"As the tree bends, so grows the tree."

Dr. Justin Kamerman-- a Sacramento chiropractor-- uses the tree analogy to describe the way a child's body structure changes when weighed down by a heavy backpack. He treats a lot of kids suffering from achy necks and backs-- even headaches-- due to wearing heavy backpacks.

A backpack should never weigh more than 10 to 20 percent of a child's weight, according to the American Academy of Pediatrics. This means, for an elementary school child weighing 50 to 60 pounds, their backpack should never reach more than about 12 pounds maximum. Ideally, it should be less.

However, many children's backpacks significantly exceed the recommended weight. The backpacks some children carry can weigh up to 50 pounds, according to Kamerman. For a smaller frame, that can be 50 percent of their body weight.

"Anytime we load the human frame, it's going to cause the body to shift where the weight is," Kamerman said. "The constant wear of a heavy backpack can really contribute to that."

Even though the weight of a backpack is linked to posture problems, heavy backpacks don't cause scoliosis, according to the American Academy of Orthopaedic Surgeons.

However, the weight of a backpack can cost you more than just a trip to the chiropractor.

Between 2013 and 2015, an annual average of 5,400 children under the age of 19 visited an emergency room for backpack related injuries, according to the U.S. Consumer Product and Safety Commission. Of the incidents, 60 percent were injuries to children ages 10 to 15.

The dangers of heavy backpacks are a concern to parents, who watch their children haul the weight every day.

Ramona Aschwanden has children who are students in the Elk Grove Unified School District. One of her kids attends Elizabeth Pinkerton Middle School, while another goes to Cosumnes Oaks High School.

"My little one almost falls over from the weight of her backpack," Aschwanden wrote on ABC10's Facebook page. "My older one always complains about it too."

Aschwanden also wrote that there are no cubbies available for her younger child.

ABC10 reached out to the Elk Grove Unified School District to find out more about their textbook policies. Xanthi Pinkerton, a spokesperson for the school district, said she hasn't heard of any parent concerns regarding heavy backpacks.

"I can't imagine kids carrying around that many textbooks," Pinkerton said.

Nearly all of the middle and high schools in the district do not allow locker use, according to Pinkerton. But, the school district rolled out a new program this year, Classlink, which allows every school within the district to access textbooks through an online portal.

"We don't want to see kids with big, bulky backpacks," Pinkerton said. "We want them safe and healthy."

Classrooms in the Elk Grove School District often times have a class set of books so students don't have transfer the books back and forth, Pinkerton said.

There are ways to help avoid the harmful effects of wearing a heavy backpack and reduce the risk of injury.

"Look at your child's posture," Kamerman said. "If they're slouched forward, check their backpacks."

Kamerman recommends children use backpacks with thicker straps, that can also clip across the chest to help distribute the weight. Kids should also tighten straps so that the backpack is not hanging off their backs, according to Kamerman.

"Putting a backpack on one shoulder is the biggest no-no," Kamerman said. "It's one of the worst, worst, worst things."

The AAP recommends parents encourage their child to tell them about any numbness, tingling, or discomfort in the arms or legs, which may mean their backpacks are too heavy. Parents should never ignore their child's back pain and should talk to the school if the textbook load is an issue, states the AAP.

Even if it's not the trendiest choice, the AAOS also suggests using a rolling backpack if the school allows it. Keep in mind, rolling backpacks still have to be carried up stairs and they may not fit in some lockers.

If a child is having severe issues with a backpack's weight, parents can always talk to a doctor about writing a note and having a school adjust to the child's needs.