The start of school is a great time for parents to take a look at — and into — their children's backpacks. Too many children are carrying packs that are too big or too heavy; both are bad for kids' backs. Fortunately, it's pretty easy to correct.

For size, a pack shouldn't be wider or longer than a child's torso (shoulders to hips). It also shouldn't hang more than four inches below their waist. If it does, they're wearing the wrong pack.

Weight can be trickier. Like the car used almost like a second home to store all sorts of things, so, too, can backpacks become repositories for weeks' worth of clothes, books and assignments. But children shouldn't be lugging around more than 15 percent of their weight — that's seven and a half pounds for a 50-pound child. Some shoes weigh that much.

Warning signs include changes in a child’s posture while wearing the backpack, difficulty putting it on, pain, tingling or red marks.

If kids won't lighten up, their parents will have to help them. Shoes, textbooks, games and whatever else collects — something has to go.

Carrying too much weight around can lead to backaches and other injuries. But way before that happens, it means children are uncomfortable during the school day and they shouldn't be uncomfortable — unless maybe they haven't done their homework. Backpacks are supposed to be a convenience, a way to carry around items that keeps the hands free, not an exercise in weight training.

Sometimes it is not enough just to look. You have to ask your child if their pack is uncomfortable.

Dr. Stanley Hyman is Director of Orthopedic Surgery at New York-Presbyterian Children's Hospital. Along with the size and weight specifications already mentioned in the hospital's public service announcement, he offers more backpack-related tips to help prevent children's back injuries:

  • Children should wear their backpack over both shoulders to spread weight evenly. That means using two straps, not one. Alternatively, a wheeled backpack may be helpful for some children.

  • Be sure that the backpack has wide, padded shoulder straps and a padded back. The more padding, the better. A padded back makes carrying the backpack more comfortable and also protects a child from being injured by the sharp edges on some school supplies.

  • If textbooks are making the bag too heavy, students or their parents should speak with the teacher(s) — sometimes books can be left at school or at home.

  • If wearing the backpack is painful, parents should talk to their pediatrician, who may recommend physical therapy to strengthen the back muscles. Warning signs include changes in a child’s posture while wearing the backpack, difficulty putting on the backpack, pain, tingling or red marks.

    • “Parents should inspect their child’s backpack from time to time. They often carry much more than they should with extra shoes, toys, electronic devices and other unnecessary items,” said Dr. Hyman.

    Backpacks are often fashion statements and status symbols. Parents need to make sure children pick out a pack that is within their budget and fits properly. Leave the rest to them.