Passive Exercise: Whole-Body Vibration and More (cont.)

That vibration, says Quist, tricks the body into thinking you're falling. "This, in turn, activates the 'stress reflex' -- an extremely rapid muscle contraction," says Quist, owner of Form and Fitness, a Milwaukee health club and rehabilitation center, where he has been training patients on the Power Plate for over a year. These muscle contractions, says Quist, are responsible for most of the benefits attributed to this type of exercise.

The Promise: According to manufacturers, those benefits include increased circulation, muscle strength, and flexibility; better range of motion; core conditioning and stability; and faster muscle recovery after working out. They say the health benefits also include enhanced metabolism, increased bone mineral density, reduction of the stress hormone cortisol, elevation of human growth hormone levels, and improved lymphatic flow. Whole-body vibration is also said to reduce cellulite and stimulate collagen production for smoother skin. Manufacturers also say MBV can provide muscle toning and conditioning for those who have health restrictions that keep them from exercising, such as multiple sclerosis, Parkinson's disease, and some forms of arthritis.

What the Experts Say: While the experts who spoke with WebMD all agreed that WBV does offer some benefits, all cautioned that the level is nowhere near the claims being made.

"I've seen some remarkable results in terms of bone density -- working better than conventional exercise -- plus good effects on circulation and muscle stimulation for those who can't do conventional exercise," says Quist. "But I don't think it can help you lose weight or impact cellulite. There is really no solid medical evidence backing up these or other health claims."

Cedric Bryant, PhD, chief science officer of the American Council on Exercise, says while whole body vibration has potential, more research is clearly needed.

"Right now, the marketing and hype is greatly outpacing the research and the scientific evidence -- but that said, from a conceptual standpoint, it could presumably improve muscle strength and stability, and an increase in bone density," says Bryant.

Indeed, in one study of 90 postmenopausal women published in the Journal of Bone and Mineral Research in 2004, a group of Belgian researchers found almost a 1% increase in hip bone density among users of the Power Plate form of WBV, along with measurable increase in muscle strength. The study participants used the machine for a total of 30 minutes three times a week for six months.

In another study, published in the Journal of the American Geriatric Society, researchers found that elderly people who were not able to participate in traditional exercise saw muscle strengthening and speed-of-movement benefits from using the Power Plate.