By the light of an orange glow, this group is sweatin' it out. The workout is called "Orange Theory Fitness".
It targets different heart rate zones - each represented by a rainbow of colors.
Here's how it works: Everyone straps on a heart rate monitor. As they exercise, their numbers are displayed on the overhead screens.
Colors change when they reach a certain zone. Most work is done in the moderate zone.
"You should be in the green zone- challenging but doable," explains Bronni Karatassos, ACE-Certified Group Fitness Instructor.
Next they take it up a notch.
"Push pace is where you move back into that orange zone - we spike that metabolism and you really start to condition your heart," Karatassos says.
The trainers say- if you reach the orange zone- and stay there for a certain amount of time, your body will burn more calories even after your workout. That theory is based on a science known as "EPOC."
"EPOC stands for Excess Post-exercise Oxygen Consumption and that is really why we breathe faster after we've had a period of strenuous activity," explains cardiologist Dr. Jyoti Sharma
of Piedmont Atlanta. "For several hours afterwards, you continue to have a really high metabolism and that's what turns into losing weight."
And with the monitors, the trainers are able to make sure everyone's where they need to be.
"This is where we move back into orange, watch those numbers come up, heart rate starts to increase here."
(Holly Firfer for CNN's Health Minute)