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ATLANTA, Georgia (NBC NEWS) --High intensity interval training tops the list of fitness trends for 2014.

Cardio, power and strength training are combined in a single one-hour session at Orangetheory Fitness.

It's interval training with a twist.

Everyone wears a heart monitor and heart rates are displayed for all to see on large TV screens.

For a third of the workout they're encouraged to get in the "orange zone", when the heart is beating at 85 percent of its max.

Dr. Karol Watson, a cardiologist at UCLA, says keeping your heart rate in this range for short periods of time is beneficial.

"We think at that level you're getting the most effective fat burning, along with the most benefit to the cardiopulmonary system without causing harm," she explains.

So far the Orangetheory franchise has seen nothing but green.

Nearly 200 gyms have been licensed nationwide and they expect to get to 500 by 2017.

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