CHICAGO - One of the largest studies of its kind shows just how sluggish American children become once they hit the teen years: While 90 percent of 9-year-olds get a couple of hours of exercise most days, fewer than 3 percent of 15-year-olds do.

What's more, the study suggests that fewer than a third of teens that age get even the minimum recommended by the government -- an hour of moderate-to-vigorous exercise, such as cycling, brisk walking, swimming or jogging.

The steady downward spiral of lethargy raises concerns about inactivity continuing into adulthood, which could endanger kids' health throughout their lives, the study authors said.

"People don't recognize this as the crisis that it is," said lead author Dr. Philip Nader, a pediatrician and professor emeritus at the University of California at San Diego.

Inactivity is linked with greater risks for many health problems, including heart disease, obesity, high blood pressure and diabetes.

The new findings come just a week after an influential pediatricians group recommended that more children have their cholesterol checked and that some as young as 8 should be given cholesterol-lowering drugs. That advice was partly out of concern over future levels of heart disease and other ailments linked to rising rates of childhood obesity.

The latest study, appearing in today's Journal of the American Medical Association, tracked about 1,000 U.S. children at various ages from 2000 until 2006.

Special gadgets were used to record their activity. The device isn't worn during swimming and contact sports, but the researchers said it's unlikely that such activity happened often enough among the children studied to skew the results.

Average levels of moderate-to-vigorous activity fell from three hours a day at age 9 to less than an hour at age 15.

Nader said he was "surprised by how dramatic the decline was," and cited schools dropping recess and gym classes and kids' increasing use of video games and computers as possible reasons.

The National Institute of Child Health and Human Development funded the research, calling it one of the largest, most comprehensive studies of its kind to date.

James Griffin, science officer for the study, said that as children mature, "You would expect somewhat of a decline [in activity] but nothing of this magnitude."

He noted that the study coincided with the rise in popularity of video games, DVDs and Internet use -- "all of the types of things that take children from outside and put them on a couch or in front of a computer."