When you load up your backpack for the first few days of school, it may not be too full of papers, books and supplies yet. But before long, as homework assignments roll in and sports practices get underway, you may be toting a whole lot of stuff, including heavy textbooks, gym clothes, sneakers, CD players, snacks, calculators and more.

Karen Jacobs, a professor of occupational therapy at Boston University, reports that it's not unusual for kids' backpacks to weigh 20, 30 or even 50 pounds!

Jacobs says that the way many kids carry a pack--slung casually over only one shoulder--adds to the pressure. Carrying a backpack that way may look cool, but it can squish the nerves and blood vessels that run through the space between your rib cage and collarbone, right below your shoulder. If you pinch a nerve in there, you can cause painful--even permanent--damage.

To protect yourself, Jacobs says, the weight of your backpack should not equal more than 15 percent of your body weight. (To find out how much this is, weigh yourself and then multiply the number by .15. For example, if you weigh 75 pounds, multiply 75by .15 to get a total of 11.25.) To find out how much your pack weighs, weigh yourself without it. Then weigh yourself with it on. Subtract the first number from the second number. The result shows how much your pack weighs.

Once you've gotten your pack down to a sensible weight, here are some additional tips for carrying it safely:

* Select a backpack with padded shoulder straps.

* Wear both straps across the shoulders and upper body to equalize the weight and reduce the chance of injury.

* Don't carry more than you need. That may mean clearing out old papers and books that you're not using.

You might even consider an alternative, such as a backpack with wheels or a backpack designed for hiking that has a rigid frame and a waist harness to distribute weight better. Some new backpacks, designed to be better for kids' backs, are starting to become available.

Now that school days are here, you have a lot of important things to get organized about. Make one of them checking the weight of your pack.