THERE'S a new oddball at the gym these days, and I'm not talking about that guy who wears the gold chains and the too-tight spandex. This one answers to the name of Bosu, and it's literally half an exercise ball and twice the challenge, throwing even the most balanced off their game.

Unusual-looking equipment at the gym is nothing new: think stretchy bands or resistance tubing. But the Bosu -- which stands for "Both Sides Up" -- might take the cake. Imagine one of those giant rubber stability balls cut in half and secured by a hard platform and you're on the right track.

Bosu is the invention of trainer David Weck, who was inspired by the idea of standing on a stability ball. "It looked like fun," says Weck in an interview from his office in San Diego. "But I thought it would be easier to do if you cut the ball in half and gave it a more stable base."

The Bosu may rest on a stable base, but it is essentially still an inflated rubber ball, an unsteady surface that shifts under your feet. Weck says staying on the ball and maintaining balance calls on muscles that don't normally get a workout during weight training and other traditional exercises. Working core muscles in the abdomen, hip, pelvis and lower back areas will improve not only balance, but also agility, stability, coordination and even posture. And that's just the beginning.

Weck's first Bosu sale, in 1999, was to the U.S. national ski team, which bought an early prototype to use while training for the 2002 Olympics. Since then, Bosu has been used as a training device by professionals in hockey, basketball and football.

Bosu is not only for the pros; weekend athletes have plenty to gain as well. Arlington resident Jay Hamilton, a 42-year-old television producer, has been sweating it out in a Bosu core conditioning class at Sport and Health's Regency Club in McLean for almost a year.

"I needed the structure of a class," says Hamilton, who admits to only going to the gym to gain an edge in his golf game, and who also credits the Bosu class for putting an added zing in his swing. "Sports is just motion and balance. Whether you're hitting a tennis ball or a golf ball, you need that core strength and balance," he says. "I don't need to go to the gym and pump iron two hours a day to achieve strength gains. The Bosu classes keep me moving; I'm toned and more flexible, and I can even see my abs now."

More and more clubs are bouncing onto the Bosu bandwagon. If you're new to the ball, you may want to try a class or session with a personal trainer to help find your footing. "Lots of people see the Bosu out on the gym floor and aren't sure what to do with it," says Justin Kelly, an American Council on Exercise certified trainer at Sport and Health in Reston. The local gym chain is in the process of adding Bosu classes at all 29 of its Washington area locations.

"I tell my clients, 'Whatever you can do on a stability ball, or on the ground, you can do on the Bosu,' " says Kelly. "And then some."

Kelly, who trains on the Bosu, likes the versatility. Bubble side up, the 12-inch inflated dome provides a cushy surface for squats, lifting weights or simply improving your balance -- try standing on one leg and closing one eye (losing depth perception) to really work balance. As for lifting, the Bosu can also be used like a weight bench or stability ball: you can lie on it, stand on it, sit on it or put one leg on it (when doing triceps kick-backs or biceps curls). The instability factor is an added plus; you can work on balance and core strength while you lift.

Turn the ball over on its round belly and you've got an unstable 25-inch platform for push-ups; the weird wobbly surface, assures Kelly, is good for working small stabilizer muscles in the shoulders.

"Bosu has become so popular because people recognize that it's a good core body workout," Kelly says. "It improves flexibility and helps strengthen smaller muscles. It can be a cardio workout and it's low impact. You can sit, stand or lie on it, or even bounce and jump on it -- there's something for everyone and every fitness level."

Hamilton's enthusiasm has trickled down to his family. "The idea that I'm exercising motivates my kids to be more active." He even bought his own Bosu for home workouts. "The kids love it, they jump on it and use it, and have tried out the classes at the gym with me."

Hamilton remembers the first time he saw the Bosu at the gym. "I thought, 'What the heck can you do on this thing?' " Now a convert, he tries to convince others at the gym, especially the men, to try classes. "I think men see these things and say, 'No thanks, I'll stick with the treadmill.' So I stand at the door and tell them, 'Listen, I know they look strange, but they really work, come in and check it out for yourself.' "

FINDING IT

Contact your local health club, YMCA or community center for classes or instruction. The Bosu is available online at www.bosu.com or by calling 800-321-9236. The half-ball, with instruction book and 30-minute video, sells for $129.95.

Robin Kirby tries to balance on a Bosu ball. Maintaining balance uses muscles that don't normally get a workout.