ATLANTA -- Are you tired of exercise machines because they bore you to death? Or are you leery of aerobic dancing because it makes you look funny? Are you thinking about giving up exercise anyway, since it's about to get cold out there? Wait a minute.

Before you give up, meet a humble old friend, the step, what stairways are made of. Of course, we're not talking about just any old step or your routine step-walking here; we're talking about step aerobics -- the Maserati of exercise in the '90s.

And it all started with an injury to a blond, energetic aerobics instructor in Atlanta, Gin Miller. To help repair a knee injury caused by high-impact aerobics, Gin's therapist put her to work on a milk crate -- simple steps up and down.

Awfully boring, Gin thought. Hey, why not put those steps to music? Develop a routine or two with some willing volunteers already burned out on high-impact aerobics? Stepping out and up on homemade wooden boxes, the first step-aerobic classes overflowed with converts, and rightly so.

"It's an excellent aerobic activity," say Peter and Lorna Francis of San Diego State University. "Why, stepping on a 10-inch box at 120 beats a minute gives you the workout of a seven mph run with the low impact of a 3 mph walk." A low-impact, high-aerobic workout. That's a nice definition of having your cholesterol-free cake and eating it, too.

"It's an excellent group activity, also," says Richard P. Boggs, who took the step from simple box to high-tech, mauve-colored exercise tool. Boggs and his partners invented and licensed the sleek, height-adjustable step now sporting the Reebok name. "With step classes," he says, "you don't have to sit there on your bicycle with your head down, hoping your time will run out."

What you have to do is think all the time -- use your mind to keep your feet and hands coordinated as you step up and down. Judith Drasner, 45, company manager for "Sheer Madness" at the Kennedy Center, says that makes the time fly. "It was getting to the point where high-impact aerobics was getting tough on my body, but I still wanted to feel strong, have aerobic endurance and feel energetic," she says.

Drasner became part of the first step-aerobics class at Somebodies Exercise Studio in Georgetown, and her reaction then (in April) was like the others in the class. "Fabulous. Because it works every muscle in your body, especially when you use hand weights," says Nina Black, 36, Somebodies' step guru.

Men particularly like step-aerobics. "It's a more macho workout," explains Black.

"It doesn't require the dance mentality that usually goes with aerobic workouts, what men feel are silly kicks and high knee lifts," says Jason Landry, 26, associate director of the aerobics activity center at the Cooper Clinic in Dallas, a leading aerobics research center.

"It gives you a great workout and a great burn," says Bob Chipperfield, 56, a consular officer at the Department of State's Bureau of Consular Affairs. Chipperfield started high-impact aerobics six years ago "because I was having a hard time walking up the street." Then he weighed 225 pounds; now he weighs 180, and three days of step-aerobics help keep the weight off.

Young people also like step-aerobics. "They learn to respect it very quickly," says Helen Samuels, 34, step instructor at the Omni Health and Racquet Club in Southampton, Long Island.

I decided to test her statement. Right there in the front line of Samuels's class was a lady named Peggy Bishop, 52, a former schoolteacher. "I didn't like step at first," she says, "because I wasn't coordinated enough. But now I'm addicted." Tanned and blond, Bishop looks determined as the step class begins. Right beside her is James Royce, 24, a recent participant in the Brooklyn half marathon.

After 30 minutes of steps and hand movements, Peggy Bishop's pulse was at 90 beats per minute and Royce's at 110. After 50 minutes, a normal class, Bishop's pulse was 120, 10 beats lower than Royce's.

Here's how you can try step-aerobics out:

Find the right step at home and do it to music. You'll need a very sturdy platform, wide enough for both feet to be on it at the same time. The Reebok platform is 43 inches by 16 inches.

If you're new to aerobics, don't make the platform more than 4 inches high. Height is what determines the difficulty of step-aerobics. "Unlike running or aerobic dancing, where you slow down as you tire," Lorna Francis says, "in step training, cadence or tempo is fixed regardless of your fatigue level. Start at a low height until you're conditioned."

How you step also is critically important. "Stepping forward off the platform increases the impact force 25 percent, something we don't recommend," says Francis. Back down off the platform in a cadence something like this: right foot up, left foot up; right foot down, left foot down. Bring your heel down when you land on the floor or platform, and keep your foot almost flat on impact. If you're going to attempt this at home, try to go for 15 minutes, checking your pulse every five minutes.

Check your pulse rate: Your maximal heart rate per minute -- the number you NEVER should go above -- is roughly 220 minus your age. Never go more than 85 percent of that maximal rate. The goal for a good sustained aerobic workout is to achieve 60 percent of your maximal rate for 30 minutes. Compare your resting heart rate to that 60 percent goal in your trial workout. Find a step-aerobics class and give it a try. Dozens of clubs are teaching it. When you call a club, ask them several questions:

Do I get a free try?

Do you have variable height steps? (Very important if you're out of shape.)

Do you have a trial membership? Don't sign up for year-long memberships without giving it some very serious thought.

It really is worth the effort: At the six clubs I've visited recently, participants in step classes use the words "step" and "fun" interchangeably.