If you’re the parent of a school-aged child you already know how backpacks have gotten bigger and heavier in recent years. Experts now recommend that a kid’s backpack weigh no more than 10 to 15 percent of a child’s body weight to avoid undue strain, though it’s not unusual for children to carry a bag equal to 22 percent of their body weight.

A recent, very small study of eight children at the University of California, San Diego, used magnetic resonance imaging (MRIs) to determine exactly how a heavy backpack affects kids’ lumbar spine.The scientists believed that the weighty bags would cause compression of the spinal discs (the shock-absorbing cushions between the vertebrae) and increase the curve of the lumbar spine. They looked at loads of nine, 18 and 26 pounds, representing roughly 10, 20 and 30 percent of the kids’ body weight.

After reviewing the MRIs, the researchers found that more weight did indeed compress lumbar discs, caused more curving in the spine to the right or left side and that the heavier the bag, the more back pain the kids felt. Since having back pain as a child increases the risk of experiencing it as an adult, there’s a risk, too, that the ache caused by too-heavy backpacks in school could have long-term effects.

For advice on choosing a backpack for your child, go to the American Academy of Pediatrics’ Backpack Safety page.

The study was reported in the January 1, 2010, issue of the medical journal Spine.