Using specially designed insoles to deliver stochastic resonance to the plantar surface of the feet has the potential to significantly improve static balance, dynamic balance, and gait mechanics in healthy, young individuals as well as elderly people and others with somatosensory deficits.
By Daniel Miranda, PhD; Wen-Hao Hsu, ScD; and James Niemi, MS
The existence of noise (in this context, a random, unwanted signal) in most systems or environments is typically considered a problem. In fact, engineers and scientists have spent decades pursuing ways to reduce or eliminate noise in a range of applications. Noise-cancelling headphones and photo-processing software are two examples of applications in which reducing noise is critical to obtaining crisp sounds and sharp images, respectively.
However, physicists and scientists from the 1980s to the mid-1990s reported applications for which the introduction of nonlinear noise to certain systems or environments enhanced the detection and transmission of weak signals.1 This phenomenon, coined stochastic resonance (SR), indicates that the flow of information through certain systems is improved by the inclusion of a specific level of noise. At the time, it was hypothesized that the sensory systems and perceptual processes may be able to take advantage of this phenomenon to improve the detection of weak stimuli.2
Collins and colleagues confirmed this hypothesis in a series of experiments involving human participants, in which subsensory SR noise improved tactile sensation by acting as a suitable pedestal for enhancing the detection of weak, normally undetectable, stimulations (Figure 1).3
Early insoles and standing balance
This was an exciting finding because it suggested the introduction of SR to the human sensory system might enhance the detection of weak stimuli in persons with sensory deficits caused by normal aging, stroke, diabetes, or other neurological disorders in addition to individuals with intact, normally functioning systems.4 From a clinical perspective, in the lower extremities, this sensory feedback provides important information to the human balance-control system that, when diminished, is associated with an increased risk of falling.
Collins hypothesized that the application of SR to the soles of the feet might improve standing balance, with eventual applications in walking and fall prevention.5 To test this hypothesis, Priplata and colleagues designed a series of experiments using a pair of large stationary devices modeled after insoles (Figure 2A).6-8
Their experiments measured sway parameters during quiet standing in young participants, elderly participants, patients with diabetes, and patients with stroke. Their findings indicated that application of SR to the plantar surface of the feet was associated with reduced postural sway parameters in each of the tested populations. Interestingly, differential effects of SR were observed between young people and those with sensory deficits (eg, elderly people, patients with diabetes, patients with stroke).
The authors suggested the young, healthy participants were already operating at a near optimum level of sensory function. Therefore, the capacity for improvement in standing balance with SR appears to depend on the baseline level of sensory impairment. The observed balance improvements lead to the question of whether the SR phenomenon would be effective in enhancing performance of activities involving dynamic balance, such as walking.
The vast majority of falls and fall-related injuries in persons with sensory deficits occur while walking or doing walking-related activities.9,10 Therefore, the early stationary SR devices needed to be redesigned for in-shoe use in order to perform studies investigating dynamic balance activities. Actuators were first placed in sandals to permit walking and then tethered with cables to the signal electronics and batteries (Figure 2B). This new iteration provided a platform to investigate the effects of SR on gait in healthy and at-risk patient populations.
Spatiotemporal parameters of gait
Certain spatiotemporal gait characteristics, which are quantifiable measures of gait function, have been associated with fall risk in people with sensory deficits. These characteristics have been traditionally separated into two categories: those associated with the rhythmic stepping parameters of gait, such as stride length and stride time; and those associated with the balance-control parameters of gait, such as stride width and double-support time (the time both feet are in contact with the ground).11
High variability in the rhythmic stepping parameters of gait is generally associated with gait instability and fall risk, as is high variability in the sway parameters measured during standing balance.12,13 Interestingly, both high and low variability in the balance-control parameters of gait have been associated with gait instability, with the understanding that some moderate level of variability is required for stability.12,14,15 For example, some step-width variability is important biomechanically because it provides a certain level of adaptability to limb movements and allows an individual to adapt and maintain stability during walking.
With low step-width variability, there is no flexibility to respond to perturbations. On the other hand, high step-width variability is typically associated with crossing one foot in front of the other during walking, which can narrow and offset the base of support and is a clinical indicator of unsteady walking.
Using SR to improve dynamic balance
From a clinical perspective, SR applied to the plantar surface of the feet could improve dynamic balance during gait in two ways: first, by reducing high variability in the rhythmic stepping parameters of gait; and second, by increasing the variability in the balance-control parameters of gait for the least variable walkers or decreasing the variability for the most variable walkers.
Using a tethered sandal device similar to the pair shown in Figure 2B, Galica and colleagues aimed to evaluate the effects of SR applied to the plantar feet on the rhythmic stepping parameters of gait in healthy young adults, elderly nonfallers, and elderly fallers as they walked at a self-selected pace on a circular track.13
At baseline, the elderly fallers had the highest variability in rhythmic stepping parameters (stride time, stance time, swing time), followed by the elderly nonfallers and the healthy, young participants. With the introduction of SR to the plantar surface of the feet, reductions in variability were observed for each of the rhythmic stepping parameters in all three participant populations. Moreover, the largest reductions were observed in the most variable walkers (elderly fallers), followed by the elderly nonfallers, and then the young participants (these participants did not experience statistically significant reductions).
This was the first study to show that SR applied to the plantar feet during the gait cycle can reduce rhythmic stepping variability in elderly participants who are at risk of falling.
Although rhythmic stepping parameters are important for the clinical potential of SR technology, it’s generally accepted that balance-control parameters of gait are more closely associated with falls and better predict fall risk. As mentioned, affecting the balance-control parameters is a two-way proposition. That is, the most variable walkers may need to reduce their variability to steady their walking, and the least variable walkers may need to increase their variability to introduce a certain level of adaptability in limb movements while walking. Therefore, proprioceptive improvements from SR that would push certain individuals into a more optimal state differ from the proprioceptive improvements that would be effective in other individuals, depending on their baseline level of balance-control variability.
In a follow-up study, Stephen and colleagues set out to test whether SR exerted a baseline-dependent effect on the balance-control parameters of gait in elderly participants.12 Using the same tethered sandals, similar to the pair shown in Figure 2B, the authors investigated the effect of SR on stride length and stride width while participants walked at a constant speed on a treadmill. Confirming their hypothesis, the results indicated a baseline-dependent effect of SR, where the least variable walkers demonstrated more variability, and reductions in variability were observed in those who were most variable at baseline.
Our recent work
To this point, the vast majority of research has focused on elderly individuals or patients with diabetes or stroke who have the potential to benefit most from the sensory improvements gained from SR applied to the plantar surface of the feet. Moreover, the observed improvements appear to be greatest for the individuals with the largest sensory deficits. This trend appears to be consistent from the standing balance studies through to more recent gait studies. Despite this, the few studies that have involved healthy, young individuals have reported only small or trending balance and gait improvements associated with SR in that population.6,13
During normal locomotion, healthy young persons are not at risk for falls or fall-related injuries, and their somatosensory systems are likely operating at an optimal state with a limited capacity to improve. This eliminates the need for sensory-enhancing SR technology during typical daily activities. However, sensory deficits do occur in healthy, young people performing vigorous activities that cause fatigue.16-18 These sensory deficits can place them at higher risk for slip-, trip-, and fall-related injuries when they are competing in long, strenuous athletic activities or military marches.
With a newly designed three-quarter-length insole device that uses a tethered control program (Figure 2C), we set out to test the effect of fatigue. We induced fatigue through a task simulating a strenuous recreational hike or military march and measured its effects on spatiotemporal gait parameters.19 Our goals were to determine if sustained vigorous walking on an inclined surface while carrying a backpack load destabilizes gait, and if SR applied to the planar surface of the feet has a stabilizing effect.
We fitted participants with a backpack weighing approximately 30% of their body weight and fitted their standard athletic shoes with the new insole devices. We then asked them to walk at a self-selected pace on a treadmill and tracked their foot position with a motion-capture system. The protocol started the participants at level ground and increased the incline by 2% every five minutes until participants reached volitional exhaustion, after which the treadmill was returned to level ground.
Throughout the protocol we applied SR to the plantar surface of the feet in a random fashion, such that pairs of trials were recorded in which SR was on for one minute and off for one minute. As in all the experiments, the subsensory level of the SR signal blinded participants the stimulus condition. We extracted spatiotemporal gait characteristics for SR-on and SR-off conditions during the baseline level-ground walking period, the period just prior to reaching volitional exhaustion, and the end level-ground walking period.
Our results indicated that, without SR, vigorous activity increased the variability in the rhythmic stepping and balance-control parameters of gait. Not surprisingly, our healthy, young participants had relatively low overall baseline variability. We concluded the undesirable increase in rhythmic stepping variability led to a compensatory increase in balance-control variability.
We believe this is a compensatory response, in which the relatively low baseline variability is pushed to a heighted state of adaptability to stabilize each participant’s gait during vigorous activity. If so, we hypothesized, the introduction of SR would enhance stimulus detection while fatigued, resulting in a reduction in rhythmic stepping variability and an increase in balance-control variability.
We observed no effect during any part of the task when SR was on compared to when it was off for the rhythmic stepping parameters. However, we did observe an increase in the variability of the balance-control parameters throughout the task when SR was turned on compared to the off condition, independent of fatigue state. Therefore, applying SR resulted in additional benefits to balance-control parameters of gait that may improve stability in healthy, young individuals who are experiencing vigorous activity and fatigue.
Finally, these results suggest that other athletic populations that become fatigued during training or competition may get injury prevention or performance benefits from a sensory-enhancing device, though additional research is needed.
We have substantial evidence that SR applied to the plantar surface of the feet improves sway parameters during quiet standing and spatiotemporal gait parameters during walking in elderly people, people with somatosensory deficits, and healthy, young individuals. In parallel, we have seen substantial engineering effort put into the sensory improvement devices. They have evolved from bulky static gel insoles to a form factor modeled after a standard three-quarter-length insole device. Despite the research promise and device development, additional research questions remain, and a fine-tuning of the device design is needed to bring this technology to the clinic or sports equipment store.
From a research standpoint, questions about the longevity of beneficial effects as well as the amplitude range of subthreshold stimulation, still remain. Each study mentioned in this review used a level 10% below each participant’s sensory perception threshold. However, would 15% or even 30% below the threshold still provide the desired effect? It is also unclear if the sensory benefits are acute or persist over a long duration. Furthermore, not much is known about how stable an individual’s sensory perception threshold is. Does the threshold vary throughout the day or over the course of multiple days? Most important, all of the research to date has been limited to measurements quantifying sway and spatiotemporal gait parameters. These metrics are associated with falling and injury risk, but it is still unclear if SR technology will translate to actual reductions in falls and injury rates among at-risk elderly people, patients with sensory deficits, athletes, or military personnel.
Some of these questions have been addressed in a recent study from Lipsitz and colleagues, who tracked a small group of elderly persons using the shoe-mounted three-quarter-length insoles (shown in Figure 2C).20 The investigators sought to determine whether the balance and gait improvements would persist throughout a day, whether sensory thresholds were consistent, and whether different levels of SR could still achieve the same beneficial effect.
The study provided strong evidence that individual sensory perception thresholds are relatively stable and that SR technology is not an acute phenomenon. Furthermore, SR levels set at 15% and 30% below the perception threshold were equal to each other in effectiveness. These findings suggest SR is not an acute phenomenon, and greatly simplify setting the subsensory threshold stimulation level of a future, commercially available device.
The future of SR devices
From a device development standpoint, recent advancements in battery, charging, and microelectronic technology could make a self-contained insole device possible in the near future. A device similar to the prototype shown in Figure 2D could contain the vibrating elements, battery, and electronics that charge the device wirelessly and allow it to communicate with a computer or smartphone. Such a device would appear indistinguishable from the replacement insoles found on drugstore shelves and sports equipment stores.
Most important, this device could be deployed in large clinical studies to establish a direct link between SR and injury risk in people with sensory deficits as well as in healthy individuals. The hope is that, soon, people will be able to come to the clinic or store, have their sensory threshold determined, and then be fitted with a device that improves their sensation with just a little bit of noise.
Daniel Miranda, PhD, is a technology development fellow; Wen-Hao Hsu, ScD, is a postdoctoral research fellow; and James Niemi, MS, is a lead senior staff engineer at the Wyss Institute at Harvard University in Boston.
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