(HealthScoutNews)-- Backpacks can take their toll on a child's back and neck, recent research finds.

"There's a fairly high incidence of back pain in children, and it appears to be greatest during the period of rapid growth -- ages 11 to 16. One U.S. study reported a back pain prevalence of 36 percent in adolescents," says Dr. Mary Ellen Franklin, a physical therapist and exercise physiologist at the Medical College of Georgia.

She recently supervised a study in which physical therapy students assessed how much backpacks, or book bags, contribute to that problem.

In the study, children aged 10 to 13 stood on a platform that measures force. The children stepped from the platform onto a high step and back down to the platform.

The children did this three times -- once without a book bag, once with the book bag carrying a load equaling 15 percent of the child's body weight, and another time with a load equaling 20 percent of the child's body weight.

The heavier the book bag, the greater the force children exerted to step up. There was also a trend toward greater impact forces when they stepped back down, the study found.

"Your body tries to keep the center of mass between the feet, so with a backpack, the trunk is in a more forward position, placing abnormal forces on the spine," Franklin explains.

"This requires shifting the head forward ... but this would mean looking down. You compensate by bringing the head up, which makes part of the neck curve to a greater extent. It's very stressful on the neck."

Franklin offers several suggestions on how to avoid problems caused by overweight backpacks:

• Lighten the load. Children should carry loads no heaver than 15 percent of their body weight.

• Rolling book bags are a good choices. But the extra weight and bulk of rolling book bags can cause problems when children lift them out of a bus or car.

• Backpacks used as book bags should have two padded shoulder straps. The worst thing for you child is a single-strap bag held on one shoulder or in one hand.

• Children should know the proper way to wear a backpack. They should use both shoulder straps and snug the pack as close as possible to their body. Backpacks that have belts that fasten at the waist or chest help with that.

• Backpacks should be appropriate for the size of the child. A backpack that's too large will sag towards the buttocks, stressing the child's lower back and shoulders.

• Look for signs -- pain, red marks from straps, poor posture -- that a backpack fits poorly or is overloaded.

More Information

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