English: Runners suffer from a painful condition variously called black toenail, jogger's toe, runner's toe, and subungual hematoma (bleeding under the nail). This is particularly common in downhill running. In a marathon, several percent of runners may be affected.^[1]

It is caused by the toe(s) being repeatedly thrust against the inside of the toebox of the shoe.^[1]

Right Column: If the shoe is too loose on the midfoot, the foot can slide forwards in the shoe, especially when going downhill. This may jam the toes into the end of the toebox.^[1]

Center Column: Feet become longer and wider when weight is put on them, because the arches flatten, and the toes also splay and bend.^[2]:p15,18 At the end of a long journey on foot, the arches flatten, the metatarsals spread, and the foot swells more than after a short one.^[2]:52 If there is not enough space around the toes, the toes will hit the toebox repeatedly.^[1]

Left Column: Wearing footwear which fits helps prevent runner's toe.^[1] Foot deformation and dysfunction can encourage calluses to form.^[1]