Across the country, trainers are increasingly taking their clients outside.

From the trailheads of the San Francisco Bay Area to the boulders of Manhattan’s Central Park, more trainers and clients are lunging up hills, playing bear-crawl tag in the grass and soaking up the experience of exercising under open skies.

What is driving this interest? After all, gyms are climate-controlled spaces designed and outfitted to optimize exercise performance. The outdoors is subject to weather, traffic and uneven terrain. What’s the appeal of leaving an ideal exercise setting and embracing an uncertain outdoor environment?

Here, several fitness professionals who have integrated the outdoors into their programs weigh in and share their motivations for moving their clients closer to Mother Nature.

Reason 1: Clients Want It

“People want to be outside,” says Kristen Horler, MS, CEO and founder of Baby Boot Camp®, which she launched 16 years ago when living in San Francisco. Now based in Sarasota, Florida, Baby Boot Camp stroller fitness is a nationwide program focusing on cardio, strength and core workouts for moms with stroller-age children. Most of the workouts happen at local parks, using equipment moms can cart in their strollers. Horler says she still gets calls weekly from people wanting to set up a Baby Boot Camp in their cities.

She started Baby Boot Camp to give moms an opportunity to work out with their children without the guilt of dropping them off at daycare or having to pay for a babysitter.

“Everything is outside as much as possible,” Horler says. “Imagine being at the park and having your workout, then afterwards your little one can come out for playtime.” (Naturally, these programs move indoors if the weather does not cooperate.)

For Hank Ebeling, owner and coach at H4 Training in Wheaton, Illinois, the weather window for outdoor activity is short but impactful. Since opening 3 years ago, H4 Training has taken clients outside in the late spring and summer.

“People love it,” he says. “You’re outside; you’ve got trees. It’s a different vibe. It gets you out of a square box inside—especially in the Midwest when you’ve been freezing all spring and winter.”

Reason 2: Science Supports It

Science backs up the idea of humans’ innate desire to connect with nature. Dasilva et al. (2011) found that when participants walked inside on a treadmill, they felt more fatigue, tension and anger than when they walked outside, notes Jennifer Flynn, PhD, visiting instructor in exercise science at Maryville College, in Maryville, Tennessee. Moreover, participants experienced less fatigue and felt happier and more invigorated outside even though their walking speed was faster than it was on the treadmill.

Several studies have shown a correlation between being outside and feeling emotionally, mentally and physically better (Coon et al. 2011; Harvard 2010; Barton & Pretty 2010).

“Being outside, it almost provides a distraction,” Flynn says. “Humans are innately connected to nature, and so that’s one potential reason why that release would occur” (see the sidebar “Health Benefits of Outdoor Exercise”).

Reason 3: It’s Simple and Flexible

Trainers can easily adapt their outdoor workouts to their training style, company philosophy and local climate.

At H4 Training, Ebeling’s personal and small-group training company, outdoor workouts occur at local parks, and clients’ spouses can go along for free (friends are welcome, too). Ebeling uses basic equipment like bands and balls and puts down old mats for clients when there is dew on the grass.

“But for the most part we try to use what’s out there,” he says. That can mean integrating monkey bars and picnic tables into the workout. He also uses body-weight exercises and fun games like bear-crawl tag and partner mirroring—where one person picks a movement like jumping jacks or squats, and the other has to copy it.

“Don’t make it too technical or too detailed,” Ebeling advises. “Make it more simple. You’re outside, and you’re active and moving.”

Rick Richey, owner of Independent Training Spot in New York, uses the outdoors to train small groups for specific goals like Spartan® or Tough Mudder® adventure races.

“I just think it’s always nice for there to be some type of goal you’re striving for, so it’s not working out for the sake of working out or for losing weight,” says Richey.

He does his small-group training in Central Park, weather permitting. He uses everything from fartlek runs—sprinting from one light post to another or one tree to the next—to step-ups on benches and stairs.

“Everything that isn’t normally a fitness tool becomes a fitness tool,” he says.

Reason 4: It’s More Fun

Exercising outside feels like being a kid again.

“It’s nice to just go outside and play,” Richey says. “[The outdoors is] more conducive to playing, and when you can get people to see their fitness as more about having fun, you’re going to get more compliance and adherence.”

Tina Vindum, MS, owner of Outdoor Fitness in San Francisco, urges trainers to find creative, unique uses of outdoor terrain. She has used mailboxes, parking meters and even trash cans to do rows.

“The more clever you can be, the more fun [it is].”

Vindum says she also gets clients to connect with the environment, feeling the trail under their feet or the grass on their legs as they lunge forward up a hill.

“Mats on the sidewalk is not outdoor fitness,” she says.

Reason 5: It Matches Their Personality

Vindum, a former competitive skier and mountain biker, launched Out-door Fitness in 1995 after her own workouts convinced her that being indoors at the gym was not cutting it.

“It didn’t make sense for me to be a mountain athlete and not be on the mountain,” she says.

She took that passion and built it into a personal training career. Today, Outdoor Fitness boasts an outdoor fitness training and certification program. Vindum trains solely outdoors, rain or shine.

When the weather is cold or wet, the key is in the warm-up, she says. “It’s like jumping in a swimming pool. Once you’re warmed up, you kind of forget.”

Reason 6: It’s Good Business

Some trainers find that moving outdoors makes their services more attractive.

“We’ve actually gotten a lot of clients off of it,” Ebeling says of his outdoor workouts, noting that they create a buzz and that people at the park often inquire and follow up with him.

He sees that being outside also teaches people what’s possible—that working out doesn’t have to happen in a gym or be done in a complex way.

“You can bring fitness anywhere,” he says.

And that makes it so much easier to attract clients and keep them coming back.