Backpacks are so ubiquitous, most parents never think about how they could injure their children. But bags that are overloaded with books and worn improperly are a long-standing problem that can cause back and neck strain. That’s why, as your kids fling their backpack over their shoulders, you should take a moment to look for signs that their backpacks could be doing damage. Some indicators include red marks on their shoulders, headaches, posture that is slouched forward or sideways, complaints of back, neck, hip, knee or leg pain and numbness or tingling in the arms or hands.

If a backpack is dragging on the floor, falling off the shoulder or hanging below the waist onto the buttocks, it could be too heavy. And if a child repeatedly asks for help carrying a backpack, or intentionally forgets it, something should be done to lighten the load.

Here are a few more tips to consider:

• Weight is an extremely important element in backpack safety. A backpack should weigh no more than 15 percent of a child’s body weight, keeping in mind that a child conditioned and fit for sports may be able to tolerate a bit more weight and an overweight child might need to carry a smaller percentage of their body weight. Weigh your children, and then weigh their backpacks. You may be surprised with the results. If you find your child’s backpack is too heavy, ask his or her teacher for an extra copy of textbooks that can be kept at home.

• Backpack construction makes a difference. A good backpack will have two padded shoulder straps, a waist strap, a chest strap and multiple pockets to distribute the weight of the contents inside. Have your children put their backpacks on and show you how they wear them. They should wear their backpack with straps on both shoulders and buckle all available chest and waist straps. The top of the backpack should be two inches below the shoulders and four inches above the waist. The backpack should not rest on your child’s buttocks.

• A backpack should be packed with the heaviest items, such as textbooks, at the rear of the bag closest to the child’s spine. Lighter objects should be placed in outer pockets. There should not be any dangling straps or charms, as these can get caught in doors and escalators.

• Share with your children the consequences of poor backpack habits. When I see patients with backpack-related problems, I put them through a series of exercises to overcome muscles spasms, pain and muscle weakness. We work on stretching, correcting poor posture and finding ways to reorganize the backpack and reduce its weight. After several weeks of physical therapy, most students end up pain-free. Teach your children the right way to carry a backpack and you'll be helping them avoid this painful problem altogether.

Clare Safran-Norton, Ph.D., is a physical therapist at Brigham and Women’s Hospital in Boston.