Photo: Stefano Moro van Wyk for Harper’s BAZAAR

If you’ve been anywhere near a gym in the past few years, you’re probably familiar with high-intensity interval training (HIIT). The practice of alternating short bursts of intense effort with low- to moderate-intensity recovery periods has been—and continues to be—extremely popular in the fitness world. It’s easy to understand why: HIIT comes with a slew of perks. Besides delivering more bang for your buck (in one study, people doing HIIT nearly doubled the number of calories they would have burned by jogging), some studies show HIIT may be particularly effective at breaking down fat, especially the fat padding the abdominal area. Years of research show a wealth of health benefits as well, including enhanced protection against heart disease, the leading killer of women in the U.S.

But maybe the best benefit—and likely the reason you’ll finally make HIIT a must-do—is how this type of training continues to burn major calories after your sweat session is finished. According to Len Kravitz, a researcher and program coordinator of exercise science at the University of New Mexico in Albuquerque, HIIT adds up to 15 percent more calories to the total torched. That means if you’ve worked off 500 calories doing HIIT, you can reasonably expect to burn at least another 75 calories post-exercise—even if you’re just sitting on the couch channel-surfing.

This after-burn effect is so appealing that it’s one of the main selling points of Orangetheory Fitness, an interval-based group class that’s taking the country by storm. (Studios are estimated to triple to 1,000 by 2017.) The 60-minute workout, which uses equipment like treadmills, rowing machines, and free weights, promises to keep your heart rate in the “orange” zone for 12 to 20 minutes over the course of the hour. That zone translates to 84 to 95 percent of a person’s estimated maximal heart rate, the maximum number of times your heart can beat in a minute without overexerting itself. When it comes to burning calories, this is the sweet spot. “It’s those spikes in your heart rate that cause a lot of calorie burn,” explains Matthew Kohn, an exercise physiologist and personal trainer with TroupeFit in New York. Because working out this intensely creates a high demand for oxygen, the body uses more energy as it restores its oxygen level to the pre-exercise state—hence the after-burn. Orangetheory claims that its workout will help you burn up to 1,000 calories, including the after-burn bonus.

So how else do you burn more calories during and after a workout? Another key is to keep your respiratory rate at a heightened level for sustained periods of time. “You really have to challenge your breathing mechanism to get the caloric burn higher,” says Kohn, who likens it to how you’d breathe when chasing after a dog that slipped its leash and took off running. “The average person should be at a 6 to an 8, with 1 being you just woke up and are still in bed, and 10 being I’m putting you on a stretcher and taking you to the hospital.”

You don’t have to be into group fitness classes to reap the rewards of HIIT; it can be incorporated into any type of exercise. Add sprints to your cardio workout, or when possible, challenge yourself on hills. Though weight training already produces an after-burn, you can strengthen it by limiting time between exercises to 15 to 45 seconds. The goal with HIIT is to reach a high level of intensity for as long as possible, says Kohn. Then, with every workout, try to beat that time. For instance, if you endured level 12 on a stationary bike for one- minute intervals, aim for 75 seconds on your next ride and 90 on the one after that. “The more you push, the stronger the after-burn,” says exercise physiologist Ellen Latham, founder of Orangetheory.

If the idea of pushing yourself super hard, even for short spurts, sounds too, well, hard, consider this: In one study, people reported much greater enjoyment with HIIT workouts than with continuously vigorous or moderate-intensity ones. And if you like your workout, you’re more likely to stick with it. That said, HIIT may increase the chances of injury and muscle soreness, so don’t go all out if you don’t have a base level of fitness. Even so, know that more HIIT isn’t always better. As the name suggests, high-intensity training is intense, so aim to do it two or three times a week, and allow for plenty of recovery time between workouts.