FAIRFIELD -- "You can do it! Keep going!" yelled Rachel Darden to her nearly two dozen clients.

The fitness instructor walked the length of the 3,000-square-foot Orangetheory Fitness studio, monitoring those pushing themselves to the limit on treadmills and those trying to reach 300 meters on rowing machines.

All were wearing heart monitors. As she walked, Darden looked up at two flat-screen televisions hanging overhead that showed the heart rate of each of the clients in varying colors, blue to green to orange and red.

On the machines, clients worked to keep their hearts beating in or near the orange zone, hence the name of the national chain of fitness studios.

"The orange zone is what we define as the fourth of the five heart-rate zones," said Mark Molina, owner of the Fairfield location, the first in the state. "It's when your heart rate is working at 83 to 91 percent of its maximum capacity, and we want you there for 12 to 24 minutes of the hour. Why? Because when you're in that orange zone for that period of time, science tells us you're not only going to burn a ton of calories during the class -- between 500 and 1,000 typically -- but you're going to get the EPOCH (excess post exercise oxygen consumption) or the afterburn. Your body is going to continue to burn and churn calories for up to 36 hours."

The studio opened late last month and already has roughly 300 members. There is no walking in and doing your own thing at Orangetheory. The intense workout -- consisting of 30 minutes of cardiovascular training and 30 minutes of strength training -- is done in classes with a fitness instructor leading the charge.

"You'll never see anybody walk in here with headphones on," Molina said.

The company has 140 studios nationwide including Molina's location at 525 Tunxis Hill Cutoff. "This is not a fad," Molina said. "This is science and technology mixed with a group dynamic."

The Wilton resident was first introduced to Orangetheory by a friend in Denver. He quickly became hooked.

A corporate and business lawyer for publicly traded companies for 20 years, Molina saw a chance to do something different. "I was looking to do something I could do for myself," he said. "I was looking to stretch my comfort zone."

In April, he signed the franchise agreement.

"Surprisingly, a lot of people around here have heard about it because they travel so much," Molina said. "But educating a new market on a new product is difficult. Fortunately, the Fairfield County market is savvy."

In the coming weeks, a second Orangetheory location will open in Shelton, and a Milford location is under development. "We are thrilled with the tremendous growth we've achieved this year," said Dave Long, CEO and co-founder of Orangetheory Fitness. "The recent opening in Fairfield has kick-started our presence in Connecticut and our continued expansion in the Northeast, a target market for the brand."

Molina said it's the intimate feel of a fitness studio -- not a gym, he asserts -- that drew him in.

"We're talking to people," he said. "We're high-fiving. We want to build something special that's about more than just churning people in and out."

The studio is open seven days a week, offering classes from 5:30 a.m. to 8 p.m. Monday through Friday and from 8:15 to 11:45 a.m. on Saturday and Sunday. For more information, call 203-883-8220.

ktorres@ctpost.com, 203-330-6321, http://twitter.com/ktorresbpt