How heavy is your kid's backpack? Study shows it could harm their health

Fourteen-year-old Lucy Chant carries about 20 pounds of school supplies in her backpack every day. Adding to the load are textbooks, snacks, and after-school soccer clothes.

"By the end of the day, it probably feels like 40 pounds," she told CBS News. "When you're tired and walking from class to class, it's a lot."

Lucy isn't unusual, and a new study shows that carrying all that weight can take its toll on a child's back.

Dr. Ken Hansraj co-authored a study that measured how wearing a backpack impacts the spine. 

"If you carry, for example, 10 pounds in your backpack, then the forces on the spine is 70 pounds of force," he said — seven times the weight. Walking uphill only adds to the impact. "So then if you're struggling uphill 20 degrees, then the forces become 120 pounds."

Hansraj said carrying too heavy a load puts children and teens at risk for long-term health problems because they're stressing areas of the spine and core that are still growing.

He recommends young kids carry no more than the equivalent of 10 percent of their body weight. That amounts to 5 pounds for a 50-pound 6-year-old. For a 150-pound teenager, the recommendation increases to 23 pounds. For a 200-pound adult, it's 40 pounds.

Carry the backpack with both straps, keeping them tight, and practice good posture to help reduce pressure on the spine. "Good posture is your ear above your shoulders and your angel wings back," he said.

Hansraj has also conducted research on how too much texting can cause neck pain. In a 2014 study, he found that bending your head at a 60 degree angle to get a better look at your phone puts 60 pounds' worth of pressure on your cervical spine, the portion of the spine above the shoulders. 

When it comes to lugging a backpack, doctors say you should listen to your body. If you start to ache, you're probably carrying too heavy of a load.

Lucy said she makes sure she carries her backpack the right way.

"It's not normal at first kind of but once you like put in the time and stuff you definitely feel a difference," she said. She's now trying to convince her friends to do the same.