One of the reasons American kids are getting fatter each year, say researchers at the Centers for Disease Control and Prevention, is that only about a quarter of them walk or ride their bicycles to school.

The lack of physical exercise worries health officials, of course, who say that parents who insist on chauffeuring their kids to school each day may be contributing to an epidemic of youthful obesity. The CDC would like to see at least half of the nation’s children going to school on foot or on bikes by 2010.

This new national health objective is commendable though impractical, I think, for a reason that health officials apparently have not taken into account. No, I’m not speaking about the perils posed by automobile traffic or child molesters on the streets, real as those concerns may be. I’m speaking about something that would make riding a bike all but impossible for the average youngster, and turn a healthful mile-long walk to school into a labor worthy of Marine boot camp.

I’m talking about backpacks, of course.

Living across the street from a middle school, I’ve been amazed at the growing backpack trend over the years – not the number of them but the sheer size and crushing weight of the things. Trudging by the house, their bodies hunched forward under the massive burdens slung from their shoulders, the kids resemble a procession of baby-faced Sherpas humping a month’s worth of supplies to the foothills of Mount Everest.

I first became aware of this weighty issue about the time my kids reached middle school. My daughter would bound up the porch steps after school, burst into the kitchen and drop her swollen backpack onto the floor with a thud that rattled the dishes and shook the pictures on the wall.

She would assure me that she was carrying books, not bricks or engine parts, in the bulging bag, and that she did indeed require every last one of them for homework.

Experts say the overloaded-backpack trend is the result of tougher academic standards, heavier homework assignments and too little time for kids to get to their lockers between classes.

Whatever the reason, doctors across the country have seen so many backpack-induced aches and pains in their young patients that they’ve begun to study the problem.

As one researcher observed in a story I read about the issue, “There are standards for how much postal workers can carry, but no standards for children.”

The Internet is loaded with ergonomic advice about the best way for kids to shoulder the heavy backpacks in order to avoid a lifetime of back problems.

One doctor even coined the name “scholar’s scoliosis” a few years back to describe the insidious modern affliction he was seeing so often in his pediatric practice.

The experts all seem to agree that the weight of a backpack should never exceed 10 percent of the weight of the child who is carrying it. Since it is not at all uncommon for today’s backpacks to weigh as much as 20 or 25 pounds, then the average middle-school child should weigh about 250 pounds in order to bear this scholarly burden without turning into a miniature Quasimodo.

The way kids are packing on the pounds these days, perhaps it will all balance out in a few years.