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. 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.
BBC News November 18, 2003