To sign up for one of the "Outside Fitness Powered by Merrell," log onto lifetimefitness.com/merrell

In the first five minutes of boot camp on a recent Monday morning, Miguel Ortiz has a class of 27 Life Time Fitness regulars exercising every muscle and audibly gasping for breath.

“Mountain climbers,” he barks over a chorus of Ga. 400 traffic and singing birds. “Let’s go. Keep those butts up. Five. Four. Three. Two. One. Nicely done.”

To most people, doing pushups, squats and sprint intervals outside in 80-degree weather doesn’t sound like a good time.

Yet, boot camp type fitness programs like this one - similar in some ways to the popular television show “The Biggest Loser” - are increasing in demand.

The reason? They provide a total-body workout that’s varied, fun and challenging, said ACE’s Chief Science Officer, Cedric X. Bryant. In addition to a great cardiovascular workout, Bryant said muscles are strengthened through high- and low-intensity exercises such as pushups, squats and lunges.

Life Time, Ortiz said, has offered outdoor fitness classes for at least the last five years. Now, in partnership with outdoor apparel and footwear company Merrell, the fitness center chain is offering free “Outside Fitness Powered By Merrell” classes through August.

“It’s a gym with a view, all fueled by fresh air and your own power,” said Emily Snayd, communications manager at Merrell.

Snayd said Merrell wanted a partner with national presence. Life Time, which has more than 100 locations, fit the mold.

“Together,” Snayd said, “we are able to motivate people to get fit, live healthy active lifestyles and appreciate all that the outdoors has to offer.”

The idea isn’t exclusive to Merrell or Lifetime.

According to an ACE survey, boot camp-style workouts, which were named the most popular workout in 2008, remained a top fitness trend in 2012.

Rising concern about obesity and sedentary lifestyles - paired with savvy marketing by fitness-related businesses - fuels the trend.

This year, the Centers for Disease Control announced a “spring into action” campaign and FamilyFun magazine has released a special “Get Outdoors” themed issue to help families spend more time in nature.

“When kids are in touch with nature, the whole world wins, since nature-loving children grow up with a greater sense of environmental responsibility,” said Ann Hallock, the magazine’s editor-in-chief.

That’s the idea behind Merrell’s and Life Time’s recent partnership, Snayd said.

“Technology, as great as it is in keeping us in touch, is making the pace of our lives increasingly faster,” she said. “We hope that these classes will, if even for an hour, get people to tune out and connect to the world around them.”

Not only has outside fitness grown in popularity, studies show its health benefits far outweigh exercising in a gym.

“Moving outside in our natural elements tend to be better for our bodies,” said Ortiz whose class attracts a fairly wide range of body types, mostly women. “It’s also more strenuous.

For instance, Ortiz said, “Treadmills stay flat. Running outside provides hills and curves and forces you to create your own momentum. And it’s more fun, especially with a team.”

Personal attention and socializing also are draws for many who turned out recently for Ortiz’s 9 a.m. class.

Lisa Farrar joined more than a year ago to shed pounds gained during pregnancy. She appreciates Ortiz’s in-you-face style.

“It’s challenging and hard and it pushes you to get out of your comfort zone,” said the 34-year-old Alpahretta mother of two. “It’s almost like a love-hate relationship.”

On this day, Ortiz, with help from trainer Chris Brown, wills his class through a total body workout, where every jump, squat and sprint is meant to build strength, endurance and agility.

“We don’t just worry about increasing the heart rate but offer zone training to make sure each member is at a level they can handle, versus working them to death,” Ortiz said. “We want to make it intense but we don’t want to kill them.”

About 15 minutes in, though, that is exactly what it looked like.

“Alright everybody,” Ortiz says pacing back and forth. “Keep those chests down, roll those shoulders back.”

Like Farrar, Tiffany Gentile, 35, had become bored with her indoor fitness classes. She’s glad she moved to outdoor bootcamp a year or so ago.

“All the baby fat is gone,” she said. “He pushes and challenges you.”