Dr. Matthew Dobbs of the Washington University School of Medicine in St. Louis and St. Louis Children's Hospital says 10 percent to 19 percent of children miss school or sports activities every year because of pain caused by heavy backpacks.

"Adolescent girls ages 11 to 16 are most at risk, which may be attributed to the rapid growth spurt they experience during this age range and the susceptibility of rapidly growing spines to back pain," Dobbs says in a statement. "In addition, girls often weigh less than boys, but still carry the same amount of backpack weight."

Children need to be careful to not pack in more weight than they can carry, Dobbs says. The more weight, especially if the packs are worn incorrectly -- by one hand or over only one shoulder -- and the longer period of time a pack is worn, the greater the risk of injury.

Dobbs advises straps be put over both shoulders so the weight is distributed evenly and over the strongest back muscles -- those in the lower back.