Carrying heavy bags to school does not increase the risk of lower back pain in pupils, according to a new British study.
Aches and pains may be psychological, study suggests
Researchers at the University of Manchester tracked more than 1,000 children aged between 11 and 14 and found little evidence of extra strain.
Instead, aches and pains were much more likely to be reported by children who had conduct problems, or were more inclined to suffer regular stomach upsets, headaches and sore throats.
The findings contradict earlier studies that suggested pupils faced the danger of long-term spinal damage as a result of carrying bags laden with books.
An Italian study, published in The Lancet 1999, suggested one in three children carried bags weighing more than 30% of their own body weight.
And in the US, some states - including New Jersey and California - have been considering imposing weight limits on back-packs to lessen the chances of damage.
Concern over the health consequences of carrying heavy schoolbags has also been growing in the UK in recent years.
To measure the impact on pupils, researchers at Manchester University's School of Epidemiology and Health Sciences, followed 1,046 children from 39 secondary schools around the north-west of England.
At the beginning of the study, all the pupils were free of back pain and completed questionnaires on their general health and behaviour.
After a year, all the children were checked again to see how many had gone on to develop low back pain.
The results, published in the US journal Pediatrics, showed those with a history of conduct problems were more than twice as likely to be in pain than those with no behaviour difficulties.
In children who regularly complained of pains in the stomach, head or throat, the risk of back pain was between 50% and 80% higher than other classmates.
But when researchers looked at the different weights normally carried by children, they could find no link with back pain.
Dr Kath Watson, who took part in the study, said it remained unclear exactly why so-called "psychosocial" problems triggered back problems when bags laden with heavy books did not.
She said: "We followed the pupils for one year, so it is possible that they are more likely to get pain as an adult (as a result of carrying bags).
"Initially, we were a bit surprised at the results. But if we went back in another year then mechanical load might be more of a risk factor for pain."
Nia Taylor, deputy chief executive of BackCare, said, despite the study, the evidence still suggested children face the threat of spinal damage from bags that are too heavy.
She added: "Kids don't always make the connection between heavy packs and back pain, so there might be some cases that just do not show up.
"Our belief is that children should not be carrying more than 10% of their own body weight because there's evidence that really heavy packs can change the postural shape of the spine."