They used to be for carrying books and lunches. When you add portable video games, gym clothes, and designer school supplies, though, backpacks are just too heavy for most kids.

Many backpacks that appeal to children are ill-designed for the task at hand—carrying a day's worth of school books, supplies, gym clothes, and kid treasures. They may look pleasing, but they may not have the proper padding and support to keep children from developing chronic problems with their backs.

According to the American Academy of Orthopaedic Surgeons (AAOS), the extra weight in backpacks can lead to medical problems for kids, with muscle fatigue and strain at the top of the list. Backpacks can also injure a child if the weight of its contents adds up to more than 15% of his or her body weight.

In a study of children in middle school, researchers found that 37% reported back pain. A third of the students said that the pain limited them from doing some activities. Researchers also found that two factors were associated with less back pain: having school lockers available and using a lighter backpack.