The right kind of workout can keep you torching calories long after you leave the gym. Here’s the real secret to melting maximum fat.
Science proves that high intensity interval training burns more fat in less time than a steady-state approach. When done strategically, HIIT can also trigger afterburn, in which your body uses calories at an increased rate for up to 24 hours post-workout. Interval training isn’t new, but exercise physiologists at Orangetheory Fitness (OTF), a hot workout studio with more than 110 locations nationwide, have packaged the concept in metabolically charged routines that guarantee you’ll keep in the zone all day.
The Science of Afterburn
Workouts that rev your metabolism long after you leave the gym may sound too good to be true, but the science is solid. “It takes oxygen and energy (calories) for 
your body to return to resting after exercise—that’s excess postexercise oxygen consumption (EPOC), or the afterburn,” says Christopher Scott, Ph.D., an exercise physiologist and associate professor at the University of Southern Maine. “The more time you spend at a high intensity, the greater the effect.” That’s because using more energy during your workout means more effort is required to return your body to resting levels. More effort means more calories gone.
A review in the Journal of Sport Sciences finds that intervals that take you to at least 85 percent of your maximum heart rate are optimal for hitting the EPOC sweet spot. That can translate to up to 15 extra calories torched for every 100 used during exercise.
OTF puts this science into practice. The studio’s precision routines maximize fat melting during class and keep your metabolism high after. According to Ellen Latham, M.S., cofounder of OTF and an exercise physiologist, you can burn more than 500 calories per class—and up to 15 percent extra in the 24 hours after you leave the studio.
The Orange Theory Experience
OTF’s goal is always a big afterburn, but the workouts themselves change daily. Here’s an example of a typical class: You strap on a heart-rate monitor (it syncs with screens in the studio that display everyone’s heart rate [HR] and tracks your time in various HR zones). The goal is to rack up a total of 12 to 20 minutes in the optimal-for-afterburn “orange” zone of about 84 to 100 percent of your maximum heart rate (MHR)—long enough to trigger fat burning during and after but not so long that you can’t sustain the workout, Latham says.
You start on a treadmill with a 27-minute routine—your coach guides you between base pace (an easy 71 to 83 percent of your MHR), push pace (an uncomfortable 84 to 91 percent of MHR) and all-out pace (a sprint at 84 to 100 percent of MHR).
Next, you switch to a rowing machine for a 12-minute routine, then finish with circuits of tone-up moves using various equipment. You may hit the orange zone during the rowing and strength segments, but that’s icing on the afterburn. Research suggests high intensity resistance training likewise contributes to EPOC. In general, however, Latham says this portion of the workout complements the cardio intervals by building power and strength, giving you a well-rounded, total-body workout that hits all the factors involved in jump-starting your metabolism.