People who chat on their mobile phone while walking could be hurting their back, according to a study.
Mobile phones mean more people walk and talk than ever before
Scientists at Australia's University of Queensland say it's all down to the way we breathe.
They say the human body is designed to exhale when our feet touch the ground. This helps to protect the spine from sudden jolts.
However, talking and walking at the same time disrupts this breathing pattern, leaving the spine exposed.
Dr Paul Hodges and colleagues at the university's school of health and rehabilitation sciences based their findings on a study of a group of volunteers on treadmills.
Some of these walked in silence while others talked, reading from prepared scripts or describing images.
The researchers measured activity in the trunk muscles that protect the spine in each of the volunteers.
They found that the trunk muscles worked properly in those people who simply walked.
However, people who talked while walking had less muscle activity in this area, potentially leaving their spines exposed.
Speaking at a Society for Neuroscience conference in New Orleans, the scientists said problems occurred because of the way the brain prioritises activities.
"Muscles often perform multiple functions at the same time and the brain prioritises these tasks according to their relative importance, which means that the accuracy of joint stability often comes second and places the person at greater risk of injury," said Dr Hodges.
The scientists said people who talk to each other while walking could be at risk of damaging their back.
But they said people who use mobile phones while walking may be particularly at risk, not least because they are likely to spend more time doing it.
Dr Matthew Bennett, a spokesman for the British Chiropractic Association, welcomed the study.
"This is completely surprising," he told BBC News Online. "This is totally new research.
"It shows that we really shouldn't be talking and walking at the same time. Talking appears to disrupt our ability to walk efficiently.
"This is something we will now have to add to our list. People with bad backs should watch the way they bend to pick things up, shouldn't sit for too long and now it would seem shouldn't talk with someone they are walking with.
"This is particularly important for mobile phone users. We already know that holding a phone to your ear for long periods is bad for you, because it can increase tension across the shoulder and cause pain."