School bags slung on one shoulder may seem stylish for students, but an expert has warned that this may cause a severe back injury.

According to Professor Gregory Kolt, a physiotherapist and health psychologist at the University of Western Sydney, school bags slung over one shoulder can catastrophically affect the spine of school children.

He said that humans are designed to be symmetrical and a poorly fitted backpack or heavy load can instantly cause muscle fatigue.

"Carrying a load unevenly over one shoulder or a poorly fitted backpack throws out this balance, places strain on muscles and can cause serious damage, which can continue into adulthood," The Age quoted Professor Kolt as saying.

"A poorly designed backpack forces muscles into an unnatural, asymmetrical, position to support the load. This causes some muscles to stretch and some to tighten to compensate, leading to muscle spasm and pain," he added.

It is necessary to have a backpack that is correct in size for the wearer, holds the load against the back, with a semi-rigid bottom panel, padded shoulder straps and panels against the back, he said.

"The longer muscles are kept in unnatural positions, the harder it is to restore balance and repair the damage," he said.

Professor Kolt recommended that parents check the weight of their child's bag regularly and talk to teachers if they believe that the child is carrying too many books.