Running through the forest on a hiking trail a group follows fitness instructor Kathy Craft, but her gym classes aren’t gym classes at all.

“Almost there, stand tall, really lower your hips, abs are tight,” hollers Craft.

A foot bridge becomes a challenge, a hiking trail instead of that stair master.

“If you are in an indoor exercise class, you are watching the clock, when is this going to end, and thinking about the rest of your day, instead of being in the moment,” explains Craft.

Outdoor versus indoor workouts? Experts say calorie-wise, outdoors takes first place.

Moving over different terrain is tougher, the wind helps burn more calories, and more vitamin D means more endorphins than inside exercise.

“Coming out here in the fresh air, it just really relieves my stress,” says Mary Ann Ransler, a member of the outdoor fitness class.

Nearly a dozen studies agree; outdoors beats indoors by almost every test: increased energy and revitalization, while decreasing depression, tension, and anger. And, if you start exercising outside, you’re more likely to stick with your regimen than your indoor counterparts.

“I think people are happier when they are exercising outdoors because they’re not staring at a screen, being told what their heart rate is. I think people will push themselves harder when they’re not reminded of what they’re doing,” says Dr. Rebecca Barker who practices internal medicine at Palo Alto Medical Foundation.

Outdoor workouts can also be “family time." Instead of yet another circuit training class, chase your child through a park. You’ll probably burn as many or more calories, and your kids will be healthier too.

And, exercising outdoors doesn’t come with monthly dues.

Stuck in the city? Try Zumba on Tuesdays or Gentle Yoga on Wednesdays, all outdoors and part of the free summer fitness classes at the Seattle Center.