If you lunch at a drive-through, instant-message instead of e-mail, and "read" books on tape, you'll relish the timesaving possibilities of slow-speed strength training. The pitch: total fitness in just one or two 20-minute workouts a week, with no warming up, stretching, or cardio required. But is this approach a gimmick or a godsend?

In truth, it's a little of both -- and it's nothing new. Extolled in such recent books as The Slow Burn Fitness Revolution and Power of 10: The Once-a-Week Slow Motion Fitness Revolution, the technique involves lifting heavy weights at about one-third of the usual speed for a single set. The logic goes like this: The key to overall fitness is building muscle mass, and strength training is a far better way to do that than aerobic exercise. Lifting weights in slow motion is the most efficient strength-training technique, because it's the fastest path to muscle fatigue (the point at which you can't move another inch).

Pushing your muscles to their limit in one set saves time, but it probably doesn't build muscle as effectively as two or three sets at normal speed. Chances are, you'll have to lift lighter weights in order to do a slow-speed set correctly. "In any strength-training regimen, you need to lift at least 50 percent of the maximum amount you can handle in one rep to improve strength and muscle size," says Gary R. Hunter, Ph.D., who researches the effects of weight training on muscle growth. For instance, if the most you could lift in one rep is 40 pounds, you'd need to use 20 pounds for your biceps curls -- more than you probably can handle at such a slow pace. A professor of human studies and nutrition sciences at the University of Alabama at Birmingham, Hunter has found that most people can lift 25 to 30 percent -- or 10 to 12 pounds, using our example -- if they're doing a slow-speed workout correctly. That will give you some strength gains, but not as much improvement as you'd see with a traditional regimen.

Even that so-so outcome may be hard to achieve. While the aforementioned books include at-home workouts, sticking to a slow-speed routine is tough without a trainer to keep you in line. The intensity makes it hard not to hold your breath, break form, and scream obscenities. Consider the slow-speed motto: Success is failure -- muscle failure. That means your muscles shake and burn on the edge of total fatigue. Wayne Westcott, Ph.D., fitness-research director at the South Shore YMCA in Quincy, Massachusetts, says that only two of the 65 people he's introduced to the method have stayed with it. Those who love it may get a high from the extreme effort and sense of accomplishment, but most folks would equate it with legalized torture.

What about slow-speed's claims that more muscle is the best way to optimal fitness -- and that everything else, even flexibility and cardiovascular work, is unnecessary? Because the reduced speed eases you into each exercise gradually, you could technically drop the warm-ups and stretching -- though doing them anyway might enhance your results. In a study of people lifting at regular speed, Westcott observed a 19 percent strength advantage in those who stretched versus those who didn't. And although strength training does beat aerobic exercise for building muscle mass, cardio offers its own benefits. "It delivers more oxygen to the muscles and uses more muscles at one time, so it's a better way to reduce your risk for heart disease, lower your blood pressure, and keep your cholesterol in check," says Walter Thompson, Ph.D., professor of kinesiology and health at Georgia State University. "If your goal is to lose weight, cardio burns more calories for most people." In other words, whether you emphasize one or the other depends on your goals -- but both are necessary.