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I always believed (and have been following this theory for at least a decade) that in order to build muscle you have to lift weights at least three or four times per week and complete at least three sets with all your muscle groups. And this doesn't even include doing cardiovascular exercise (i.e., walking, biking, etc.). Let me tell you, staying fit is hard work -- not to mention the time commitment.

Lately, I've been seeing all this hype about getting that "super body you've always wanted" with a 20-minute workout only three times per week. Or even better, I know places that promise results with just 20 minutes ONCE a week. Are we talking about an oil change here or a workout?

Regardless of the program, when you're trying to build muscles, "the amount of repetitions, sets and the intensity are not as important as taking the muscle to complete exhaustion. That's how you get the best results," says Stephen Rice, M.D., Ph.D., M.P.H., a sports medicine specialist at the Jersey Shore Medical Center in Neptune, N.J. 

Working your muscle to fatigue (when you literally can't move another muscle) during exercise actually causes microscopic injuries to the muscle. As the muscle repairs itself, it becomes stronger by building larger fibers to prevent future injury. This is how you sculpt your body. We use weights to help do this, because it takes much longer to exhaust the muscles through calisthenics or other types of exercise. 

Very slow workouts have become popular as a way of saving time in the quest to fatigue muscles. The idea is that each part of the movement's repetition takes about 20 seconds (as opposed to the traditional, which takes six), working each muscle to complete exhaustion. Because of the intensity of the movement, you only complete one set per muscle group with about four to six repetitions.

"Working out slowly allows your body to just focus on the muscles themselves, without 'cheating' by using momentum," says Adam Zickerman, author of Power of 10 (Quill, $14.95). But some experts don't think this is as important. "You only lose about 20 percent of the exercise's effectiveness to momentum, which isn't so bad. Risk of injury is the only real concern," says Susan Lewis, M.D., director of the Center for Sports Medicine at Saint Francis Memorial Hospital in San Francisco.

And what about the claim that very slow workouts are more effective at helping you lose weight and sculpt your body? The results are mixed, but recent studies have shown you burn fewer calories and gain less muscle, especially if you only do it once per week. Also, almost all the studies that have been completed had subjects do their workouts THREE times per week, not once.

Nonetheless, Zickerman says his plan is more realistic for people's tight schedules. And he doesn't discount the importance of watching your food choices. "The fitness industry advocates working out several hours per week with cardio and weight training -- meanwhile most are sitting in front of their TV eating chips -- all I'm saying is, watch your diet, work out once per week doing exercises slowly and correctly, and see what happens," adds Zickerman.

Doing shorter, slower training sessions may seem like a breeze, but it may not be what you think. "Working your muscles until they are completely busted, right at their limit, is not a simple task and is also quite painful. Not only that, but it can also be difficult to do without a trainer or coach standing on top of you," adds Rice.

Many in the "slow workout camp" also claim that you don't need to do any aerobic or cardiovascular exercise -- this 20-minute session each week "will be all the exercise you will ever need. But don't throw out your treadmill yet. "Yes, you might improve the strength of your heart muscle through strength training, but it is not necessarily a functional improvement -- the heart needs to practice pumping blood to become functionally better," says Rice.

The other types of shortened workouts that have become increasingly popular are express or "quickie" workouts designed as a circuit with workout "stations."  They are meant to be done three times per week and take approximately 20 to 30 minutes per session. Town Sports International (in Boston, New York, Philadelphia, and Washington, DC) launched the XpressLine in which you go through a series of machines completing one set of each exercise. You're done in just 22 minutes. Others, such as the trendy Curves for Women, even incorporate cardio into their half-hour workouts -- aerobic activities like jogging in place are done at the stations between weight machines. Some experts warn that these types of workouts encourage "bad form" because you're doing them quickly, trying to keep within the time frame.

So how will you get the best results from your workout? That depends on your goals and your preferences.

"For some people, a workout needs to be quick or they won't do it at all. Anything is better than nothing!" says C. Jessie Jones, Ph.D., professor of kinesiology at California State University, Fullerton.

Bottom line: Doing any kind of strength training regimen is better than not doing it. And yes, just once a week is great for beginners, but you should probably increase to two or three times per week after six months. Keep in mind that you need to increase the weights or vary your workouts about every 12 to 15 weeks -- your muscles need new challenges, or they will stop growing. Also, if you're trying to lose weight, remember that larger muscles mean increased strength, improved appearance, and more calories burned. Studies have shown that one pound of muscle burns between 30 to 50 calories per day even when you're not using it -- whereas a pound of fat only burns two to five calories per day.

All experts agree that if you like doing something, and you get results, you will sustain it for a longer period of time. And for most of us, that's the biggest challenge of all.