"Vibration works ... but we're still trying to figure out how to use it best and I think we're a number of years away before we do that," said Bill Amonette, one of the study authors and a fitness expert at the University of Houston-Clear Lake. "I think we have to be cautious of some of the claims. ... With aggressive marketing, sometimes they claim things that aren't necessarily true."

Some researchers are also concerned that high-amplitude vibration can be dangerous over time since it can send jarring waves throughout the body, said Andrew Abercromby, another researcher at Johnson Space Center. "I believe, and I think quite a few other people believe, the jury is still out on it," Abercromby said.

Clinton Rubin, a biomedical engineering professor at State University of New York at Stony Brook, said he has asked Power Plate to stop citing his research in its promotional materials. His work has led to a vibration device before the Food and Drug Administration approval for prevention and reversal of bone loss from osteoporosis, but that device uses much gentler vibrations than Power Plate, Rubin said.

He believes the Power Plate's vibration levels could cause low back pain, cartilage damage, blurred vision, hearing loss and even brain damage.

"I think they are cavalier in dismissing the dangers of chronic exposure," he said. "I'm a scientist. I worry that people are going to use this device based on a misrepresentation of science."

Power Plate warns pregnant women and people with retinal detachment, blood clots, bone tumors and other medical conditions not to use the machines.

Soloflex recommends using its device only 30 minutes a day because too much use would exceed industry standards for safe vibrations in the workplace. (The Occupational Safety and Health Administration doesn't set safety levels for fitness equipment, an agency spokesman said.)

"I own a steel factory so I know whole body vibration has allowable exposure limits," Wilson said. "But too much aspirin is bad for one too."

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