Only about one-quarter of American teens aged 12–15 years engaged in moderate-to-vigorous physical activity for at least 60 minutes daily, according to recently released data from the National Center for Health Statistics.

The findings from the 2012 National Health and Nutrition Examination Survey (NHANES) and the NHANES National Youth Fitness Survey also showed that the percentage of male teens who were physically active for at least 60 minutes daily decreased as weight status increased.

These numbers are troubling to me because Inactivity is the hidden bomb that will hurt our children as they age.  Exercise when young makes one's bones thicker and sturdier and will delay osteoporosis later in life.  Regular exercise prevents, to some extent, obesity, and definitely makes the muscles better developed and the cardiovascular system more able to deal with stress.

The American Academy of Pediatrics recommends that children participate in daily moderate-to-vigorous physical activity for at least 60 minutes. The activity should increase the heart rate and cause hard breathing some of the time

But most of our children do not walk anywhere, they are driven. They sit rather than play outside. I never joined the Little League, but it was easy to stay active. We all played basketball in the school yard and baseball on the back lawn of the telephone building everyday after school until sunset. Sometimes the play went on long after (or at least, until our mothers caught us for supper).

My mother is almost 98, lives alone and takes no medicine. She used to walk from around 7th and Pine to 48th and Walnut (about 4 miles, I believe) for high school to save the 5 cents each way on the trolley. She's been active throughout her whole life. At one point, she used to swing dance for several hours a day. In the last year or so, her spine has started to subside from gravity, but she can climb by herself into our SUV and do her own housework. (She refuses to get a maid.) It is not only because of her lifelong healthful activity, but it would be difficult to do so well without this activity.

American children started to become obese in 1963 at a time when over 85 percent of school children either walked or rode bicycles to school. In 2009, less than 20 percent of children get themselves to school, and now over 30 percent are very overweight and 20 percent are obese.  Parents, even in the best neighborhoods, are afraid to let children play on their own.  Playgrounds are much safer, but not nearly as exciting as they use to be and not as enticing for children to spend prolonged time there.  When is the last time you saw a self-propelled children's carousel?

A good day for a child, I have always said, is at least one hour of reading and one hour of running.  The average child in the United States does neither.  Some unanticipated consequences have occurred from our children being overweight and obese. Obese children start and finish going through puberty earlier than in the past because the fat cells actually make various estrogen and testosterone like chemicals. Unfortunately, if you start and finish puberty earlier, you end up shorter than expected.  Americans used to be the tallest people in the world.  Now northern Europeans are taller than us.

Children have thinner bones from just sitting and weigh more so they are less sturdy. And they will have a greater chance of broken bones as they age.

So get your children outside. Let them run, dance and climb. Yes, they might fall, but they will soar physically and I think mentally.  In a world where 1 in 60 adult women live to be a 100 years old, we had better prepare their bodies to live that long.

A version of this previously appeared on Thomas Jefferson University Hospitals' @Jeff blog

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