Studies weight use of vibration training as a bone therapy

The popularity of vibration training stems from its image as a fitness secret of the stars, endorsed by celebrities from Madonna to Lance Armstrong. But image isn't everything. In fact, the technology's greatest potential may be realized in elderly patients and other special populations at risk for fracture.

Whereas researchers have struggled to demonstrate statistically or clinically significant effects of vibration training in healthy individuals (see “Athletes embrace vibration training despite mixed reviews,” November-December, page 20), studies of seniors and other patients with low bone density have suggested that vibration may strengthen both muscles and bones—fighting fracture risk on two fronts—in these individuals.

“Most currently available treatments for osteoporosis and osteopenia are drugs, and they are very specific, usually targeting a single pathway in the skeletal metabolism,” said Douglas Kiel, MD, an associate professor of medicine at Harvard Medical School and director of medical research at the Hebrew SeniorLife Institute for Aging Research, both in Boston. “The nice thing about the possibility of a mechanical approach is that if it improves several different domains that contribute to falling and fracturing, it would clearly be superior to any single approach.”

Approach with caution

That said, even the researchers whose findings suggest vibration can stimulate increases in strength and bone density have serious reservations about whether it can be used safely, even at the lowest magnitudes.

“Nobody in the world wants to see low-level mechanical signals translated to the clinic more than me,” said Clinton J. Rubin, PhD, professor and chairman of biomedical engineering at the State University of New York in Stony Brook and co-developer of the vibration technology currently being marketed as the Juvent1000. “I think it has great potential. But the vast majority of the data on the human response to vibration, which primarily comes from occupational health and safety research, suggests that vibration flat out is a pathogen. So it has to be approached with great if not grave caution, because it can do irreversible damage.”

In a study¹ presented in poster form at the 2006 meeting of the American Society of Bone and Mineral Research, Rubin and colleagues reported a wide range in accelerations produced by three different vibrating platforms in a 160 lb subject standing on the device. The low-magnitude vertical Juvent platform provided the least amount of acceleration, at 0.3 times gravitational acceleration (0.3g) at a frequency of 33 Hz; the horizontal Vibrafit platform ranged from 2.46g at 27 Hz to 8.63g at 51 Hz: and the popular high-magnitude vertical PowerPlate platform ranged from 8.46g to 18g depending on its power setting, both at 34 Hz.

The International Standards Organization's threshold value limits for human exposure to whole body vibration² suggest that exposures to accelerations above 2g for longer than 1 minute are potentially unsafe, with that duration of exposure time decreasing as accelerations increase. According to Rubin, 0.3g of acceleration is low enough to be considered safe for up to four hours of exposure.

Because other studies^3,4 have cited PowerPlate accelerations at as low as 2.16g and training regimens often incorporate exposure times as short as 30 seconds, the extent to which the Stony Brook findings offer an indictment of high-magnitude vibration is unclear. But when talking about devices that individuals can purchase for use in their own homes, and not necessarily under the supervision of a practitioner or even a coach, the potential for abuse is undeniable.

And for researchers studying the elderly or other potentially vulnerable patient populations, safety is a primary concern. Kiel says that's one reason why he has chosen to use the low-magnitude Juvent device in his work, including a multi-year randomized, double-blinded, placebo-controlled trial of subjects 65 and older that has been funded by the National Institute on Aging.

“We felt that the low magnitude device had the best scientific background,” Kiel said. “Seniors, especially, may be fragile, or they may have trouble standing, so we didn't want anything that would be too jarring.”

Sweet and low

Interestingly, it is vibration at the lowest—and safest—levels of magnitude that appears to have the most significant clinical impact. In a 2004 study published in the Journal of Bone and Mineral Research,⁵ Rubin and colleagues analyzed 70 postmenopausal women who were randomized to two 10-minute sessions per week of standing on a either a platform that vibrated at 30 Hz or a placebo platform that did not vibrate (both devices emitted a low-frequency audible sound throughout the 10-minute session to suggest that even the placebo devices were “on”). Although the investigators found no significant between-group differences in bone density at one year when using an intention-to-treat analysis, they did find significant differences when adjusting for patient compliance and body weight (a risk factor for low bone density). In those who were more than 85.9% compliant, exposure to vibration was associated with a statistically significant relative benefit of 2.17% at the femoral neck (+0.4% in the intervention group vs -2.13% in the placebo group). And in those who were more than 85.9% compliant and weighed less than 65 kg, the researchers found a statistically significant relative benefit of 3.35% at the lumbar spine, with the intervention group experiencing a 0.18% gain while those in the placebo group had a 3.17% loss.

In the same issue of JBMR, researchers from the University of Manchester in the U.K. performed a similarly-designed study⁶ in 20 children with disabilities that limited their mobility (thus decreasing the amount of loading their bones would experience on a daily basis). The children stood on a low-magnitude or placebo vibration platform for 10 minutes per day, five days per week, for six months. Despite poor (44%) compliance, the investigators found that the children who stood on active platforms had a 6.3% gain in volumetric trabecular bone mineral density at the proximal tibia, compared to a loss of 11.9% in the placebo group (a statistically significant difference). The researchers also observed a positive net benefit of treatment at the spine, but that was not statistically significant.

Most recently, investigators from Children's Hospital of Los Angeles and the University of Southern California studied 48 adolescent girls with low bone mineral density and a history of skeletal fracture. In that study, published in the September 2006 issue of JBMR,⁷ the use of low-magnitude vibration for 10 minutes per day was associated with statistically significant increases in cancellous bone in the lumbar vertebrae (2.1% vs 0.1%) and cortical bone in the femoral midshaft (3.4% vs 1.1%) compared with controls who did not receive vibration. As in the 2004 study by Rubin et al, the Los Angeles team found that results improved with compliance—however, they also found that no additional benefit was experienced above a threshold level of two minutes of vibration per day.

The challenge of treating low bone mass in children is particularly daunting, said Tishya Wren, PhD, an assistant professor of research in the orthopedic center of Childrens Hospital Los Angeles and a co-author of the September 2006 study, who also recently completed a study of low-magnitude vibration in children with cerebral palsy. “In kids we don't really talk about osteoporosis per se, but what's happening in kids is that they're behind in their gain of bone mass, which later in life translates into osteoporosis,” Wren said. “All of the medications for osteoporosis were developed for the elderly, so we really don't have a very good idea of what the effects are in kids, especially because you're talking about really long term use.”

Higher power

Research using higher vibration magnitudes has produced conflicting results with regard to bone density. In a study presented at the most recent meeting of the ASBMR in September,³ investigators from the University of Oklahoma found no significant effects of vibration on bone mineral density, bone mineral content, or bone biomarkers in 54 estrogen-deficient post-menopausal women randomized to receive vibration plus resistance training, resistance training alone, or no intervention. The 22 subjects in the vibration group used a PowerPlate device, on which they performed dynamic movements in three positions – standing, seated, and standing off the device but connected to it by straps – in sequences that began with one set of 15-second sessions and progressed to two sets of one-minute sessions, three times per week for eight months.

Study author Ian J. Palmer, PhD, who said the acceleration of the platform was 2.16g, believes his group might have seen different results with a low-magnitude device.

“It's got to come down to the magnitude,” said Palmer, now research coordinator for Oklahoma Sports Science and Orthopedics in Oklahoma City. “Vibration just seems to have a better effect when it's that lower magnitude. Also, maybe more time spent on the platform and greater number of days in the week could be key issues.”

Researchers from Leuven, Belgium, published a study in March 2004⁸ suggesting that performing static and dynamic knee extensor exercises on a PowerPlate vibration device (2.28-5.09g) three times a week significantly increased bone mineral density at the hip in 70 post-menopausal women after six months. But in a one-year trial presented by the same authors at the 2006 ASBMR meeting,⁹ bone mineral density at the femoral neck decreased by a nonsignificant 0.6% in 42 postmenopausal women randomized to receive vibration compared with a loss of 1.6% in control subjects; the between-group difference was not statistically significant.

More positive results have been associated with the high-magnitude Galileo device, whose accelerations have been estimated to range between 2.5g and 14g. At the 2006 ASBMR meeting, Japanese researchers reported a 3.7% increase in calcaneal bone mineral density in 40 female subjects between age 60 and 80, who used the device for three one-minute durations, three times per week, for six months.¹⁰ And in a study published in the November 2006 issue of the online journal BMC Musculoskeletal Disorders,¹¹ investigators from Spain found significantly greater increases in bone mineral density at the femoral neck but not the lumbar spine in 14 post-menopausal women who experienced vibration, compared to 14 in a control group. The vibration protocol involved six one-minute doses per session, three times per week for eight months.

Strong stuff

One positive aspect of the two Power Plate studies is that, despite not finding significant effects of vibration on bone health, both did find that performing resistance exercise on the vibration platform significantly improved muscle strength. Those strength increases, theoretically, could lead to improvements in bone density over a longer period of time, as greater strength would enable subjects to intensify their resistance training. Because poor strength has been shown to be a risk factor for falls and fracture,¹² strength increases may also help reduce falls. (The same may be said for improved balance, which research^13,14 suggests may be another benefit of high-magnitude vibration training in older adults.)

But lower magnitudes of vibration have also been associated with strength gains, at least in younger subjects. In their study of healthy adolescent girls with low BMD,⁷ Wren and colleagues found that the cross-sectional area of the paraspinous musculature was 4.9% greater in those who underwent vibration.

In the just-competed CP study, however, Wren's team found no statistically significant benefit of vibration with regard to either muscle or skeletal health, possibly because of the heterogeneous nature of that patient population. They have no plans to continue to study vibration in the near future, she said.

“Our feeling is that it has an effect, but it's not as overwhelming as we had hoped,” Wren said. “So we're not sure whether we'll keep pursuing it or not.”

Kiel and Rubin, on the other hand, will be immersed in vibration technology for at least the next few years. Kiel's VIBES study, which is currently enrolling patients, will install low-magnitude vibration devices in at least six independent living facilities in the Boston area. More than 200 patients will be randomized to stand on either an active platform or a placebo platform for 10 minutes per day, for two years.

At the completion of the study, currently projected for 2010, the researchers hope they will have further evidence that when it comes to vibration, more isn't always better.

“If I shine really bright light in your eyes, you don't see more clearly,” Rubin said. “Just because one aspirin is good for you, doesn't mean that you should take 50 aspirin per day. The same is true for mechanical stimulation as well.”

Jordana Bieze Foster is a freelance writer based in Massachusetts and the former editor of BioMechanics magazine.

References

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2.International Standards Organization. Evaluation of human exposure to whole-body vibration. Geneva: ISO 2631:2631-2634.

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