If parents would make sure their school-age children walk, play and generally exercise every day, far fewer kids would be hurt when they embark on strenuous organized sports.

Dr. Lyle J. Micheli, director of sports medicine at Children's Hospital in Boston, says more kids are suffering from badly torn knee ligaments, for example. And what causes these injuries, he says, seem quite minor: perhaps just a wrong turn in a basketball game.

"There's nothing wrong with organized sports, but [kids] should be getting their exercise in a lot more ways, too," he says. "Like walking to school, riding the subways, playing pickup sports and helping around the house."

Parents should make sure kids exercise about an hour every day.