Running toenails turning black should be considered whether it is caused by trauma, over-exercise, or fungal infection. You should choose comfortable running shoes, or reduce the amount of exercise and take braking rest. 1. Trauma: If there is a heavy object falling from a high place and hitting the toes or kicking a hard object during exercise, or repeated mild injuries over a long period of time, it will cause capillary rupture under the toenail and cause bruising, which will be black in color. Treatment should suspend running, brake rest, reduce activities. 2. Excessive exercise: too much exercise or wearing shoes that are not suitable for running exercise, resulting in excessive friction between the toes and the shoe surface and causing local capillary breakage, appearing black nail performance. 3. Fungal infections: If the athletes love sweaty feet, and they do not pay attention to hygiene, do not wash their feet, it is easy to lead to fungal hyperplasia, some fungi will produce pigment, sludge under the nails, can be black. You should pay attention to hygiene and wash your feet regularly. If the above symptoms occur and continue to aggravate, you should immediately seek medical advice and standardize treatment under the guidance of a doctor.