Black Toe Nails

A barefoot runner with a black toe nail

When a runner develops one or more black toe nails, the cause is generally assumed to be running shoes that are too small in the toe box area. Recently, however, I came across a barefoot runner who seems to have a couple of toe nails that are consistently black. I'm absolutely sure that this particular runner never runs with shoes, so we can discard a constricted toe box when considering possible causes of his condition. Since it doesn't seem to be painful for him, maybe it's just not worth worrying about.