Stochastic Resonance Reduces Sway and Gait Variability in Individuals With Unilateral Transtibial Amputation: A Pilot Study

Long-range cross-correlations between center of pressure velocity and colored noises provided during quiet standing

Unperceivable electrical noise stimulation has been applied to improve postural control through the enhancement of somatosensory feedback. It has been observed that stimulation with a pink noise (1/f) structure is more effective than stimulation with other noise structures. In addition, the 1/f structure embedded in the postural control system may have a superior effect on postural control stabilization. However, the direct relationship between the long-range correlations of the pink-noise signal applied to somatosensory receptors and those of the postural control system has not been elucidated. Thus, we aimed to explore a common long-range correlation factor shared in the time series of the provided noise and foot center of pressure (CoP) during quiet standing. Sixteen young adults stood quietly on the force platform for 65 s. Four noise conditions (no stimulation and stimulation of knee joints with white-, pink-, and red-noise-like signals) were employed during the standing trials. The detrending moving-average cross-correlation analysis revealed that in each of the anteroposterior and mediolateral directions, the CoP velocity time series displayed significant long-range cross-correlations with the white and pink noise signals provided at that time, whereas such an effect was not observed in the red noise signal. This result indicates that pink and white noise signals would alter the temporal behavior of the CoP during quiet standing, although the mechanism remains to be elucidated.

Stochastic resonance in the sensory systems and its applications in neural prosthetics

Noise is generally considered to be detrimental. In the right conditions, however, noise can improve signal detection or information transmission. This counterintuitive phenomenon is called stochastic resonance (SR). SR has generated significant interdisciplinary interest, particularly in physics, engineering, and medical and environmental sciences. In this review, we discuss a growing empirical literature that suggests that noise at the right intensity may improve the detection and processing of auditory, sensorimotor, and visual stimuli. We focus particularly on applications of SR in sensory biology and investigate whether SR-based technologies present a pathway to improve outcomes for individuals living with sensory impairments. We conclude that there is considerable evidence supporting the application of SR in developing sensory prosthetics. However, the progression of SR-based technologies is variable across the sensory modalities. We suggest opportunities for further advancements in each modality, considering the best approaches to maximise benefits and capitalise on progress already made. Overall, SR can offer opportunities to improve existing technologies or to motivate innovations.

Socket-residuum coupling integrity affects perception of external stimuli: Effects of altering the transtibial interface using vacuum-assisted suspension

BACKGROUND

Relative movement between the socket and residual limb can impair function in prosthesis users. It is plausible that, in addition to its mechanical effect, the integrity of the socket-residuum interface influences the ability of an individual to sense tactile cues through the prosthesis. Vacuum-assisted suspension (VAS) has been shown to reduce relative movement at this interface, providing a means to test this premise. The purpose of this pilot study was to assess the effects of altering socket-residuum interface integrity through the VAS pressure level on the thresholds of perception of an externally applied vibration stimulus.

METHODS

Seven unilateral transtibial prosthesis users participated. Socket-residual limb integrity was altered using the VAS subatmospheric (vacuum) pressure level. Vibration perception tests were conducted at low, mid, and high vacuum levels, targeting 0, 8, and 19 in Hg respectively, and performed in partially loaded and fully loaded conditions. Vibration intensity was increased using a dial until participants delivered a verbal signal indicating it was perceptible, and the nominal intensity was recorded.

RESULTS

Intensity thresholds decreased (ie, sensitivity increased) from low to high vacuum settings when fully loaded ( P = 0.008). Differences when partially loaded were nonsignificant and variable across participants.

CONCLUSION

This study provides preliminary evidence that altering the integrity between the socket and residual limb by modifying the vacuum level affects sensation related to the external environment experienced through the prosthesis, although translation of these findings to real-world stimuli remains to be tested.

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.

The effect of mechanical vibration-based stimulation on dynamic balance control and gait characteristics in healthy young and older adults: A systematic review of cross-sectional study

BACKGROUND

A good dynamic balance control and stable gait played an important role in the daily ambulation, especially for older adults with sensorimotor degeneration. This study aimed to systematically review the effects and potential mechanisms of mechanical vibration-based stimulation (MVBS) on dynamic balance control and gait characteristics in healthy young and older adults.

METHOD

Five bioscience and engineering databases, including MEDLINE via PubMed, CINAHL via EBSCO, Cochrane Library, Scopus, and Embase, were searched until September 4th, 2022. Studies published between 2000 and 2022 in English and Chinese involving mechanical vibration related to gait and dynamic balance were included. The procedure was followed via the preferred reporting items for systematic reviews and meta-analysis method. The methodological quality of included studies was assessed using the NIH study quality assessment tool for observational cohort and cross-sectional studies.

RESULTS

A total of 41 cross-sectional studies met the inclusion criteria and were included in this study. Eight studies were good-quality while 26 were moderate-quality and 7 were poor-quality. There were six categories of MVBS at various frequencies and amplitudes utilized in included studies, including plantar vibration, focal muscle vibration, Achilles tendon vibration, vestibular vibration, cervical vibration, and vibration on nail of hallux.

SIGNIFICANCE

Different types of MVBS targeting different sensory systems affected the dynamic balance control and gait characteristics differently. MVBS could be used to provide improvement or perturbation to specific sensory systems, to induce different sensory reweight strategies during gait.

Different types of plantar vibration affect gait characteristics differently while walking on different inclines

Background

Plantar vibration has been widely used to strengthen the sensation of the somatosensory system, further enhancing balance during walking on a level surface in patients with stroke. However, previous studies with plantar vibration only involved the level surface, which neglected the importance of inclined/declined walking in daily life. Thus, combining the plantar vibration and inclined/declined walking might answer a critical research question: whether different types of plantar vibration had different effects on gait characteristics during walking on different inclines.

Methods

Eighteen healthy young adults were recruited. Fifteen walking conditions were assigned randomly to these healthy adults (no, sub-, and supra-threshold plantar vibration × five different inclines: +15%, +8%, 0%, -8%, -15% grade). A motion capture system with eight cameras captured 12 retro-reflective markers and measured the stride time, stride length, step width, and respective variabilities.

Results

A significant interaction between vibration and inclination was observed in the stride time (p < 0.0001) and step width (p = 0.015). Post hoc comparisons found that supra-threshold vibration significantly decreased the stride time (-8%: p < 0.001; -15%: p < 0.001) while the sub-threshold vibration significantly increased the step width (-8%: p = 0.036) in comparison with no plantar vibration.

Conclusions

When walking downhill, any perceivable (supra-threshold) vibration on the plantar area decreased the stride time. Also, the increase in step width was observed by non-perceivable (sub-threshold) plantar vibration while walking uphill. These observations were crucial as follows: (1) applying sub-threshold plantar vibrations during uphill walking could increase the base of support, and (2) for those who may need challenges in locomotor training, applying supra-threshold vibration during downhill walking could reach this specific training goal.

Effects of subthreshold electrical stimulation with white noise, pink noise, and chaotic signals on postural control during quiet standing

BACKGROUND

The stochastic resonance (SR) phenomenon has been used to improve postural control through the application of imperceptible noise to the somatosensory system. White noise signals have been applied in numerous SR studies on postural control. However, because the SR effect depends on the noise structure, the stimulation effects of signals with different structures, such as pink noise and chaotic signals, on postural control, must be determined to achieve better clinical applications of SR technology.

RESEARCH QUESTION

During quiet standing, how is postural control affected by subthreshold electrical stimulation to the knee joints when signals with different structures (white noise, pink noise, and chaotic signals) are used?

METHODS

Sixteen healthy young adults stood quietly for 40 s with their eyes closed. To evaluate postural sway, we calculated the mean velocity, root mean square (CoP_RMS), and range (CoP_Range) values for the center of pressure (CoP) in the anteroposterior direction. The standing task was conducted under subthreshold electrical stimulation with white noise, pink noise, and chaotic signals based on the Lorenz system, in addition to the no-stimulation condition. The four stimulation conditions were randomized within each set and repeated seven times.

RESULTS

Significant effects of stimulation were observed in the CoP_RMS and CoP_Range values. The CoP_RMS value under the pink noise signal was significantly lower than that under the no-stimulation condition. The CoP_Range value also tended to decrease under the pink noise signal compared with the no-stimulation condition; however, the differences were not statistically significant. No significant changes were found with the white noise and chaotic signals compared with the no-stimulation condition.

SIGNIFICANCE

We demonstrated that the pink noise signal was more effective in reducing postural sway than the white noise and chaotic signals based on the Lorenz system during quiet standing.

Subthreshold Vibration Influences Standing Balance but Has Unclear Impact on Somatosensation in Persons With Transtibial Amputations

Stochastic resonance has been successfully used to improve human movement when using subthreshold vibration. Recent work has shown promise in improving mobility in individuals with unilateral lower limb amputations. Furthering this work, we present an investigation of two different signal structures in the use of stochastic resonance to improve mobility in individuals with unilateral lower limb amputations. Cutaneous somatosensation and standing balance measures using spatial and temporal analysis were assessed. There were no differences in the somatosensation measures, but differences in the temporal characteristics of the standing measures were seen with the various vibration structures when compared to no vibration, one of which suggesting mass may play an important role in determining who may or may not benefit from this intervention. Stochastic resonance employed with subthreshold vibration influences mobility in individuals with unilateral amputations, but the full direction and extent of influence is yet to be understood.

Vibration Sensitivity Is Associated With Functional Balance After Unilateral Transtibial Amputation

Objectives

To evaluate differences in vibration perception thresholds between adults with transtibial amputation and age-matched adults without amputation and to examine associations between vibration perception thresholds and balance performance. We hypothesized that adults with transtibial amputation would demonstrate lower thresholds compared with adults without amputation and that lower thresholds would be associated with better functional balance.

Design

Prospective cross-sectional study.

Setting

National conference, clinical practice, and university laboratory.

Participants

Adults (N=34) with a nondysvascular, unilateral, transtibial amputation and 43 age-matched controls without amputation.

Interventions

Participants' vibration perception thresholds were evaluated bilaterally by applying a vibration stimulus to the midpatella and recording their verbal response to conscious perception of stimulus. Functional balance was assessed with the Berg Balance Scale and the Four Square Step Test.

Main Outcome Measures

Residual and sound limb (right and left for controls) vibration perception thresholds, Berg Balance Scale, and Four Square Step Test.

Results

For participants with transtibial amputation and controls, there were no significant between-group (P=.921) or interlimb (P=.540) differences in vibration perception thresholds. Overall, robust regression models explained 35.1% and 19.3% variance in Berg Balance Scale scores and Four Square Step Test times, respectively. Among adults with transtibial amputation, vibration perception thresholds were negatively associated with Berg Balance Scale scores (P=.009) and positively associated with Four Square Step Test times (P=.048). Among controls, average vibration perception thresholds were not significantly associated with functional balance (P>.050).

Conclusions

Adults with nondysvascular, transtibial-level amputation demonstrated similar vibration detection compared with adults with intact limbs, indicating that vibration detection is preserved in the amputated region postamputation. These findings suggest a unique relationship between vibration perception and functional balance post-transtibial amputation.