Recently published in the Journal of the American Medical Association, the study reveals that while nine-year-olds exercise on average about three hours per day, by age 15, that number drops to 49 minutes on weekdays and just 35 minutes or less on Saturdays and Sundays. (Those additional minutes on Monday through Friday are no doubt forced on the kids by a reviled gym teacher in spandex shorts, a whistle around his neck, and an annoying gum-chewing habit.)

Combine that with a fast-food diet, the Internet for entertainment, and parents who don't encourage exercise, and you've got a sure-fire recipe for sloth.

These are interesting numbers, but I'm not sure they're cause for alarm across the board. Teenagers are tired a lot -- it's part of adolescence. When I was a kid, I was outside as much as possible: riding bikes, swimming, playing in the yard, or skateboarding.

By 15, I was more concerned with hitching a ride to the record store to buy the latest Jane's Addiction CD than I was with my neglected 10-speed. One of my greatest high school coups was obtaining an open-ended doctor's note excusing me from gym -- which I milked for my entire sophomore year.

But that got old, and I re-discovered the joys of having arms, legs, and a cardiovascular system. Now I run marathons...

Obviously, this won't be everyone's tale, but I do think a certain degree of laziness among America's teens is inevitable, and possibly for the best. You have to want to exercise, and when you choose to do it as an adult, it means so much more than when you were forced to as a surly kid. Some of the most active people I know were morbidly obese when they were teenagers. They clearly got tired of it and wanted to feel better.

There's also the matter of time. By the time a kid is 15, he or she has a lot busier schedule than a nine-year-old. There's more homework, after school jobs, and a more active social life.

Moreover, some classmates of mine growing up who played every sport they could are now inactive and overweight.

In short, studies like this can be good for discussion, and keeping people aware of children's health, but they don't necessarily mean anything in the long run. In the end, it comes down to the individual, and the fact that nothing is permanent.

[Image Credit: David Joles, Star Tribune]