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

doi:10.1016/j.gaitpost.2018.04.031

Gait & Posture

Volume 63, June 2018, Pages 27-32

https://doi.org/10.1016/j.gaitpost.2018.04.031 Get rights and content

Highlights

•
We tested several different levels of noise stimulation during balance tests.
•
We compared the effect of noise stimulation above and below sensory threshold.
•
We tested the presence of a Stochastic Resonance effect due to noise stimulation.
•
We found that stimulation above sensory threshold can be optimal.

Abstract

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.

Introduction

Noise is an inherent characteristic of all signals in the sensorimotor system [1]. Several studies in the past two decades have shown that adding mechanical or electrical noise to the sensorimotor system during static and dynamic tasks can be beneficial to motor task performance [2], [3], [4], [5], [6], [7]. An abundance of research has tested the effects of both mechanical and electrical noise stimulation during the performance of postural balance tasks. Mechanical noise stimulation applied to the plantar surface of the foot and to joints of the lower extremity has been shown to improve postural balance by decreasing sway in both healthy [8], elderly [9] and impaired individuals [10]. Similar effects have been observed using electrical noise stimulation applied to joints [11] and muscles [12], [13] in healthy individuals and after orthopedic injuries [14], [15], [16]. The efficacy of noise stimulation in the sensorimotor system is reportedly related to the stochastic resonance (SR) phenomenon [17]. SR is characterized by an improvement in the reception of weak stimuli in non-linear systems in the presence of a particular optimal level of additive noise [18], [19], [20], [21], [22], [23].

Most of the previously published studies on noise stimulation during postural balance tasks have tested only a few levels of noise below the sensory threshold (ST). Testing noise intensities that a person cannot feel, in fact, rules out confounding factors affecting motor task performance related to the increase in attention that localized tactile sensations may cause. On the other hand, noise stimulation has several times been proposed as a possible aid during physical therapy for persons affected by a peripheral sensorimotor deficit [10], [15], [16], [24]. In this perspective it is irrelevant whether an increase in sensorimotor acuity and a relative increase in performance is due to a SR effect or co-caused by improved perceptual attention [24].

In this work we performed an experiment with the purpose of clarifying the effect of different levels of electrical noise stimulation, above and below ST, on postural balance during a mildly challenging task performed by healthy individuals. The task consisted in a one-legged stance exercise where participants were asked to lean forward and maintain a fixed position. Performance in this task critically depends from proprioceptive information coming from the spindles of the tibialis anterior (TA), that we targeted using electrical noise stimulation. We analyzed if a SR-like behavior, characterized as a U-shaped curve mapping performance to noise intensity [18], is observable when testing different levels of stimulation. We also investigated the different effects of electrical noise stimulation intensities below and above ST with the aim of assessing if there are clear differences between stimulation intensities that individuals can and cannot feel.

Section snippets

Participants

Fifteen healthy individuals (6 females, age = 23.2 ± 0.7 years) participated in this study. Inclusion criteria consisted of the absence of impairments that would affect the performance of the experimental motor task. All individuals agreed to participate in the study by signing an informed consent form. All procedures were conducted in accordance with the policies of the Human Research Ethics Committee of UCD and with the Declaration of Helsinki.

Experimental procedures

Participants were asked to perform a set of

Stimulation parameters and optimal stimulation

All participants except one were able to perform all the tasks successfully. One participant was not able to perform the task on numerous trials and was excluded from this analysis. We observed an average value of stimulation intensity for ST of the participants equal to 430 ± 110 μA. None of the stimulation levels tested induced muscular contractions in the TA in any of the subjects. For 12 out of 14 participants the values of AP-SD at the OS level were lower than their equivalent without

Discussion

In this study we observed that stimulation applied to the TA muscle improves postural balance performance in the AP direction. We also observed that OS values are not only found in the sub-sensory range, but almost half of our subjects presented OS values above the sensory threshold.

It is well known in literature that sub-sensory stimulation can positively affect postural balance by reducing postural sway. This result has been observed using both electrical [11], [12], [13], [28] and mechanical

Conflict of interest

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Acknowledgements

We would like to thank Jack O’Keeffe for his contribution to this study. This study was partially funded by the UCD Seed Funding grant SF1303.

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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.
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?
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.
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Full length articleEffect of noise stimulation below and above sensory threshold on postural sway during a mildly challenging balance task

Highlights

Abstract

Background

Research question

Methods

Results

Significance

Introduction

Section snippets

Participants

Experimental procedures

Stimulation parameters and optimal stimulation

Discussion

Conflict of interest

Acknowledgements

Lancet

Clin. Biomech. (Bristol, Avon)

Gait Posture

J. Biomech.

Arch. Phys. Med. Rehabil.

Neurosci. Res.

Noise in the nervous system

Nat. Rev. Neurosci.

Noise in human muscle spindles

Nature

Enhancing sensation in diabetic neuropathic foot with mechanical noise

Diabetes Care

Noise-enhanced tactile sensation