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The New Zealand Chiropractors Association (NZCA) has warned against the classic money-saving technique of buying new-entrants one bag to last through to university following a US study which showed how serious the damage to children can be.
"Two key spinal measurements change as the backpack load increases," NZCA spokesman Dr Simon Kelly said.
"Heavier weights cause compression of the intervertebral discs, which act as a cushion between the vertebrae (bones of the spine). In the lower spine, the disc height became smaller (reflecting greater disc compression) at heavier backpack weights. Heavier loads were also associated with increased curvature of the lower spine, either to the right or the left."
Bags should be no more than 10% of the child's weight, but a recent survey by the Chiropractors Association of Australia found almost half carried bags well over that mark.
Dr Kelly said parents needed to take responsibility for preserving their children's spines.
"Don't buy a small child a big bag hoping they'll grow into it. They will always try and fit as much in there as they can.
"Make sure they carry their backpacks on both shoulders to spread the load and always check that the bag you're buying meets their approval. If it's not cool, they won't wear it!"