Back Whack.

Ever feel like you're carrying the weight of the world on your shoulders? Blame it on your backpack!

U.S. doctors are reporting a rash of back pain complaints from backpack-toting teens. "Backpack stress is a universal problem," says Marian Jacoby, a back specialist in Baton Rouge, Louisiana.

When you wear a backpack properly, your back and stomach muscles--among the strongest in your body--support the pack's weight. But when you overload your bag, lift it incorrectly, or sling the pack Over one shoulder, you can strain the soft tissues in your back, including your back muscles, spinal cord, nerves, and blood vessels. "Soft tissues help bones support your body in an upright position," explains Jacoby.

How can a bookbag make your back ache? The most common problem is toting a bag that's too heavy. When you do, your center of gravity (the imaginary center of your body that balances your weight) shifts. To counter added weight from your pack, you lean forward and round your shoulders, which pulls your back out of its normal position and can permanently alter your posture.

Picking up a heavy book bag is like a crane hoisting a large object--only the human body is far more breakable! As you lift your bag, you generate a force of rotation called torque. Torque is equal to the weight of your bag times the bag's distance to your waist. Your waist acts as a pivot, or folding point. "If you bend over at the waist to pick up a 5-kilogram (11 pound) bag, it places 50 kilograms (110 pounds) of weight on the soft tissue of the spine!" says Jacoby.

How can you combat backpack stress? Use both shoulders and make sure your bag weighs no more than 15 percent of your total body weight. And, "when picking up your backpack, keep your back straight, and squat," adds Jacoby.