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What's more, short, intense workouts also get you fitter in less time. In a side-by-side comparison, researchers at McMaster University in Ontario measured fitness gains in eight interval exercisers—20- to 30-minute cycling workouts that included four to six 30-second sprints—against eight volunteers who pedaled at a lower intensity for 90 to 120 minutes. After two weeks, the interval group was every bit as fit as those who worked out three to four times as long.

One reason intervals are effective is that they target more of your muscle, says Canadian study author Martin Gibala, PhD, associate professor of kinesiology at McMaster University. The average exerciser uses a lot of slow-twitch (endurance) muscle fibers and way too few fast-twitch (speed and power) fibers. Working more fast-twitch fibers not only gives you firm, shapely muscle tone but also fast-tracks your fitness gains. "Your body thinks, 'Whoa! That was hard work,' and it responds by increasing your ability to use oxygen and burn fat," Gibala says.

Few among us can squeeze in the recommended amount of exercise—at least 30 minutes most days of the week. So here's a comparison any overscheduled woman could love: Do a 30-minute walk at a steady, moderate pace, and you'll shed 112 calories. But mix in eight 30-second speed-walking sprints, and you'll burn 165 calories. After an interval session, your metabolism can stay elevated for a full day, and you'll burn two to three times the calories you could expect from lower-intensity exercise.

You'll feel younger, as well. Fast-twitch fibers are the first to go with age, largely because neurons stop communicating with them. "Our research finds that you can increase neuron firing rates after just one week of training," says Christopher Knight, PhD, a neuromuscular researcher at the University of Delaware in Newark.

With more muscles buzzing at attention, you have more energy for life, says Melanie Buchholz, 42-year-old mother of three from Jackson, Tennessee, who started interval training five years ago. Yes, she shrank from a size 12 to a 6, but she gets more pleasure from the vigor she gained: "Friends talk about how tired they are. This workout gives me great energy. And I sleep very soundly."

As a reminder, always consult your doctor for medical advice and treatment before starting any program.

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