Grades aren't the only things parents of children and teens should be keeping an eye on. Their physical activity should be scrutinized as well, considering that from ages nine to 15, some kids could fall into a steady downward spiral of lethargy.

It shouldn't be surprising that an uptick in video game playing, television watching and computer surfing is probably to blame for the fact that as kids grow older, their time spent moving may greatly decrease, according to a recent study.

In research in the Journal of the American Medical Association, 1,000 children were given accelerometers to track their daily movements, starting at age nine in 2000 and ending in 2006 when the participants turned 15.

Things started out well -- the nine-year-olds engaged in moderate to vigorous physical activity about three hours a day on weekdays and weekends.

But as they got older, that amount of time started to diminish -- weekdays by 38 minutes per year, and weekends by 41 minutes per year.

By the time the study subjects reached 15, they were moving only 49 minutes on weekdays and 35 minutes on weekends. That's far less than the recommended one hour a day of moderate to vigorous activity.

"This is a reflection of what's happened in society in general," says James Griffin, the science officer for the study.

Griffin says a growing interest in computers, television and video games may not have been the only culprits. As kids grow into teens, they begin to shun family activities such as hiking. In early adolescence, some become interested in team sports and others not, giving the latter fewer options for exercise.

Griffin points out that even some who participate in soccer or baseball are often shuttled by car to and from games, spend little time actually playing, and chow down on fast food afterward.

"I think the message," says Griffin, "is about wanting to keep up the good habits that kids have around ages nine to 11 to prevent that drop-off.

"That can be as simple as encouraging them to take walks, as well as organized sports or bike riding."

Los Angeles Times