A new study in U.S. published Wednesday suggested that kids cut back dramatically on exercise as they hit the teen years.

Researchers tracked 1,000 U.S. children when they were at age 9 and then again at ages 11, 12, and 15. The results showed that average levels of moderate-to-vigorous activity fell from three hours a day at age 9 to less than an hour at age 15.

What's more, fewer than a third of teens got even below the recommended 60 minutes a day for good health. Girls fell below the recommended time by about age 13.1 for weekday activity and boys, at age 14.7.

The sharp physical activity dipping may continue 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 study didn't go into why activity declines, but Nader said that it's "probably a combination of things."

He noted that kids today don't go outside as much to play, partly because of safety concerns. "Teens today may have competing things, such as computers and other technology that keeps them inactive."

(Xinhua News Agency July 16, 2008)