At the age of 39, FDR contracted Polio. Soon after he would become paralyzed from the waist down for the rest of his life.
In order to help overcome his disability, he wore steel leg braces to walk short distances. However he was mostly confined to a wheelchair for the rest of his life.
This is one of only a few photos showing him and his leg braces.
With his pant legs rolled up, of the few in existence, this one most clearly shows his condition.