High-intensity interval training (HIIT) describes any workout that alternates between intense bursts of activity and fixed periods of less-intense activity or even complete rest.

Efficient. Super-efficient HIIT is the ideal workout for a busy schedule—whether you want to squeeze in a workout during your lunch break or to get in shape for a fast-approaching event. Research shows you can achieve more progress in a mere 15 minutes of interval training (done three times a week) than the girl jogging on the treadmill for an hour. (A 10-minute fat-blasting cardio routine that burns more calories than a half hour on the treadmill!) And according to a 2011 study presented at the American College of Sports Medicine Annual Meeting, just 2 weeks of high-intensity intervals improves your aerobic capacity as much as 6 to 8 weeks of endurance training.

Burn More Fat. Not only do you burn more calories during a HIIT workout, but the effect of all that intense exertion kicks your body’s repair cycle into hyperdrive. That means you burn more fat and calories in the 24 hours after a HIIT workout than you do after, say, a steady-pace run.

Healthier Heart. Most people aren’t used to pushing into the anaerobic zone (that lovely place where you can’t breathe and you feel like your heart is trying to jump out of your chest). But in this case, extreme training produces extreme results. One 2006 study found that after 8 weeks of doing HIIT workouts, subjects could bicycle twice as long as they could before the study, while maintaining the same pace.

Lose Weight, Not Muscle. Anyone who has been on a diet knows that it’s hard to not lose muscle mass along with fat. While steady state cardio seems to encourage muscle loss, studies show that both weight training and HIIT workouts allow dieters to preserve their hard-earned muscles while ensuring most of the weight lost comes from fat stores.

Increase Metabolism. In addition to increased fat burning and more muscle preserved, HIIT stimulates production of your human growth hormone (HGH) by up to 450 percent during the 24 hours after you finish your workout. This is great news since HGH is not only responsible for increased caloric burn but also slows down the aging process, making you younger both inside and out .

Challenging. This is not a workout you can do while chatting with your friend. Because it’s so short, you will be working hard the whole time. The trade-off is this format offers seasoned exercisers a new challenge and new exercisers a quick way to see results. You may be in pain, but you definitely won’t be bored.

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