Among the machines getting a big push: the $3,500 Power Plate (the company says Madonna bought one after she fell off a horse), the $2,000 Ironman Resolution and the $495 Soloflex Whole Body Vibration Platform. "This is the greatest discovery since the Greeks and Romans perfected barbell exercises," says Soloflex founder Jerry Wilson.

"Experiencing vibrations during a workout isn't the be-all and end-all, but rather a great complement to what you're already doing," says Jasper Sidhu, president of The WAVE (Whole Body Advanced Vibration Exercise), whose $5,000 in-home version of a commercial platform arrives in January.

Simply standing on one isn't going to turn you into the Incredible Hulk. But if you're willing to stretch, lift weights or otherwise strain yourself on the platform, your muscles will get an added workout.

Rooted in research conducted during the 1960s space race, vibration machines work off the principle that if muscles are worked while being shaken, they call upon neighboring fibers, building mass faster. Technological advances have made it possible to build these devices, once found largely in hospitals, at reasonable costs.

Sidhu says his machine's price reflects "the quality of the vibration." But Soloflex's Wilson, whose product is smaller and markedly less expensive, shrugs off the notion: "When you're only on it for 30 minutes a day, what we offer works."

Experts say the key is the consistency of the vibration and the size of the platform, "both of which usually come with products that cost a few thousand dollars," says John Porcari, a fellow with the American College of Sports Medicine.

"There's no question that your muscles, through the contraction provided by the vibration, will get stronger" if you work out on the platform, says Porcari. "If you can lift 100 pounds easily, doing so while standing on this machine will soon get you to lift 110."

Sports medicine and rehab expert Dan Hamner says his New York office's hospital-grade Power Plate has helped get his patients up and moving.

"I usually come from the 'no pain, no gain' camp," he says. "But I've been amazed how this painless device has managed to help those people who didn't want to do anything.