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If you think that teenagers are becoming weaklings, you're right.

Less than half of youths ages 12 to 15 are even close to being aerobically fit, according to data released Wednesday by the Centers for Disease Control and Prevention.

That's down from 52 percent of youths in 1999 to 2000, the last time this survey was conducted. It measures "adequate" levels of cardiorespiratory fitness, which children need not only for sports but for good health.

And that was true regardless of a child's race and family income.

Girls were particularly out of shape, with just 34 percent of them having adequate cardiovascular health, compared with 50 percent of boys. But 50 percent isn't so great.

"It's frightening," Dr. Stephen Pont, an assistant professor at the Dell Children's Medical Center in Austin, tells Shots. Children tend to be less physically active as they become teenagers, Pont says, because there are fewer opportunities for organized sports, and less gym class too.

But given the nation's obesity epidemic, teenagers need to be more active, not less, says Pont, who was not involved in the study. "This may result in our childhood obesity epidemic getting worse."

The increase in childhood obesity may have peaked, according to datareleased last year by the CDC, at least in very young children. Even if that's true, Pont says, "the place they're stabilizing is still a horrible place to be."

As medical director for the Austin Independent School District, Pont has been active in making the point that adding physical activity to the school day actually improves standardized test scores and reduces behavior problems. But many school districts haven't gotten that message.

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