To help kids prevent injury when using a backpack:

Lighten the load. No matter how well-designed the backpack, less weight is always better. Use the bathroom scale to check that a pack isn't over 10% to 15% of your child's body weight (for example, the backpack of a child who weighs 80 pounds shouldn't weigh more than 8 to 12 pounds).

Use and pick up the backpack properly. Make sure kids use both shoulder straps. Bags that are slung over the shoulder or across the chest — or that only have one strap — aren't as effective at distributing the weight as bags with two wide shoulder straps, and therefore may strain muscles. Also tighten the straps enough for the backpack to fit closely to the body. The pack should rest evenly in the middle of the back and not sag down to the buttocks.