Like adorable turtles, their little limbs poking out from under their shells, kids shuffle their ways to school bearing on their shoulders ever-heavier backpacks. Even high–schoolers have to bend forward beneath books and binders to cart their cargo to and from school. They’re burdensome (and can be goofy-looking), but are they dangerous?

Overloaded Backpacks Present Health Risk To Back to School StudentsYes, say many experts.

“Since at least 1998, we’ve noticed backpacks getting bigger and heavier, and not in proportion to the kids’ sizes,” says Dr. Karen Jacobs, a clinical professor at Boston University and spokesperson for the American Occupational Therapy Association (AOTA), which sponsors a school backpack awareness day. Jacobs says crowded schools and scant locker space appear to be driving the phenomenon.

A 2010 study from the University of California, San Diego, concludes, “backpack loads are responsible for a significant amount of back pain in children.” The same study says a full third of kids aged 11 to 14 report back pain. Other research from 2011 came to a similar conclusion.

“Kids are saying ‘My back hurts, my neck and my shoulders hurt,’” Jacobs says. “A heavy backpack can also contribute to headaches and problems concentrating at school.”

Like the frame of a house, the spine what keeps your child’s body sturdy and upright. Put too much weight on this frame while a young body is still developing, and it could change a kid’s posture, compress his spine, and impair growth, says Rob Danoff, a doctor of osteopathic medicine and a certified family physician with Philadelphia’s Aria Health System. “It also might contribute to back problems or injuries when your child’s older,” Danoff says.

How heavy is too heavy? “As a general rule, research shows the backpack should be no more than 10 to 20 percent of your child’s body weight to avoid pain or potential injury,” Jacobs says. “We like to err on the side of caution and recommend 10 percent.” (Danoff’s recommendation—no more than 15 percent—falls in line with Jacob’s.)

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This article originally posted on TIME.com.