A study which involved 1930 people found that those with pronated feet were also the people with higher incidences of lower back pain. These findings suggest that pronated foot function may contribute to low back symptoms. Interventions that modify foot function, such as orthotics, may therefore have an important role in the prevention and treatment of low back pain.
To read more about this study here are two links. The Reuters link is an article based on an interview with the researchers and others. The Rheumatology link is the actual study for those who are keen to read a little deeper.