I'm sure you've seen it happen before at some point in your life. Your dad bends down to pick up a crate of firewood, and instead of lifting with his knees like he should, lifts with his back, and then suddenly ... stops. As you come running over to see what happened, your father tells you to tell your mother to call the doctor; he threw his back out.

Now, as an adult, you're starting to get the small symptoms that may cause you the same pain that dear ol' dad suffered thirty years ago. One of the biggest reasons for chronic back pain in adults is improper posture and uneven weight distribution, particularly when carrying backpacks.

"Back pain is pervasive in our society," American Chiropractic Association (ACA) President James A. Mertz said. "Eighty percent of all Americans will suffer from it at some point in their lives, and 50 percent of us will suffer from low-back pain this year alone. Much of this suffering is brought on by bad habits initiated during our younger years — such as carrying overweight backpacks to school."

Lands' End Direct Merchants discovered in a survey that more than 96 percent of 8- to 12-year-old children will carry a backpack to school, and one-third will wear it improperly. Also, a study in Italy found that children carry backpacks that are far too heavy for them, in relation to their body weight.

In response, the ACA offers the following advice to help prevent future back pain for your children, and temporary back pain for you (if you use a backpack):

• Your child's backpack should weigh no more than 5 percent to 10 percent of his or her body weight. A heavier backpack will cause your child to bend forward to support the weight.

• The backpack should never hang more than four inches below the waistline. A backpack that is too low causes increased weight on the shoulders, thus forcing the individual to lean forward.

• Individualized compartments help position the contents effectively. Place pointy or bulky objects away from the part of the pack that will rest on the child's back. Uneven surfaces cause painful blisters.

• Buy the best-designed backpack available for the child. Bigger packs mean more stuff, which equals heavier weight.

• Wear both shoulder straps, because carrying the pack by one shoulder can cause disproportionate weight distribution, leading to neck and muscle spasms, and low back pain.

• Unpadded straps can dig into your child's shoulders, so buy a pack with padded straps.

• Adjustable straps fit to your child's body. Shoulder straps that are too loose can cause the backpack to dangle and eventually will cause spinal misalignment and pain.

• If the pack is still too heavy, ask his/her teachers if the child can leave the heaviest books at school and only bring home hand-outs or workbooks.

• Look into packs-on-wheels if back pain persists.

The ACA also offered this advice for hikers wearing hiking-style backpacks:

• When getting fitted for a backpack, the pack should accommodate your dorsal length — from your upper back to the bottom of your ribs — and not your entire body.

• When hiking, the shoulder straps should rest in the center of each clavicle or collarbone. Shoulder straps are for increased stability and not for carrying weight. You should be able to fit two fingers, comfortably, under the straps.

• Hip belts should fit around the hips and above the pelvis, where your pants would rest, to carry most of the load in the pack.

• Pack the heaviest items on the sides of the pack or in the bottom portion. Too much weight on top can make you top-heavy and cause a disproportionate weight shift, resulting in back pain.

Address: American Chiropractic Association, 1701 Clarendon Boulevard, Arlington, VA 22209; (800) 986-4636, AmeriChiro@aol.com, www.amerchiro.org.