Most inventors treat their new products like children to be babied. But Ray Irwin wants you to step all over his. Irwin, chief financial officer of Atlanta-based Sports Step and a co-owner of the Sportslife chain of health clubs, claims paternity for a lightweight, adjustable platform that has fostered a craze in indoor exercise: step aerobics.

Stepping, in which exercisers step on and off a low platform while making vigorous arm movements, offers an effective cardiovascular workout with a lower risk of injury and joint stress than high-impact aerobics. It's taught at most major health clubs, and since the only equipment you need is a pair of sneakers and a stepper, you can do it almost anywhere. Among Sports Step's products are The Step II ($59.95), a popular home model, and The Step XT ($79.95), whose sleek design is suitable for travel. Both come with instructional videos.

There are a few stepping dos and don'ts. Knees should not bend more than 90 degrees. The foot should be placed in the center of the platform with each step. Heels should not hang off the platform. Experts advise against using handheld weights. And, as with any hard exercise, you should warm up first.

What's the big deal about stepping? Fitness experts contend that it combines the best of aerobic and calisthenic exercise, working out the heart and lungs while promoting coordination and muscle tone. A 1990 San Diego State University study found that step aerobics provides the benefits of a 7.5-mile-per-hour run with the low impact of a 3-mile-per-hour walk.

IF THE SHOE FITS. The right shoes are important for step aerobics. They should offer good forefoot flexibility and provide adequate heel lifts to accommodate stress on the Achilles' tendons and calves. Cross-training and aerobics shoes are acceptable because they offer superior cushioning and stability. Reebok International, which markets a stepper, and Ryka, which makes women's athletic shoes, both sell shoes specifically made for stepping.

The usual rash of celebrity exercise videos has followed the stepping craze. You can step along with Cher, Kathy Smith, and, of course, Jane Fonda. Step aerobics won't give you a movie star's body overnight. But steppers are fun, relatively cheap, and a lot more compact than most home gyms.

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