons might help to control spasms after SCI.

Relationship between the Diabetic Polyneuropathy Index and the Neurological Findings of Diabetic Polyneuropathy.

Objective To achieve an accurate quantification in diabetic polyneuropathy (DPN), we developed a new electrophysiological index that we called the DPN index. The relationship between the DPN index and the neurological findings in diabetic patients was assessed. Methods The DPN index was calculated by the mean value of percentages of four parameters (tibial compound muscle action potential amplitude / F wave minimum latency, sural sensory nerve action potential amplitude / sensory nerve conduction velocity) against the mean normal values. Twenty healthy subjects were recruited as a control group. Patients A total of 348 diabetic patients who were hospitalized in our hospital during the period from December 2016 to August 2019 were retrospectively studied. The correlations between the DPN index and five neurological findings (subjective sensory symptoms, diminished or absent Achilles tendon reflex, impaired tactile and vibration sense, low coefficient of variation of R-R interval) were evaluated. Results The DPN index in healthy subjects was 129.3±32.7%. The DPN index in diabetic patients with one or more neurological findings was significantly lower than that in diabetic patients without any neurological findings (p<0.01: 89.3±27.8% vs. 118.4±21.2%). For each of the five neurological findings, the DPN index in the group with an abnormality was significantly lower than that in the group without any abnormality (each p<0.01). Spearman's correlation coefficients indicated that a greater number of neurological findings resulted in a lower DPN index (r=-0.711, p<0.01). Conclusion Our study suggested that the DPN index is useful for evaluating the severity of DPN.

Perception of postural verticality in roll and pitch while sitting and standing in healthy subjects

Assessment of verticality perception is increasingly used in patients with disorders of upright body orientation, e.g. in pusher behavior after hemispheric stroke or in retropulsion in neurovascular or degenerative diseases. The subjective postural vertical (SPV) assesses the individual’s perceived upright body orientation in space and can be determined in a sitting or a standing position. As somatosensory input differs between sitting and standing, we suppose a different representation of verticality. Thus, the aim of this study was to compare the SPV while sitting and while standing, and to determine the influence of proprioceptive and visual stimulations on the SPV in both positions.Twenty healthy subjects (18–40 years, 9 female) participated in two experiments. In experiment 1 the SPV was assessed in the pitch and roll planes while sitting and standing. In experiment 2 we additionally applied somatosensory Achilles tendon vibration and visual optokinetic stimulation.Results of experiment 1 revealed no difference of the SPV error between sitting and standing. Though, we found an increased SPV variability while sitting. Experiment 2 showed no effect of sensory manipulation in the pitch plane, but an effect of optokinetic stimulation in the roll plane. The tilt in direction of the visual stimuli was more pronounced while sitting. Optokinetic stimulation also resulted in a larger SPV variability in roll, indicating larger insecurity in verticality estimation during stimulation.In conclusion, even though the SPV assessment while sitting and standing resulted in similar mean values, the larger variability and the higher sensitivity to visual stimuli while sitting suggest different strategies for verticality estimation while sitting and while standing.

The Effect of Achilles Tendon Vibration and Proprioceptive Neuromuscular Facilitation on Gait Performance in Ambulatory Stroke Patients

The Effect of Achilles Tendon Vibration and Proprioceptive Neuromuscular Facilitation on Gait Performance in Ambulatory Stroke Patients

Aftereffects of prolonged Achilles tendon vibration on postural control are reduced in older adults.

To assess the change in the contribution of proprioceptive signal from leg muscles in postural control with ageing. Fifteen young (~23yr) and fifteen older adults (~68yr) participated in Experiment 1, which consisted of recording the mean position of the centre of pressure (CoP), CoP path length, CoP velocity, and the amplitude of the Hoffmann (H) reflex and maximal M wave (MMAX) in the soleus muscle during upright standing, before and after 1h of bilateral Achilles tendon vibration applied in seated posture. Eight young (~24yr) and eight older adults (~67yr) participated in Experiment 2 consisting of recording H-reflex and MMAX in seated posture before and after the 1-h vibration procedure used in Experiment 1. Immediately after the 1-h vibration, the mean CoP position shifted forward in both groups (p<0.05), with a greater magnitude of change (% pre-vibration) in young [mean(SD); 74(41)%] than older adults [44(40)%; p<0.05]. The CoP path length and velocity only increased in young adults after vibration (p<0.05). The H-reflex amplitude decreased only in young adults after vibration [before: 35(12); after: 16(13)% Mmax, p<0.05] during upright standing (Experiment 1), whereas it decreased similarly (p>0.05) in young [before: 47(12)% Mmax; after: 28(17)% Mmax] and older adults [before: 34(13)% Mmax; after: 21(14)% Mmax] in seated posture (Experiment 2). Prolonged Achilles tendon vibrations lead to lesser postural perturbation in older than in young adults, supporting the assumption of a decreased reliance on leg muscle proprioception in postural control with ageing.

Balance control mechanisms do not benefit from successive stimulation of different sensory systems.

In humans, to reduce deviations from a perfect upright position, information from various sensory cues is combined and continuously weighted based on its reliability. Combining noisy sensory information to produce a coherent and accurate estimate of body sway is a central problem in human balance control. In this study, we first compared the ability of the sensorimotor control mechanisms to deal with altered ankle proprioception or vestibular information (i.e., the single sensory condition). Then, we evaluated whether successive stimulation of difference sensory systems (e.g., Achilles tendon vibration followed by electrical vestibular stimulation, or vice versa) produced a greater alteration of balance control (i.e., the mix sensory condition). Electrical vestibular stimulation (head turned ~90°) and Achilles tendon vibration induced backward body sways. We calculated the root mean square value of the scalar distance between the center of pressure and the center of gravity as well as the time needed to regain balance (i.e., stabilization time). Furthermore, the peak ground reaction force along the anteroposterior axis, immediately following stimulation offset, was determined to compare the balance destabilization across the different conditions. In single conditions, during vestibular or Achilles tendon vibration, no difference in balance control was observed. When sensory information returned to normal, balance control was worse following Achilles tendon vibration. Compared to that of the single sensory condition, successive stimulation of different sensory systems (i.e., mix conditions) increased stabilization time. Overall, the present results reveal that single and successive sensory stimulation challenges the sensorimotor control mechanisms differently.

Is reintegration of ankle proprioception information different from vestibular information in balance control?

Introduction During balance control, proprioceptive, vestibular and visual afferences are continuously reweighted. To assess the role of a sensory system, it is common to induce sensory illusions and measure the effect on body sway. The objective was to assess sensory reintegration during and after the stimulation of one sensory system (single condition - SS) or successive stimulation of different senses (mix condition - MX). In SS, ankle proprioception or vestibular signal was altered. In MX, VIB (EVS) was followed by EVS (VIB). Material and methods Participants stood on a force platform with eyes closed, head turned at ∼90° and tilted upward ∼18°, while EVS and Achilles tendon vibration induced backward body sway. The scalar distance between the center of pressure and the center of gravity (COP-COG), stabilisation time (ST), and linear impulse during and following sensory alteration were calculated. We contrasted VIB and EVS in SS, and compared SS and MX. Results: In SS, COP-COG during sensory reintegration was larger in VIB compared to EVS (P = 0.018). No difference was found during stimulation (P Discussion/conclusion Reweighting sensory information was poorer following alteration in ankle proprioception than vestibular information. Increase in ST in the MX compared to the SS revealed that the sensorimotor integration mechanisms did not benefit from successive stimulation of different sensory systems.

Soleus single motor units show stronger coherence with Achilles tendon vibration across a broad bandwidth relative to medial gastrocnemius units while standing

To probe the frequency characteristics of somatosensory responses in the triceps surae muscles, we previously applied suprathreshold noisy vibration to the Achilles tendon and correlated it with ongoing triceps surae muscle activity (recorded via surface EMG) during standing. Stronger responses to tendon stimuli were observed in soleus (Sol) relative to medial gastrocnemius (MGas) surface EMG; however, it is unknown whether differences in motor unit activity or limitations of surface EMG could have influenced this finding. Here, we inserted indwelling EMG into Sol and MGas to record the activity of single motor units while we applied noisy vibration (10-115 Hz) to the right Achilles tendon of standing participants. We analyzed the relationship between vibration acceleration and the spike activity of active single motor units through estimates of coherence, gain, phase, and cross-covariance. We also applied sinusoidal vibration at frequencies from 10 to 100 Hz (in 5-Hz increments) to examine whether motor units demonstrate nonlinear synchronization or phase locking at higher frequencies. Relative to MGas single motor units, Sol units demonstrated stronger coherence and higher gain with noisy vibration across a bandwidth of 7-68 Hz, and larger peak-to-peak cross-covariance at all four stimulus amplitudes examined. Sol and MGas motor unit activity was modulated over the time course of the sinusoidal stimuli across all frequencies, but their phase-locking behavior was minimal. These findings suggest Sol plays a prominent role in responding to disturbances transmitted through the Achilles tendon across a broad frequency band during standing.NEW & NOTEWORTHY We examined the relationship between Achilles tendon stimuli and spike times of single soleus (Sol) and medial gastrocnemius (MGas) motor units during standing. Relative to MGas, Sol units demonstrated stronger coherence and higher gain with noisy stimuli across a bandwidth of 7-68 Hz. Sol and MGas units demonstrated minimal nonlinear phase locking with sinusoidal stimuli. These findings indicate Sol plays a prominent role in responding to tendon stimuli across a broad frequency band.