According to Lori Rubenstein, PT, MApp, Sc, Instructor of Clinical Physical Therapy at University of Southern California, children should carry no more than 10 percent of their body weight in their backpacks. In her research, conducted at the University of South Australia, Adelaide, Australia, Ms. Rubenstein discovered that youngsters were carrying up to 30 percent of their body weight.

"Even pack animals are not allowed to carry more than 25 percent of their body weight," said Ms. Rubenstein.

Shoulder and low back pain, muscle and neck spasms, and tingling and numbness in the hands and fingers are symptoms you might experience when your backpack is over loaded. If you're like most teenagers, you probably stack books and other heavy items at the bottom of the pack and your lunch and other fragile items on top. You then sling the whole thing over one shoulder by a thin strap and head off for school. These heavy and uneven loads distort your posture, causing muscle strain and fatigue.

Here are some suggestions to help protect your musculoskeletal health when using a backpack:

1. Wear the backpack on both shoulders. Your spine does not have the strength to balance a heavy weight carried on one side of your body. You'll also want to make sure the weight is equally distributed across your back.

2. Adjust the shoulder straps to allow the top of the backpack to be at shoulder height and the lowest point no lower than the hollow of the lower back. Straps should be wide and heavily padded.

3. The bag should not be wider than the width of your chest.

4. Pack items evenly with the heaviest items closest to the spine. This allows your legs to support the additional weight.

5. A waist belt will help keep the bag close to the spine and a hip belt helps to distribute the load more evenly to the hips.

6. The surface of the bag should be padded and in contact with your spine.

7. Don't use your backpack as a locker. Schools should provide lockers and enough time for students to go to and from their lockers between classes. This helps avoid the unnecessary transporting of heavy books. One school, in La Puente, Calif., offers luggage carts for their students to transport their book bags between classes.

8. Talk to your teachers and ask them to tell you which books you'll need the following day. This way you can leave any unneeded books at home.

Watch the scale
High school students may not carry the weight of the world on their shoulders, but sometimes it seems that way. A study done by a group of Florida high school journalism students confirmed Lori Rubenstein's research. They weighed all the book bags brought to school for one week. The average backpack weighed 19.8 pounds and the heaviest - containing nothing but textbooks and a notebook - weighed 30.9 pounds! And the youngster carrying that backpack could not have weighed more than 95 pounds.