Now that the kids have headed back to school, it won’t be long before they start complaining about the heavy load they have to bear – not all the academic work, but all the books they have to lug around. The overloaded backpack is a real pain, they may soon be saying.

Don’t believe it, says a University of Michigan medical school professor. It’s not carrying a full bookbag that is giving school kids a backache, says Andrew Haig, medical director of the university’s Spine Program. It is a lack of exercise and the extra body weight that they carry around that causes the backs of many youngsters to hurt, he found.

In a study released earlier this year, Dr. Haig found that students, few of whom walk or bike to school, didn’t carry backpacks long enough to do any damage to their backs. Most carry a pack to the bus stop, sling it on the bus floor and then pick it up again for the walk from the bus to the school door. Few carried packs between classes. Hence, they didn’t carry the pack long enough to cause anything more than temporary discomfort, Dr. Haig concluded. About a third of the kids studied complained of back pain.

Most telling in his study was the fact that the children he studied in middle school were less active, watched more TV, played more video games and were more likely to be overweight than their elementary school counterparts. The older students, who did carry a heavier load in their packs, were more likely to complain of back pain.

Another study at the Alfred I. DuPont Hospital for Children in Delaware found that students who routinely carried packs were more likely to report back pain. But, Dr. Richard Kruse, an orthopedic pediatrician at the hospital, said being overweight, not exercising and watching a lot of television also raised the risks.

Of course, this doesn’t mean that a child should be burdened with a bookbag that weighs half as much as he does. As so much else in life, common sense should rule. Most experts, including the American Occupational Therapy Association and the American Academy of Pediatrics, recommend that students carry only 10 to 15 percent of their body weight in their bags. Backpacks should fit properly to distribute the weight, have padded shoulder straps and should be warn with both straps over the shoulder (although Dr. Haig says this doesn’t really matter either).

Plus, kids should get outside and exercise more, perhaps by walking to school. When at home, turn off the TV and do away with the sugary, high-fat foods. Now, if you could only get the kids to actually read the books they complain about lugging around.