Effectiveness of electrical noise in reducing postural sway: a comparison between imperceptible stimulation applied to the anterior and to the posterior leg muscles

The effects of subsensory electrical noise stimulation on the reactive control of balance during support surface perturbations

BACKGROUND

The ability to respond effectively to balance perturbations is crucial for fall prevention. Subsensory electrical stimulation (SES) applied to the skin leads to improved proactive balance control but there is limited evidence on the SES effect on reactive balance control.

RESEARCH QUESTIONS

To test the efficiency of SES in improving reactive balance control against unpredictable support surface perturbations and to compare the effects of SES applied to the trunk and the lower legs.

METHODS

Twenty-three young adults stood on a treadmill while recovering from 15 forward and 15 backward surface translations of increasing magnitude to determine the backward and forward stepping thresholds (BSTh and FSTh). Then, they recovered from three repetitions of forward and backward perturbations of fixed magnitude to determine the characteristic of the compensatory step (i.e., step time, step length, step delay and Margin of Stability - MOS). Each test was conducted with no stimulation (NS), leg stimulation (LS), or trunk stimulation (TS) equal to 90 % of the sensory threshold. Repeated-measures ANOVA and Tukey post-hoc tests were used to analyze the main and interaction effects of stimulation and repetition.

RESULTS

TS and LS increased the BSTh by 31.5 % (p=0.002) and 16.4 % (p=0.028), respectively, with greater effects of TS; (ii) during backward perturbations, TS reduced compensatory step time by 9.0 %, step length by 17.1 %, and MOS at compensatory heel strike by 17.7 % (p<0.016); and (iii) during forward perturbations, LS and TS reduced the step time by 4.5 % and 3.5 % (p<0.017), and increased the minimum MOS by 7.8 % and 4.5 %, respectively (p<0.048).

SIGNIFICANCE

This is the first study that showed how the application of SES affects reactive balance control during support surface perturbations. TS was more effective than LS during backward perturbations. TS may be an effective strategy to enhance balance control during reactive postural tasks, thus potentially reducing fall risk.

Subsensory stochastic electrical stimulation targeting muscle afferents alters gait control during locomotor adaptations to haptic perturbations

Subsensory noise stimulation targeting sensory receptors has been shown to improve balance control in healthy and impaired individuals. However, the potential for application of this technique in other contexts is still unknown. Gait control and adaptation rely heavily on the input from proprioceptive organs in the muscles and joints. Here we investigated the use of subsensory noise stimulation as a means to influence motor control by altering proprioception during locomotor adaptations to forces delivered by a robot. The forces increase step length unilaterally and trigger an adaptive response that restores the original symmetry. Healthy participants performed two adaptation experiments, one with stimulation applied to the hamstring muscles and one without. We found that participants adapted faster but to a lesser extent when undergoing stimulation. We argue that this behavior is because of the dual effect that the stimulation has on the afferents encoding position and velocity in the muscle spindles.

Decrease in current perception thresholds of A-beta fibers by subthreshold noise stimulation using transcutaneous electrical nerve stimulation

Developing effective supplements and rehabilitation of the impaired tactile and proprioception sensation is a significant challenge. One potential method for improving these sensations in clinical practice is using stochastic resonance with white noise. While transcutaneous electrical nerve stimulation (TENS) is a simple method, the effect of subthreshold noise stimulation via TENS on sensory nerve thresholds is currently unknown. This study aimed to investigate whether subthreshold TENS can alter afferent nerve thresholds. The electric current perception thresholds (CPT) of A-beta, A-delta, and C fibers were assessed in 21 healthy volunteers during both subthreshold TENS and control conditions. Subthreshold TENS was found to have lower CPT values compared to the control condition for A-beta fibers. No significant differences were observed between subthreshold TENS and control for A-delta and C fibers. Our findings indicated that subthreshold TENS might selectively enhance the function of A-beta fibers.

The Effect of Subliminal Electrical Noise Stimulation on Plantar Vibration Sensitivity in Persons with Diabetes Mellitus

Subliminal electrical noise (SEN) enhances sensitivity in healthy individuals of various ages. Diabetes and its neurodegenerative profile, such as marked decreases in foot sensitivity, highlights the potential benefits of SEN in such populations. Accordingly, this study aimed to investigate the effect of SEN on vibration sensitivity in diabetes. Vibration perception thresholds (VPT) and corresponding VPT variations (coefficient of variation, CoV) of two experimental groups with diabetes mellitus were determined using a customized vibration exciter (30 and 200 Hz). Plantar measurements were taken at the metatarsal area with and without SEN stimulation. Wilcoxon signed-rank and t tests were used to test for differences in VPT and CoV within frequencies, between the conditions with and without SEN. We found no statistically significant effects of SEN on VPT and CoV (p > 0.05). CoV showed descriptively lower mean variations of 4 and 7% for VPT in experiment 1. SEN did not demonstrate improvements in VPT in diabetic individuals. Interestingly, taking into account the most severely affected (neuropathy severity) individuals, SEN seems to positively influence vibratory perception. However, the descriptively reduced variations in experiment 1 indicate that participants felt more consistently. It is possible that the effect of SEN on thick, myelinated Aβ-fibers is only marginally present.

I Can Step Clearly Now, the TENS Is On: Transcutaneous Electric Nerve Stimulation Decreases Sensorimotor Uncertainty during Stepping Movements

Transcutaneous electric nerve stimulation (TENS) is a method of electrical stimulation that elicits activity in sensory nerves and leads to improvements in the clinical metrics of mobility. However, the underlying perceptual mechanisms leading to this improvement are unknown. The aim of this study was to apply a Bayesian inference model to understand how TENS impacts sensorimotor uncertainty during full body stepping movements. Thirty healthy adults visited the lab on two occasions and completed a motor learning protocol in virtual reality (VR) on both visits. Participants were randomly assigned to one of three groups: TENS on first visit only (TN), TENS on second visit only (NT), or a control group where TENS was not applied on either visit (NN). Using methods of Bayesian inference, we calculated the amount of uncertainty in the participants' center of mass (CoM) position estimates on each visit. We found that groups TN and NT decreased the amount of uncertainty in the CoM position estimates in their second visit while group NN showed no difference. The least amount of uncertainty was seen in the TN group. These results suggest that TENS reduces the amount of uncertainty in sensory information, which may be a cause for the observed benefits with TENS.

Subsensory electrical noise stimulation applied to the lower trunk improves postural control during visual perturbations

BACKGROUND

Low levels of sensory noise applied to the skin through electrical stimulation (ES) can improve balance control through a mechanism called stochastic resonance (SR). Little is known regarding the extent subsensory ES can improve reactive control of balance after unanticipated balance perturbations and the best location where to apply the stimulation.

RESEARCH QUESTIONS

How efficient is subsensory ES in improving reactive control of balance following visual perturbations delivered in a virtual reality (VR) environment? 2) Does lower trunk stimulation have greater effects than lower legs stimulation?

METHODS

Eighteen healthy young adults stood on a force plate while wearing a Valve Index VR headset in eyes closed (EC), eyes open (EO), eyes open with anteroposterior visual perturbations (AP) and eyes open with mediolateral visual perturbations (ML) conditions. No-stimulation (NS), leg stimulation (LS), or trunk stimulation (TS) equal to 90% of the sensory threshold (ST) was applied. The 95% confidence ellipse area (95%EA), the lengths of AP and ML sway path (APPath, MLPath), and the AP and ML 50% and 95% power frequencies (APPF50, MLPF50, APPF95, and MLPF95) were calculated. Repeated-measures ANOVA and Tukey post-hoc tests were used to analyze the main and interaction effects of stimulation and visual conditions.

RESULTS

During AP perturbations, participants showed higher frequencies, longer paths, and larger ellipse areas. TS caused lower APPF50, MLPF50, MLPF95, APPath and EA while LS caused lower MLPF50 and EA. During ML perturbations, TS reduced APPF50 and both LS and TS caused reduction of MLPF95. Higher instability following AP perturbations was associated with greater effects of TS and LS.

SIGNIFICANCE

The application of subsensory ES improved postural control during AP perturbations and TS reduced postural sway more effectively than LS. TS may be an effective strategy to enhance balance control during reactive postural tasks, thus potentially reducing fall risk.

Transcutaneous spinal direct current stimulation (tsDCS) does not affect postural sway of young and healthy subjects during quiet upright standing

Transcutaneous spinal direct current stimulation (tsDCS) is an effective non-invasive spinal cord electrical stimulation technique to induce neuromodulation of local and distal neural circuits of the central nervous system (CNS). Applied to the spinal cord lumbosacral region, tsDCS changes electrophysiological responses of the motor, proprioceptive and nociceptive pathways, alters the performance of some lower limb motor tasks and can even modulate the behavior of supramedullary neuronal networks. In this study an experimental protocol was conducted to verify if tsDCS (5 mA, 20 minutes) of two different polarizations, applied over the lumbosacral region (tenth thoracic vertebrae (T10)), can induce changes in postural sway oscillations of young healthy individuals during quiet standing. A novel initialization of the electrical stimulation was developed to improve subject blinding to the different stimulus conditions including the sham trials. Measures of postural sway, both global and structural, were computed before, during and following the DC stimulation period. The results indicated that, for the adopted conditions, tsDCS did not induce statistically significant changes in postural sway of young healthy individuals during quiet standing.

The optimal exploitation of sensory electrical stimulation for regulating postural balance depends on participants' intrinsic balance abilities

BACKGROUND

The sensory electrical stimulation applied to the postural muscles provides additional sensory information that improves postural balance but this improvement seems to be highly subject-dependent.

RESEARCH QUESTION

The first aim was to analyse the effects of sensory electrical stimulation on postural balance and the second aim was to analyse these effects depending on intrinsic postural balance abilities of subjects.

METHODS

Twenty healthy young male participants completed a monopedal postural task with sensory electrical stimulation (1 ms; 10 Hz; 7 ± 2 mA i.e., twice the intensity corresponding to the sensory threshold) and without sensory electrical stimulation. Pearson's product-moment correlations were performed on centre of pressure parameters to assess whether the participant's balance abilities at baseline were related to the beneficial effects of sensory electrical stimulation.

RESULTS

The results showed positive correlations for all the variables measured (i.e., with r² from 0.32 to 0.35). Evidence suggests that subjects' abilities to take advantage from electrically induced additional afferents depended on participants' intrinsic balance abilities. In fact, subjects who exhibited the worst postural balance at baseline (i.e. without stimulation) benefited more from the effects of sensory electrical stimulation than subjects who displayed the best postural balance at baseline.

SIGNIFICANCE

In physically impaired subjects, as part of functional rehabilitation, sensory electrical stimulation would be particularly interesting in order to limit their risk of falling.

Sensory electrical stimulation and postural balance: a comprehensive review

PURPOSE

Sensory electrical stimulation (SES)-i.e., low-intensity electrical currents below, at, or just above the sensory threshold but below the motor threshold-is mainly used to restore/improve postural balance in pathological and healthy subjects. However, the ins and outs of its application as well as the neurophysiological effects induced are not yet well known. Hence, the aim of this paper was to address the effects of SES on postural balance based on these considerations.

METHOD

The immediate/concurrent effects (SES applied during postural balance measurements), the acute effects (SES durably applied before measuring postural balance) and the chronic effects (SES included in training/rehabilitation programs, i.e., measurements performed before and after the programs) were analysed with a comprehensive review.

RESULT

SES can lead to the improvement of postural balance using any of the three applications (immediate/concurrent, acute and chronic), notably in pathological subjects. The beneficial effects of SES can take place at the peripheral (sensory receptors sensitivity), spinal (spinal motoneural excitablity) and supra-spinal (cortex reorganisation or adaptation) levels. In healthy subjects, SES appears interesting, but too few studies have been conducted with this population to report clear results. Moreover, the literature is relatively devoid of comparative studies about the characteristics of the stimulation current (e.g., location, current parameters, duration).

CONCLUSION

In practice, SES appears to be particularly useful to reinforce or restore the postural function in the immediate/concurrent, acute or chronic application in pathlogical populations while its effects should be confirmed in healthy sujects by future studies. Moreover, future research should focus on the different characteristics of stimulation.

Effect of the application of somatosensory and excitomotor electrical stimulation during quiet upright standing balance

Somatosensory (which activates sensory neurons only) and excitomotor (which activates both motoneurons and sensory neurons) electrical stimulations applied on the musculature of the lower-limb are likely to facilitate and disturb balance control respectively. The aim of this study was to compare the possible balance control modifications induced by somatosensory (SS) and excitomotor (EX) electrical stimulations applied on the quadriceps femoris in quiet standing condition. Kinetics and kinematics parameters were recorded with a force platform (displacements of center of foot pressure) and a 3D analysis system (hip, knee and ankle angles) respectively during a postural task. Twenty healthy young male participants carried out a monopedal postural task (i.e., unilateral stance) in three conditions: SS stimulation (1ms; 10Hz; 7±2 mA i.e., twice the intensity corresponding to the sensory threshold), EX stimulation (400 µs; 50 Hz; 20 ± 5 mA i.e., twice the intensity corresponding to the motor threshold), and a control (CONT) condition without stimulation. The results showed no significant differences between the three conditions except for the knee' angle which was higher in the EX condition (167.3±11.6 vs 164.3±5.8 and 163.9±8) (p < 0.005) than in the two other conditions (SS stimulation and CONT). This means that the EX stimulation induced a postural position change (i.e., a slight knee extension) during the monopedal postural task without altering balance control. Overall, on the basis of the stimulation parameters used in the present work, neither the SS stimulation, nor the EX stimulation facilitated or disturbed postural balance.

Sensory Sub- and Suprathreshold TENS Exhibit No Immediate Effect on Postural Steadiness in Older Adults with No Balance Impairments

Transcutaneous electrical nerve stimulation (TENS) has been reported to attenuate postural sway; however, the results are inconclusive, with some indicating the effect and others not. The study aimed to evaluate the effect of sensory sub- and suprathreshold low-frequency TENS applied through the plantar surface and posterior aspect of shanks on postural sway. In a group of healthy community-dwelling older adults, TENS was delivered with two different current intensities: (1) subsensory which is below conscious perception and (2) suprasensory threshold which is within the range of conscious perception. Frequencies of the TENS stimulation were sweeping from 5 to 180 Hz and were delivered through the plantar surface and posterior shanks of both legs. Postural sway was measured with a force platform in eyes-open and eyes-closed conditions. To evaluate potential fast adaptability to TENS stimuli, the results were evaluated in two time intervals: 30 seconds and 60 seconds. The results indicated that TENS with the chosen frequencies and electrode placement did not affect postural sway in both the sub- and suprathreshold intensities of TENS, in eyes-open and eyes-closed conditions, and in 30-second and 60-second time intervals. In conclusion, given that in this study sub- and suprathreshold TENS applied via the plantar surface of the feet did not attenuate postural sway, it would be easy to conclude that this type of electrical stimuli is ineffective and no further research is required. We must caution against this, given the specificity of the electrode placements. We recommend that future research be performed consisting of individuals with balance impairments and with different positions of electrodes.

Analysis of the Effectiveness of Sub-sensory Electrical Noise Stimulation During Visuomotor Adaptations in Different Visual Feedback Conditions

Sub-sensory electrical noise stimulation has been shown to improve motor performance in tasks that mainly rely on proprioceptive feedback. During the execution of movements such as reaching, proprioceptive feedback combines dynamically with visual feedback. It is still unclear whether boosting proprioceptive information in tasks where proprioception mixes with vision can influence motor performance. To better understand this point, we tested the effect of electrical noise stimulation applied superficially to the muscle spindles during four different experiments consisting of isometric reaching tasks under different visual feedback conditions. The first experiment (n = 40) consisted of a reach-and-hold task where subjects had to hold a cursor on a target for 30 s and had visual feedback removed 10 s into the task. Subjects performed 30 repetitions of this task with different stimulation levels, including no stimulation. We observed that trials in which the stimulation was present displayed smaller movement variability. Moreover, we observed a positive correlation between the level of stimulation and task performance. The other three experiments consisted of three versions of an isometric visuomotor adaptation task where subjects were asked to reach to random targets in <1.5 s (otherwise incurring in negative feedback) while overcoming a 45° clockwise rotation in the mapping between the force exerted and the movement of the cursor. The three experiments differed in the visual feedback presented to the subjects, with one group (n = 20) performing the experiment with full visual feedback, one (n = 10) with visual feedback restricted only to the beginning of the trajectory, and one (n = 10) without visual feedback of the trajectory. All subjects performed their experiment twice, with and without stimulation. We did not observe substantial effects of the stimulation when visual feedback was present (either completely or partially). We observed a limited effect of the stimulation in the absence of visual feedback consisting in a significant smaller number of negative-feedback trials and a significant smaller movement time in the first block of the adaptation phase. Our results suggest that sub-sensory stimulation can be beneficial when proprioception is the main feedback modality but mostly ineffective in tasks where visual feedback is actively employed.

Perception of threshold-level whole-body motion during mechanical mastoid vibration

BACKGROUND

Vibration applied on the mastoid has been shown to be an excitatory stimulus to the vestibular receptors, but its effect on vestibular perception is unknown.

OBJECTIVE

Determine whether mastoid vibration affects yaw rotation perception using a self-motion perceptual direction-recognition task.

METHODS

We used continuous, bilateral, mechanical mastoid vibration using a stimulus with frequency content between 1 and 500 Hz. Vestibular perception of 10 healthy adults (M±S.D. = 34.3±12 years old) was tested with and without vibration. Subjects repeatedly reported the perceived direction of threshold-level yaw rotations administered at 1 Hz by a motorized platform. A cumulative Gaussian distribution function was fit to subjects' responses, which was described by two parameters: bias and threshold. Bias was defined as the mean of the Gaussian distribution, and equal to the motion perceived on average when exposed to null stimuli. Threshold was defined as the standard deviation of the distribution and corresponded to the stimulus the subject could reliably perceive.

RESULTS

The results show that mastoid vibration may reduce bias, although two statistical tests yield different conclusions. There was no evidence that yaw rotation thresholds were affected.

CONCLUSIONS

Bilateral mastoid vibration may reduce left-right asymmetry in motion perception.

Association Between Plantarflexion Torque Variability In Quiet Stance And During Force And Position Tasks

This study examined the association between plantarflexion torque variability during quiet bipedal standing (QS) and during plantarflexion force- and position-matching tasks (FT and PT, respectively). In QS, participants stood still over a force plate, and the mean plantarflexion torque level exerted by each subject in QS (divided by 2 to give the torque due to a single leg) served as the target torque level for right leg FT and PT (performed with the participants seated with their right knee fully extended). During FT participants controlled the force level exerted by the foot against a rigid restraint, while during PT they controlled the angular position of the ankle when sustaining equivalent inertial loads. Standard deviation (SD) of plantarflexion torque was computed from torque signals acquired during periods with and without visual feedback. Significant correlations were found between plantarflexion torque variability in QS and FT (r = 0.8615, p < 0.0001 and r = 0.8838, p = 0.0003 for visual and no visual conditions, respectively) as well as between QS and PT (r = 0.8046, p = 0.003 and r = 0.7332, p = 0.0103 for visual and no visual conditions, respectively), regardless of vision availability. No significant differences were found between the correlations for Qs vs FT and QS vs PT (t(8) = 0.4778, p = 0.6455 and t(8) = 1.6819, p = 0.1310 for visual and no visual conditions, respectively), as assessed by "Hotelling-Williams" tests for equality among dependent correlations. The results indicate that simple measurements of plantarflexion torque fluctuations during FT and PT may be used to estimate balance ability. From a practical standpoint, it is suggested that rehabilitation protocols designed to regain/improve balance function may be based on the performance of FTs or PTs executed in a seated position.

Fingertip-Coupled Spindle Signaling Does Not Contribute to Reduce Postural Sway Under Light Touch

The details of how light touch (LT) of a stable surface reduces postural sway are still not well known. We hypothesized that removal of feedback provided by muscle afferents of the touching fingertip would increase postural sway in standing subjects. Eleven participants stood upright on a force plate with eyes closed and on an unstable surface. The experimental conditions involved two different finger positions: with partial muscle afferents (PMA), which includes sensory information from the fingertip flexor muscles, and no muscle afferents (NMA), without information from either fingertip flexor or extensor muscles. In the control condition, the participants kept the same posture, but with no finger touch (NT). Postural sway in both anteroposterior (AP) and mediolateral (ML) axes were recorded. Results showed that LT decreased all sway quantifiers as compared with the NT condition. The withdrawal of information from the touch finger muscle afferents (NMA condition) did not increase postural sway. Actually, there was a small, albeit statistically significant, decrease in the variability of center of pressure displacement in the AP direction. These results indicate that in some cases, muscle afferent input may either not contribute or even worsen the overall quality of sensory feedback from a given body segment, leading to no improvement or even a slightly decreased performance of the motor control system (evaluated by means of levels of postural sway in the present investigation). The results suggest that non-spindle fingertip afferents provide the bulk of the sensory feedback associated with the fingertip that is touching a ground-referenced object during quiet standing under LT.

The Promise of Stochastic Resonance in Falls Prevention

Multisensory integration is essential for maintenance of motor and cognitive abilities, thereby ensuring normal function and personal autonomy. Balance control is challenged during senescence or in motor disorders, leading to potential falls. Increased uncertainty in sensory signals is caused by a number of factors including noise, defined as a random and persistent disturbance that reduces the clarity of information. Counter-intuitively, noise can be beneficial in some conditions. Stochastic resonance is a mechanism whereby a particular level of noise actually enhances the response of non-linear systems to weak sensory signals. Here we review the effects of stochastic resonance on sensory modalities and systems directly involved in balance control. We highlight its potential for improving sensorimotor performance as well as cognitive and autonomic functions. These promising results demonstrate that stochastic resonance represents a flexible and non-invasive technique that can be applied to different modalities simultaneously. Finally we point out its benefits for a variety of scenarios including in ambulant elderly, skilled movements, sports and to patients with sensorimotor or autonomic dysfunctions.

Stochastic resonance stimulation improves balance in children with cerebral palsy: a case control study

Background

Stochastic Resonance (SR) Stimulation has been used to enhance balance in populations with sensory deficits by improving the detection and transmission of afferent information. Despite the potential promise of SR in improving postural control, its use in individuals with cerebral palsy (CP) is novel. The objective of this study was to investigate the immediate effects of electrical SR stimulation when applied in the ankle muscles and ligaments on postural stability in children with CP and their typically developing (TD) peers.

Methods

Ten children with spastic diplegia (GMFCS level I- III) and ten age-matched TD children participated in this study. For each participant the SR sensory threshold was determined. Then, five different SR intensity levels (no stimulation, 25, 50, 75, and 90% of sensory threshold) were used to identify the optimal SR intensity for each subject. The optimal SR and no stimulation condition were tested while children stood on top of 2 force plates with their eyes open and closed. To assess balance, the center of pressure velocity (COPV) in anteroposterior (A/P) and medial-lateral (M/L) direction, 95% COP confidence ellipse area (COPA), and A/P and M/L root mean square (RMS) measures were computed and compared.

Results

For the CP group, SR significantly decreased COPV in A/P direction, and COPA measures compared to the no stimulation condition for the eyes open condition. In the eyes closed condition, SR significantly decreased COPV only in M/L direction. Children with CP demonstrated greater reduction in all the COP measures but the RMS in M/L direction during the eyes open condition compared to their TD peers. The only significant difference between groups in the eyes closed condition was in the COPV in M/L direction.

Conclusions

SR electrical stimulation may be an effective stimulation approach for decreasing postural sway and has the potential to be used as a therapeutic tool to improve balance. Applying subject-specific SR stimulation intensities is recommended to maximize balance improvements. Overall, balance rehabilitation interventions in CP might be more effective if sensory facilitation methods, like SR, are utilized by the clinicians.

Trial registration

ClinicalTrials.gov identifier NCT02456376; 28 May 2015 (Retrospectively registered); https://clinicaltrials.gov/ct2/show/NCT02456376.

Subliminal electrical and mechanical stimulation does not improve foot sensitivity in healthy elderly subjects

OBJECTIVE

Deterioration of cutaneous perception may be one reason for the increased rate of falling in the elderly. The stochastic resonance phenomenon may compensate this loss of information by improving the capability to detect and transfer weak signals. In the present study, we hypothesize that subliminal electrical and mechanical noise applied to the sole of the foot of healthy elderly subjects improves vibration perception thresholds (VPT).

METHODS

VPTs of 99 healthy elderly subjects were measured at 30 Hz at the heel and first metatarsal head (MET I). Participants were randomly assigned to one of five groups: vibration (Vi-G), current (Cu-G), control (Co-G), placebo-vibration (Pl-Vi), and placebo-current (Pl-Cu). Vi-G and Cu-G were stimulated using 90% (subliminal) of their individual perception thresholds for five minutes in a standing position. Co-G received no stimulation. The placebo groups were treated with mock stimulation. VPTs were measured twice before the intervention (baseline (BASE) and pre-measurement (PRE)), and once after the intervention (post-measurement (POST)).

RESULTS

Significant differences were found between measurement conditions comparing BASE and POST, and PRE and POST. VPTs between groups within each measurement condition showed no significant differences. Vi-G was the only group that showed significantly higher VPTs in POST compared to BASE and PRE, which contradicts previous studies.

CONCLUSION

We analyzed increased VPTs after subliminal mechanical stimulation. The pressure load of standing for five minutes combined with subliminal stimulation may have shifted the initial level of mechanoreceptor sensitivity, which may lead to a deterioration of the VPT. The subliminal electrical stimulation had no effect on VPT.

SIGNIFICANCE

Based on our results, we cannot confirm positive effects of subliminal electrical or mechanical stimulation on the sole of the foot.

Effect of noise stimulation below and above sensory threshold on postural sway during a mildly challenging balance task

BACKGROUND

Mechanical and electrical sub-sensory noise stimulation applied to the sensory receptors has been shown to improve performance during postural balance tasks. This improvement has been linked with the Stochastic Resonance (SR) phenomenon. It is not clear if noise levels above sensory threshold can also lead to a reduction in postural sway.

RESEARCH QUESTION

The aim of this study was to investigate the different effects of sub- and super-sensory electrical noise stimulation applied to the Tibialis Anterior muscle during several repetitions of a mildly challenging single-leg postural balance task.

METHODS

Fifteen healthy individuals participated in this study. Participants performed 25 repetitions of a balance tasks where they leaned forward and maintained a pre-determined position for 20 s. Each participant experienced 5 different stimulation levels (no-stimulation, 70%, 90%, 110% and 130% of their sensory threshold ST) for 5 times in a randomized order. Optimal stimulation (OS) was defined as the stimulation intensity minimizing the standard deviation of postural sway in the anteroposterior direction.

RESULTS

∼57% of the participants presented levels of OS below ST. We did not observe a clear SR-effect, characterized by a U-shaped relationship between the performance metric and the stimulation intensity. OS led to a selective improvement in all the anteroposterior posturographic parameters analyzed. Stimulation below ST led to an improvement in most of the balance features, while stimulation above ST led to an increase in postural sway.

SIGNIFICANCE

Our results suggest that OS can be found both below and above ST although stimulation below ST appears to be more effective in reducing postural sway.

Effects of White Noise Achilles Tendon Vibration on Quiet Standing and Active Postural Positioning

Applying white noise vibration to the ankle tendons has previously been used to improve passive movement detection and alter postural control, likely by enhancing proprioceptive feedback. The aim of the present study was to determine if similar methods focused on the ankle plantarflexors affect the performance of both quiet standing and an active postural positioning task, in which participants may be more reliant on proprioceptive feedback from actively contracting muscles. Twenty young, healthy participants performed quiet standing trials and active postural positioning trials designed to encourage reliance on plantarflexor proprioception. Performance under normal conditions with no vibration was compared to performance with 8 levels of vibration amplitude applied to the bilateral Achilles tendons. Vibration amplitude was set either as a percentage of sensory threshold (n = 10) or by root-mean-square (RMS) amplitude (n = 10). No vibration amplitude had a significant effect on quiet standing. In contrast, accuracy of the active postural positioning task was significantly (P = .001) improved by vibration with an RMS amplitude of 30 μm. Setting vibration amplitude based on sensory threshold did not significantly affect postural positioning accuracy. The present results demonstrate that appropriate amplitude tendon vibration may hold promise for enhancing the use of proprioceptive feedback during functional active movement.

Electro-tactile stimulation of the posterior neck induces body anteropulsion during upright stance

Sensory information conveyed along afferent fibers from muscle and joint proprioceptors play an important role in the control of posture and gait in humans. In particular, proprioceptive information from the neck is fundamental in supplying the central nervous system with information about the orientation and movement of the head relative to the rest of the body. The previous studies have confirmed that proprioceptive afferences originating from the neck region, evoked via muscle vibration, lead to strong body-orienting effects during static conditions (e.g., leaning of the body forwards or backwards, depending on location of vibration). However, it is not yet certain in humans, whether the somatosensory receptors located in the deep skin (cutaneous mechanoreceptors) have a substantive contribution to postural control, as vibratory stimulation encompasses the receptive field of all the somatosensory receptors from the skin to the muscles. The aim of this study was to investigate the postural effect of cutaneous mechanoreceptor afferences using electro-tactile stimulation applied to the neck. Ten healthy volunteers (8M, 2F) were evaluated. The average position of their centre of foot pressure (CoP) was acquired before, during, and after a subtle electro-tactile stimulation over their posterior neck (mean ± SD = 5.1 ± 2.3 mA at 100 Hz—140% of the perception threshold) during upright stance with their eyes closed. The electro-tactile stimulation led to a body-orienting effect with the subjects consistently leaning forward. An average shift of the CoP of 12.1 ± 11.9 mm (mean ± SD) was reported, which significantly (p < 0.05) differed from its average position under a control condition (no stimulation). These results indicate that cutaneous mechanoreceptive inflow from the neck is integrated to control stance. The findings are relevant for the exploitation of electro-tactile stimulation for rehabilitation interventions where induced anteropulsion of the body is desired.

Improved proprioceptive function by application of subsensory electrical noise: Effects of aging and task-demand

The application of subsensory noise stimulation over the lower limbs has been shown to improve proprioception and postural control under certain conditions. Whereas the effect specificity seems to depend on several factors, studies are still needed to determine the appropriate method for training and rehabilitation purposes. In the current study, we investigated whether the application of subsensory electrical noise over the legs improves proprioceptive function in young and older adults. We aimed to provide evidence that stronger and age-related differential effects occur in more demanding tasks. Proprioceptive function was initially assessed by testing the detection of passive ankle movement (kinesthetic perception) in twenty-eight subjects (14 young and 14 older adults). Thereafter, postural control was assessed during tasks with different sensory challenges: i) by removing visual information (eyes closed) and; ii) by moving the visual scene (moving room paradigm). Tests performed with the application of electrical noise stimulation were compared to those performed without noise. The results showed that electrical noise applied over the legs led to a reduction in the response time to kinesthetic perception in both young and older adults. On the other hand, the magnitude of postural sway was reduced by noise stimulation only during a more challenging task, namely, when the optical flow was changing in an unpredictable (nonperiodic) manner. No differential effects of stimulation between groups were observed. These findings suggest that the relevance of proprioceptive inputs in tasks with different challenges, but not the subjects' age, is a determining factor for sensorimotor improvements due to electrical noise stimulation.

Effects of different lower-limb sensory stimulation strategies on postural regulation-A systematic review and meta-analysis

Systematic reviews of balance control have tended to only focus on the effects of single lower-limb stimulation strategies, and a current limitation is the lack of comparison between different relevant stimulation strategies. The aim of this systematic review and meta-analysis was to examine evidence of effects of different lower-limb sensory stimulation strategies on postural regulation and stability. Moderate- to high- pooled effect sizes (Unbiased (Hedges’ g) standardized mean differences (SMD) = 0.31–0.66) were observed with the addition of noise in a Stochastic Resonance Stimulation Strategy (SRSS), in three populations (i.e., healthy young adults, older adults, and individuals with lower-limb injuries), and under different task constraints (i.e., unipedal, bipedal, and eyes open). A Textured Material Stimulation Strategy (TMSS) enhanced postural control in the most challenging condition—eyes-closed on a stable surface (SMD = 0.61), and in older adults (SMD = 0.30). The Wearable Garments Stimulation Strategy (WGSS) showed no or adverse effects (SMD = -0.68–0.05) under all task constraints and in all populations, except in individuals with lower-limb injuries (SMD = 0.20). Results of our systematic review and meta-analysis revealed that future research could consider combining two or more stimulation strategies in intervention treatments for postural regulation and balance problems, depending on individual needs.

A Human-machine-interface Integrating Low-cost Sensors with a Neuromuscular Electrical Stimulation System for Post-stroke Balance Rehabilitation

A stroke is caused when an artery carrying blood from heart to an area in the brain bursts or a clot obstructs the blood flow to brain thereby preventing delivery of oxygen and nutrients. About half of the stroke survivors are left with some degree of disability. Innovative methodologies for restorative neurorehabilitation are urgently required to reduce long-term disability. The ability of the nervous system to reorganize its structure, function and connections as a response to intrinsic or extrinsic stimuli is called neuroplasticity. Neuroplasticity is involved in post-stroke functional disturbances, but also in rehabilitation. Beneficial neuroplastic changes may be facilitated with non-invasive electrotherapy, such as neuromuscular electrical stimulation (NMES) and sensory electrical stimulation (SES). NMES involves coordinated electrical stimulation of motor nerves and muscles to activate them with continuous short pulses of electrical current while SES involves stimulation of sensory nerves with electrical current resulting in sensations that vary from barely perceivable to highly unpleasant. Here, active cortical participation in rehabilitation procedures may be facilitated by driving the non-invasive electrotherapy with biosignals (electromyogram (EMG), electroencephalogram (EEG), electrooculogram (EOG)) that represent simultaneous active perception and volitional effort. To achieve this in a resource-poor setting, e.g., in low- and middle-income countries, we present a low-cost human-machine-interface (HMI) by leveraging recent advances in off-the-shelf video game sensor technology. In this paper, we discuss the open-source software interface that integrates low-cost off-the-shelf sensors for visual-auditory biofeedback with non-invasive electrotherapy to assist postural control during balance rehabilitation. We demonstrate the proof-of-concept on healthy volunteers.

Improving postural control by applying mechanical noise to ankle muscle tendons

The application of subthreshold mechanical vibrations with random frequencies (white mechanical noise) to ankle muscle tendons is known to increase muscle proprioceptive information and to improve the detection of ankle movements. The aim of the present study was to analyze the effect of this mechanical noise on postural control, its possible modulation according to the sensory strategies used for postural control, and the consequences of increasing postural difficulty. The upright stance of 20 healthy young participants tested with their eyes closed was analyzed during the application of four different levels of noise and compared to that in the absence of noise (control) in three conditions: static, static on foam, and dynamic (sinusoidal translation). The quiet standing condition was conducted with the eyes open and closed to determine the subjects' visual dependency to maintain postural stability. Postural performance was assessed using posturographic and motion analysis evaluations. The results in the static condition showed that the spectral power density of body sway significantly decreased with an optimal level of noise and that the higher the spectral power density without noise, the greater the noise effect, irrespective of visual dependency. Finally, noise application was ineffective in the foam and dynamic conditions. We conclude that the application of mechanical noise to ankle muscle tendons is a means to improve quiet standing only. These results suggest that mechanical noise stimulation may be more effective in more impaired populations.

D1 and D2 Inhibitions of the Soleus H-Reflex Are Differentially Modulated during Plantarflexion Force and Position Tasks

Presynaptic inhibition (PSI) has been shown to modulate several neuronal pathways of functional relevance by selectively gating the connections between sensory inputs and spinal motoneurons, thereby regulating the contribution of the stretch reflex circuitry to the ongoing motor activity. In this study, we investigated whether a differential regulation of Ia afferent inflow by PSI may be associated with the performance of two types of plantarflexion sensoriomotor tasks. The subjects (in a seated position) controlled either: 1) the force level exerted by the foot against a rigid restraint (force task, FT); or 2) the angular position of the ankle when sustaining inertial loads (position task, PT) that required the same level of muscle activation observed in FT. Subjects were instructed to maintain their force/position at target levels set at ~10% of maximum isometric voluntary contraction for FT and 90° for PT, while visual feedback of the corresponding force/position signals were provided. Unconditioned H-reflexes (i.e. control reflexes) and H-reflexes conditioned by electrical pulses applied to the common peroneal nerve with conditioning-to-test intervals of 21 ms and 100 ms (corresponding to D1 and D2 inhibitions, respectively) were evoked in a random fashion. A significant main effect for the type of the motor task (FT vs PT) (p = 0.005, η²_p = 0.603) indicated that PTs were undertaken with lower levels of Ia PSI converging onto the soleus motoneuron pool. Additionally, a significant interaction between the type of inhibition (D1 vs D2) and the type of motor task (FT vs PT) (p = 0.038, η²_p = 0.395) indicated that D1 inhibition was associated with a significant reduction in PSI levels from TF to TP (p = 0.001, η²_p = 0.731), whereas no significant difference between the tasks was observed for D2 inhibition (p = 0.078, η²_p = 0.305). These results suggest that D1 and D2 inhibitions of the soleus H-reflex are differentially modulated during the performance of plantarflexion FT and PT. The reduced level of ongoing PSI during PT suggests that, in comparison to FT, there is a larger reliance on inputs from muscle spindles primary afferents when the neuromuscular system is required to maintain position-controlled plantarflexion contractions.