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Subsensory Vibratory Stimulation Improves Balance

By Jamie Santa Cruz

Subsensory vibratory stimulation applied to the soles of older adults' feet can improve key indicators of balance, potentially reducing fall risk.

Falls create a substantial concern among elderly patients, with, in each year, one in three older adults experiencing a fall.1 New research, however, indicates that subsensory vibratory stimulation applied to the sole of the foot can improve key indicators of balance, which holds promise for reducing falls among older adults.2

The study, published online in October in Archives of Physical Medicine and Rehabilitation, involved a group of 12 healthy volunteers ranging in age from 65 to 90. Imperceptible vibrations were delivered to the soles of their feet by means of foam insoles that had been embedded with piezoelectric actuators. Each subject was then evaluated on three separate days on several balance measures, including gait variability (the variations in step from one foot to the next) and postural sway (the area of the ellipse over which a person sways while standing). While wearing the vibrating insoles, subjects experienced significant improvements on both measures, with gait variability reduced by an average of about 9% and postural sway reduced by about 15%. These findings are significant because balance problems, which become more pronounced with age, are known to increase the risk of falls.3

The volunteers' functional performance was also measured through a timed up-and-go test that required standing up from a seated position, walking three meters, and then returning to sit down. On this test, the average reduction in time to completion was 3% to 4%. Importantly, the improvements in balance and function persisted through repeated measurements throughout the day.

The vibration technology used in the insoles relies on the counterintuitive principle of stochastic resonance, which holds that small amounts of white noise within certain systems improve the ability to detect a signal within that system. Noise is usually thought to increase the challenge of focusing, but the proper type of noise at the proper level can improve the ability of detection, says lead author Lewis Lipsitz, MD, director of the Institute for Aging Research at Hebrew SeniorLife. In this case, the vibratory white noise applied to the feet helps the subjects better sense the surface on which they're standing and better perceive where their feet are in space.

The study builds on previous work by James Collins, PhD, a core faculty member at the Wyss Institute for Biologically Inspired Engineering at Harvard University and a professor of medicine and biomedical engineering at Boston University, which had previously demonstrated the efficacy of vibratory stimulation for improving standing balance.4 In a 2003 study on the topic, Collins and his colleagues compared measures of standing balance between elderly subjects and younger subjects prior to receiving vibratory stimulation and found that the difference was relatively small—only a 10% difference in balance between the two groups. After the elderly subjects were directed to stand on gel-based insoles that emitted subsensory vibration, they were able to match the performance of the younger subjects.

"The technology sort of closes the gap," says James Niemi, lead senior staff engineer at the Wyss Institute, who was involved in designing the insoles for both studies. "I hesitate to suggest that these insoles, when you wear them, make 73-year-olds perform like 23-year-olds in all balance metrics, but that's essentially what it did in lab measurements." The improvements, he says, illustrate the nonlinear behavior of the human neuromotor system. "Two plus two doesn't always equal four when you're dealing with a system like that. Small amounts of input or direction can make big differences."

Though previous studies had shown that vibratory stimulation could influence balance, the vibratory devices used in the earlier research required such large energy sources that they could not be inserted into a shoe. The current study involved redesigned technology that relies on two piezoelectric actuators inserted into the medial arch region of commonly available insoles, enabling greater portability.

Given the association between balance and fall risk, Niemi and Lipsitz are both optimistic that the balance improvements seen in their studies will translate to meaningful reductions in fall risk. However, no data are available on that question yet. Lipsitz stresses that predicting the impact on fall risk is difficult because the risk of falls is compounded by a variety of other factors such as the type of terrain. "What's really needed is a very large classically funded trial design," Niemi says. "Long term, over months and years, do people fall less with the insoles on or fall more when the insoles are turned off?"

Potential Use for Patients With Diabetes, Stroke?
One consequence of the choice to use smaller vibratory devices in the current study was that the study could include only healthy older adults. Initially, the researchers tried to include overweight and impaired individuals, but the vibrators used in the insoles were not sufficiently powerful. One of the keys to the technology is establishing the threshold of sensation; the vibratory stimulation must be turned up to the point where the subject can feel it, and then reduced to a level just below the sensory threshold. However, individuals with impairments and those who are overweight require a higher level of stimulation in order to reach the sensory threshold—a level not achievable with the shoe insoles.

The group's previous research, however, shows that the principle of subsensory vibration improves balance even in less-healthy populations. A 2006 study published in the Annals of Neurology, which involved larger, more powerful vibrators, included individuals with diabetic neuropathy or stroke, and both groups showed significant balance improvements after receiving stimulation.5 Thus, Lipsitz and Niemi remain optimistic that the technology has potential for use with impaired individuals.

Lipsitz believes that one group that might especially benefit from the technology is patients with diabetes, not only for balance improvement but also for injury prevention. Nerve damage caused by diabetes results not only in gait and balance problems but also leads patients with diabetes to injure themselves without realizing it; this, in turn, can lead to infections and amputations. "If we could provide a device that enabled people to improve their sensation, then maybe they would be aware of small injuries and be able to treat them earlier before they become something more serious," Lipsitz says.

Logistics create the most significant issue in using the technology for people with impairments. "The more vibration you have to drive, the bigger the batteries have to be—the bigger the power source," Niemi says, noting there is a limit to the size of a battery patients will be willing to carry around.

Beyond its use for fall prevention and for patients with diabetes, the technology could have a wide variety of other potential applications. For instance, Lipsitz says vibrating insoles could be used to improve the athletic performance of runners or jumpers; there could also be military applications, such as use among soldiers exposed who have been exposed to Agent Orange and have developed neuropathy.

Commercial Development
The researchers behind the insoles seek to partner with a third-party manufacturer to enable wider availability and facilitate larger clinical trials. To date, however, identifying an appropriate manufacturer has presented a challenge because medical device companies tend to perceive the insoles as footwear while footwear companies typically view them as medical devices. The fact that the vibration is subsensory and doesn't give a foot massage that can be felt has also proven to be a "head-scratcher" for footwear companies that might consider developing the product, according to Niemi.

However, Niemi says that if and when the insoles become commercially available, they will be cost-effective. The design used in the current study involves relatively expensive add-ons such as a go-to-sleep feature and the ability to link to an iPhone, which drive up the cost. Many of these features may be superfluous for most elderly individuals, however, and less feature-rich versions could be very affordable, according to Niemi. "It's certainly not a $5 item, but it's not $500 technology," he says.

For now, though, the product, still in early development, is not yet ready for market. "I get all these calls from people wanting to buy this," says Lipsitz, "but it's very early in its development and requires a lot more engineering before it's ready to be commercialized."

Meanwhile, patients and clinicians should not look to conventional foot massagers as a substitute. Niemi has heard from some patients who assume the stimulation might be comparable, but the type of vibration delivered by a foot massager is uniform, unlike the noisy, irregular stimulation delivered by the vibrating insoles. "It provides fundamentally a very different kind of vibration, and we don't feel like it will actually work for balance," Niemi says.

— Jamie Santa Cruz is a freelance writer based in Englewood, Colorado.


1. Tromp AM, Pluijm SM, Smit JH, Deeg DJ, Bouter LM, Lips P. Fall-risk screening test: a prospective study on predictors for falls in community-dwelling elderly. J Clin Epidemiol. 2001;54(8):837-844.

2. Lipsitz LA, Lough M, Niemi J, Travison T, Howlett H, Manor B. A shoe insole delivering subsensory vibratory noise improves balance and gait in healthy elderly people [published online October 24, 2014]. Arch Phys Med Rehabil. doi: http://dx.doi.org/10.1016/j.apmr.2014.10.004.

3. Rubenstein LZ, Josephson KR. Falls and their prevention in elderly people: what does the evidence show? Med Clin North Am. 2006;90(5):807-824.

4. Priplata AA, Niemi JB, Harry JD, Lipsitz LA, Collins JJ. Vibrating insoles and balance control in elderly people. Lancet. 2003;362(9390):1123-1124.

5. Priplata AA, Patritti BL, Niemi JB, et al. Noise-enhanced balance control in patients with diabetes and patients with stroke. Ann Neurol. 2006;59(1):4-12.

Sidebar: Steps to Fall Prevention
Although vibration technology is not yet widely available, the new research underscores the importance of sensation and its role in fall prevention. Lewis Lipsitz, MD, director of the Institute for Aging Research at Hebrew SeniorLife, provides several reminders of options that are currently available to clinicians and patients to help improve sensation and minimize falls among older adults, including the following:

• Intervene on any known issues that may impact sensation, such as vitamin B12 deficiency, diabetes, or back problems that may affect nerves extending to the feet.

• Encourage older adults with decreased sensation to inspect their feet every day and wash them well, examining them for cuts, scratches, etc. Patients should also be encouraged to wear good shoes with strong ankle support and wide, flat soles. "Padded, spongy soles are not particularly good because you don't feel the ground," Lipsitz says.

• Clinicians can also work with patients to help improve other systems that compensate for sensory loss in feet. For example, patients with decreased sensation should make sure their vision is good, that there is adequate lighting in their rooms, and that their living spaces are cleared of cords, rugs that can slip, and scattered objects. Also, beware of sedatives that compromise alertness. When foot sensation is compromised, "You need other systems to be heightened to prevent falls," Lipsitz says.