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, like cycling, brisk walking, swimming or jogging.

The sharp drop 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, (more...)