iStock/ThinkstockBY: DR. TIFFANY YEH

(NEW YORK) — From kindergarten to high school to our first job or a big trip, we carry our lives in our backpacks. But while they’re meant for carrying everything we might need, too often, we include things we don’t. Over time, the strain caused by these items can lead to problems with the spine, and a new study illustrates just how easily it can happen.

What’s the problem with a heavy backpack?

Carrying excessive weight in a backpack can cause wear-and-tear on the joints, ligaments, and muscles across the entire back and in the hips. Oftentimes, these body parts work to compensate for the extra weight, but because they cannot sustain that strength for an extended period of time, they begin to degenerate, which can cause stiffness, a loss of range of motion and pain. These effects can spread to other parts of the body as other muscles work to compensate for the strain.

This gradual degeneration can lead to chronic back pain, compressed discs in the spine (herniated discs), neck pain, an altered posture and gait and even pain in the feet, according to Dr. Kenneth Hansraj, lead author of the study and a spinal and orthopedic surgeon at the New York Spine Surgery and Rehabilitation Medicine Center.

“People everywhere have struggled to assess the impact of objects in a backpack to the body in general, and the spine in particular,” Hansraj said in a press release.

How heavy can it really be?

Hansraj’s study used a computer model to determine the amount of stress that’s put on a spine when it’s made to carry a backpack ranging in weight from 1 to 100 pounds — a range the study noted can include everything from school books to hiking gear.

Based on the model, Hansraj and his colleagues found that the amount of force placed on a spine in a neutral position is about 7.2 times the weight of the backpack. If the spine is slumped forward about 20 degrees, the amount of force increases to 11.6 times the weight of the bag.

The reason the backpack causes so much strain? Even in a neutral position, the spine is not totally straight — it’s somewhat S-shaped if you look at it sideways. So, even though from the outside the backpack may seem like it only pulls downward, inside the spine, the weight causes different pressure forces depending on where on the spine it’s hitting.

The study was based on a “physiologically accurate” simulation of the human spine, so it’s unclear how much of the data can be applied to actual people. However, based on the analysis, a 50-pound child who is carrying a 5 pound backpack would be putting about 36 pounds of pressure on their spine if it was in a neutral position. If they were slumped forward 20 degrees, the bag would put more pressure on their spine than they weighed — 58 pounds.

What can I do to prevent these problems?

The American Academy of Pediatrics notes that children should never carry a backpack weighing more than 10 to 20 percent of their body weight. In the press release, Hansraj goes further, citing previous studies, to say that for young adults, backpacks shouldn’t be over 13 to 15 percent of their body weight and for college-aged adults, 15 to 20 percent of their body weight.

While the above might be the best way to prevent injuries and longer-term musculoskeletal problems, here are some other easy ways to carry your bag safely:

• Pay attention to proper posture: Keep your ears above your shoulder, your shoulders back, your chest open, and make sure your back stays straight.
• Lift your bag up from the ground by bending your knee. Don’t just bend over, as this will put more strain on the lower back.
• Pack only what is necessary.
• Wear and tighten both straps of the backpack to distribute weight evenly.
• Use a waist or chest strap if your backpack has one.
• Pack heavier items closer to the center of the back.
• Use digital textbooks if your school has them.
• Use lockers or available storage space at work or school to avoid carrying everything at once.
• Make multiple trips if you have to carry several heavy objects.

If you are already experiencing pain and you think it’s from carrying a heavy backpack, see a doctor as soon as possible.

Dr. Tiffany Yeh completed pediatrics residency at Brown University, and is currently an endocrinology fellow at New York-Presbyterian Weill Cornell Medical Center and a member of the ABC News Medical Unit.

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