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We're all aware of the health benefits of exercise, but for many it has become the eat-your-vegetables activity of adulthood—something we know is good for us, though we don't particularly enjoy it. But as children, fitness was not a routine, it was simply routine—a natural, integral part of our lives. It had nothing to do with reps or sets or goals of any kind; it was about climbing a jungle gym, running through a sprinkler, and jumping rope just for the fun of it. Movement that increased strength, endurance, and flexibility had its own reward: a good time.

Around the country, growing numbers of fitness enthusiasts have begun to put a childlike spirit of fun back into exercise. Alongside high/low aerobics and Spinning, you may also find classes called Recess and P.E. 101, where participants get their pulses racing in vigorous games of leapfrog, hopscotch, or tag. Other people, in groups or by themselves, are simply unleashing the child within while walking, running, or biking. By adding an element of play to workouts, getting and staying in shape can become something you might actually look forward to.

"Play is a very motivating part of exercise," says fitness instructor Mindy Mylrea. "When you're playing, you can exercise longer and harder without being aware of it. After a fun workout, you're not thinking, 'Oh, I'm glad it's over.' You're feeling alive and energetic."

Stacey Powells of Santa Clarita, California, tries to put a little bounce in her step during her daily walks with her two dogs. "I run up hills, walk backward down them, run zigzag across the street, create little obstacle courses out of trees and creek beds—anything to make my walk interesting," says Powells, 41. "I've taken step classes and found myself staring at the clock—doing something fun is much more appealing."

It was people like Powells who pushed the play trend into gyms several years ago. Many beginners complained that choreographed classes weren't cutting it for them. Although they liked the motivation and craved the camaraderie, they weren't crazy about the intimidating, hard-to- follow movements.

Next: Why "play" provides such amazing fitness benefits