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Students are returning to class this week lugging backpacks filled with books and binders. But how much is too much to carry?

Debra Cameron, an assistant professor in the University of Toronto’s department of occupational science and occupational therapy, says the acceptable weight for a backpack depends on the child’s age and stage of growth. Younger children should carry no more than 10 per cent of their body weight, while older teenagers should carry no more than 15 per cent of their body weight.

There are other ways to know if your child is carrying too heavy a load.

“I think if it induces a person’s posture to change drastically, it’s too heavy,” says Mark Erwin, an assistant professor in the divisions of neurological and orthopaedic surgery at �山ǿ��. “If someone has red marks on their shoulders when they take their backpack off, or their arms or fingers fall asleep, it’s too heavy.  If you’re struggling to get the backpack on, or you’re leaning forward or you’re pulled off to one side, that’s clearly out of balance.”

The result could be back or neck pain and poor posture, says Erwin, who is also a scientist in the division of genetics and development at Toronto Western Research Institute.

“A heavy backpack can pull you backwards,” he says. “As a response, you may lean forward. This brings your torso forward and your head down. But to walk, you’ve got to pull your head up to see where you are going. That results in extending your neck, and compressing the posterior part of your neck. And then, people tend to shrug their shoulders up to try and support the heavy backpack better.”

So, how do you pack smarter?

“You want some of the heavier items to be carried closer to the top of the backpack because that keeps the weight closer to your body. If the heavy items are at the bottom, the backpack tends to swing out and be further away from your body,” says Cameron. “You really want to balance weight over both shoulders to make sure you’re carrying the backpack the way the it was designed to be carried.”

Backpacks can vary greatly in size, and Cameron also says it’s a balance between finding one large enough without overwhelming the person wearing it. As well, the pack should be worn in line with the hip bones rather than being higher up the back or lower down on the buttocks.

Finally, says Erwin, “It’s another reason why increasing the overall fitness of young people may help decrease problems.”

Erin Howe is a writer with the Faculty of Medicine at the University of Toronto.