Has the nature of being a kid changed?

CHICAGO (WLS) -- Childhood is a carefree time filled with hours of running and playing outside. Or at least it was.

Today, a child's leisure time may look far different, and a new report indicates a majority of kids are now unfit.

CDC researchers put more than 600 kids aged 12 to 15 through the paces on treadmills to measure their heart and lung fitness. They found only 42-percent of these kids met the definition for fit, which is down more than 10 percentage points from 54-percent in 2000.

It's not just weight, according to the report. Barely half of normal weight kids could pass the fitness test.

Experts believe TV, video games and texting and chatting online take time away from exercise. Kids should be active for at least one hour a day, they say.

The best way to encourage exercise? Parents should get out there, too.