Minneapolis — New research is charting the roots of the nation's obesity epidemic: Between the ages of 9 and 15, kids' daily physical activity drops from an average of three hours to less than one.

On weekends it's worse. Fifteen year olds, on average, move around only 35 minutes a day on Saturdays and Sundays, according to a study of 1,000 kids across the country.

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The results, published Wednesday in the Journal of the American Medical Association (JAMA), put physicians on notice that they need to pay more attention to whether their young patients are spending too much time online and on their cell phones, and not enough at the pool or park.

"There is a lot of pressure on physicians to start addressing this," said Dan Halvorsen, an expert in pediatric exercise physiology at the University of Minnesota.

It is the latest in a year of alarming news about childhood obesity and the diseases that come with it. Nearly a third of the nation's kids are too heavy and increasingly sedentary. Just last week the American Academy of Pediatrics caused consternation among doctors and parents when it issued guidelines for more aggressive use of cholesterol drugs in at-risk children to protect them from the heart disease they may have later in life.

National health guidelines recommend that both teenagers and adults get a least an hour of moderate physical activity every day. As the study published this week shows, most teenagers have a long way to go.

The study also reveals at what ages doctors have to start paying attention to the drop in activity — around age 13 for girls and 14-and-a-half for boys. That's when the amount of time spent being physically active drops precipitously.

"What shocked me was the dramatic decline. It drops off really fast," said Dr. Philip Nader, a pediatric cardiology researcher at the University of California, San Diego, who conducted the study.

Most research studies that track kids' physical activity rely on surveys and reports by the kids themselves, which, as any parent knows, can be rather inaccurate. Nader did it differently.

He and his fellow researchers put an accelerometer — a device that measures physical movement — on a total of about 1,000 kids in ten different cities. The kids wore it on their waists for seven days while it logged how much they biked, walked, jumped or sat around.

The younger they were, the more active they were. Nine year olds logged an average of three hours a day of moderate to vigorous activity. By age 12 that had dropped to about an hour and a half on weekdays and 83 minutes on weekends. By age 15 it was down to 49 minutes during the week and 35 minutes on the weekends. At every age, boys were somewhat more active than girls.

At age 15 only 28 percent kids were physically active for an hour or more each weekday. On weekends the rate dropped to 14 percent.

Dr. Angela Fitch, a pediatrician with the Fairview Eagan Clinic, sees ways kids are socializing and using technology that explain the study's findings.

She said a co-worker recently recounted a moment that would be familiar to many parents in the age of texting, e-mail and instant messaging. The co-worker's 12-year-old daughter came home early from a school dance and went straight to the computer to message her friends. When her parents asked why she didn't go over to someone's house or have a sleepover, she replied: "'We don't have to. We can talk like this,'" Fitch said. "Instead of pillow fighting they are sitting in front of a computer messaging each other."

Halvorsen said that the kids who keep moving are often the ones who make it to increasingly elite sports teams. Officials from the Minnesota State High School League estimate that about 38 percent of the state's 253,000 tenth through twelfth graders participate in at least one sport at school — one of the highest participation rates in the country. But that still leaves 62 percent of high school students needing other ways to be active.

"Right around the ages of 12, 13 or 14, the fun sports are becoming competitive sports," Halvorsen said. The kids who make it onto the athletic sports teams tend to stay active, he said. "Sadly, the rest of them don't have many options."

Kids would benefit if schools and community organizations started offering more varieties of organized sports — everything from table tennis to ultimate Frisbee — for the kids who don't choose traditional sports like hockey or basketball, Halvorsen said.

Ultimately, exercise has to be a family affair and in order to become a lifelong habit it has to start before adolescence, said Dr. Gigi Chawla, a pediatrician at Children's Hospital in St. Paul.

"You can't just expect your child to go out to the park and run around while you watch," she said. "You need to bike to the park together."