If you're looking for a gym that offers high-intensity interval classes, it used to be that CrossFit was your one-stop shop. Didn't like their weight-heavy philosophy? Well, you could always do intervals at home. While CrossFit still has the market cornered, there's a new brand of like-minded gyms – and they're growing fast.

Orangetheory Fitness, which opened its first workout studio in Florida in 2010, now boasts 88 exercise franchises in the U.S., with 175 more to come by the end of this year alone. You can credit at least some of that exercise mania to Orangetheory's claims that they have something CrossFit doesn't: constant monitoring so you can't cheat. "It's basically highly motivating group personal training in a structured scientific environment," says Orangetheory co-founder David Long.

Orangetheory members take 60-minute group classes that rotate through high-intensity intervals, while running on treadmills, indoor rowing, suspension training (like TRX), and using free weights. Everyone wears a heart rate monitor to make sure he or she spends at least 12 to 20 minutes in Zone 4/5, otherwise known as the "Orange Zone," or 84 percent or higher than your max heart rate. These intense intervals then lead to something called EPOC, excess post-exercise oxygen consumption, or when your body eats up more oxygen and calories in the 24 to 36 hours after you work out, thereby increasing your overall fat burn. "It's pretty much the only one of its kind," says Long of the Orangetheory concept. "We're getting people in that EPOC state, and we're completely changing the workout every day. And the fact we're in the group setting, it adds a ton of motivation."

Matt Fitzgerald, a trainer, coach, and author of more than a dozen books on fitness, says Orangetheory looks great on paper. "It looks like a good workout, and I'm sure it produces results for folks who get into it," he says, adding Orangetheory's strength component looks safer than that of CrossFit (which often demands very heavy or erratic lifting and pulling).

But Fitzgerald cautions against doing high-intensity intervals every day – so that you have time to recover. "I think HIIT sessions are indispensable to a fitness program aimed at maximizing results, but moderate-intensity aerobic workouts are, too, because they are complementary to HIIT," he says. "It's also psychologically difficult to go all-out day after day. The burnout rate in CrossFit is high and I wouldn't be surprised to hear it's high in Orangetheory, too. Sometimes it's nice to do a workout that feels good the whole way through."