Most medical experts in fitness and physiology advocate a more varied regime but concede that weight training has become more medically accepted in the last 15 years. They reject the notion of discarding cardio workouts entirely.

The latest, shorter regimes rely on the basic principle of replacing fat with muscle because muscles burn calories at rest, and fat does not.

''Muscles are like an investment in the bank, earning you money,'' Mr. Cruise said. ''Fat is like a job you go to and once you leave, it stops paying. Once you get off that treadmill, you stop burning calories, whereas muscle keeps burning all day long.''

Mr. Cruise's workout can be done at home, using free weights. Jim Karas, who is Diane Sawyer's trainer and the author of ''Flip the Switch: Discover the Weight-Loss Solution and the Secret of Getting Started'' (Harmony Books, 2002), trains his clients with inflated rubber balls, free weights and resistance bands. He prescribes two to three sessions of 20 minutes each a week, with five minutes of cardio to warm up.

Mr. Zickerman and Fredrick Hahn, the author of ''The Slow Burn Fitness Revolution: The Slow Motion Exercise That Will Change Your Body in 30 Minutes a Week'' (Broadway Books, 2002), subscribe to the slow-motion school of lifting, which is performed, ideally, on customized weight training machines, though their books tell how to do the workouts at home. The Hahn workouts consist of six or seven exercises performed for three minutes each.

''By moving heavy weights at a slow pace, you eliminate any momentum that might help get the weights up faster and make it easier on the muscle,'' said Mr. Hahn, who owns Serious Strength, a gym on the Upper West Side.