USING WEIGHT MACHINES, he works through a half dozen muscle groups, diligently exhausting each one. Then he gets on with his life. "When I was running," he recalls, "the next day I would feel I was run over by a truck."

The new routine never leaves him feeling bonked, but that's not the best part. Alexander has shed some 20 unwanted pounds since switching regimens, and his waist has shrunk by four inches.

Could fitness be this simple? For three decades we've heard endlessly about the virtues of aerobic exercise. Medical authorities have touted running and jumping as the key to good health, and millions of Americans have taken to the treadmill (however sporadically) to reap the rewards. But the story is changing. Everyone from the American Heart Association to the Surgeon General's office has recently embraced strength training as a complement to aerobics. And as weight lifting has gone mainstream, so has the once obscure practice known as "Super Slow" training.

Enthusiasts claim that by pumping iron at a snail's pace--making each "rep" last 14 seconds instead of the usual seven--you can safely place extraordinary demands on your muscles, and elicit an extraordinary response. Slow lifting may not be the only exercise you need, as some proponents believe, but the benefits are often dramatic.

Almost anyone can handle this routine. The only requirements are Zenlike focus and a tolerance for deep muscular burn. For each exercise--leg press, bench press, shoulder press and so on--you set the machine to provide only moderate resistance. But as you draw out each rep, depriving yourself of momentum, the weight soon feels unbearable. Defying the impulse to stop, you keep going until you can't complete a rep. Then you sustain your futile effort for 10 more seconds while the weight sinks gradually toward its cradle.

Intense? Uncomfortable? Totally. But once you embrace muscle failure as the goal of the workout, it can become almost pleasurable. "When you do this right," says Dr. M. Doug McGuff, an emergency-room physician who runs an exercise studio in Seneca, S.C., "a brief workout is all you can stand."

BURNING ALL THE TIME

The goal is not to burn calories while you're exercising but to make your body burn them all the time. Running a few miles may make you sweat, but it expends only 100 calories per mile (roughly two Oreo cookies), and it doesn't stimulate much bone or muscle development. Strength training doesn't burn many calories, either.

But when you push a muscle to failure, you set off a cascade of physiological changes. As the muscle recovers over several days, it will thicken and the new muscle tissue will demand sustenance. By the time you add three pounds of muscle, your body requires an extra 9,000 calories a month just to break even. Hold your diet steady and, presto, you're vaporizing body fat.

When Rona Ostrow took up slow-motion training 14 months ago, she had battled breast cancer for nearly five years. The treatments had damaged her thyroid and sent her abruptly into menopause, leaving her weak, overweight and discouraged about restoring her body.

The 52-year-old librarian couldn't face the gym scene, so she signed on with Adam Zickerman, founder of an individual-training studio called InForm Fitness, for a brief weekly dose of slow lifting.

She has since lost four inches from her chest, waist and hips and regained some faith in her body. On a recent icy morning, she slipped and fell on the sidewalk. "I just jumped back up like a hockey player," she marvels.

Ostrow might have benefited from any strength-training program. But proponents insist the slow technique is safer and more effective than traditional methods. And preliminary studies suggest they have a case.

In 1993 and again in 1999, Wayne Westcott, fitness research director at the South Shore YMCA in Quincy, Mass., assigned untrained, middle-aged volunteers to one of two regimens. Both groups performed the same round of exercises. But while one group did 10-rep sets, spending seven seconds on each repetition of the exercise, the other group did five-rep sets, extending each rep for 14 seconds. Both groups put in the same amount of time, but over periods of eight to 10 weeks, the slow lifters gained 50 percent more strength than the controls.

JOINT-KILLING LUNACY?

Slow lifting isn't just for the infirm or the soft-of-stomach. A number of professional sports teams have adopted the drill, and bodybuilders are discovering that they too can gain by slowing down. The question is whether this is all the exercise a person needs to stay healthy.

Ken Hutchins, the Florida-based trainer who founded the Super Slow movement (and patented the name), claims adamantly that it is. In screeds with titles like "Why NOT Aerobics?" and "Aerobics is Dead," he dismisses anything beyond purely recreational running, jumping or dancing as joint-killing lunacy.

"By performing [aerobic] activities on your off days," he says, "you compromise the progress you could be making."