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Gaining weight can do that to you: Michelle Falkenstein, a 45-year-old teacher and mother of three from Saskatchewan, felt discouraged, demoralized, and depressed. But she also had determination. Like millions of women, she had struggled with her weight and self-esteem much of her adult life. Then her doctor added high cholesterol to her list of woes. "That was it," she recalls. "I needed to change what I was doing."

Except that she was doing everything right. Falkenstein spent 45 minutes a day on cardio machines at her gym and another 30 or 40 minutes lifting weights. She was dieting, but her weight wouldn't budge. To cut down on gym trips, she bought an elliptical trainer. She had no idea how much time she would save.

Falkenstein's elliptical machine was programmed with workouts from the book Ready, Set, Go!, by Phil Campbell, which promised quick weight loss. Falkenstein decided to forgo her usual steady, moderate gait and try the 20-minute fast-paced workouts, which included eight 30-second intervals—full-speed efforts—followed by slower-paced breaks for recovery. (Most exercise machines offer some version of an interval workout.) She did it three days a week, along with lifting heavier weights—choosing amounts she could heft no more than 12 times. Six weeks and one dress size later, Falkenstein was beaming. "My clothes fit and my cholesterol is normal," she says. "And I feel a lot better about myself."

Speed Works
While most experts keep telling us we need to exercise longer if we want to lose weight—we're supposed to exercise 60 to 90 minutes a day just to maintain weight loss—a fast-growing body of research indicates that intensity, not duration, is really the missing piece in our fitness puzzle:

• Last December, Canadian researchers reported that just two weeks of interval training boosted women's ability to burn fat during exercise by 36 percent.
  
  
• Levels of human growth hormone—which assists in building muscle and eliminating fat—skyrocketed 530 percent in subjects after just 30 seconds of sprinting as fast as they could on a stationary bike, according to a British study.
  
  
• Australian fitness researchers had 18 women perform 20 minutes of interval training on a stationary bike—eight-second sprints followed by 12 seconds of recovery throughout the workout, three days a week. The women lost an average of five and a half pounds over 15 weeks without dieting, while a similar group performing 40 minutes of moderate cycling three days a week actually gained a pound of fat over the same period. Two of the heavier women who did intervals dropped 18 pounds.