Hey, Runatic!
Runners sometimes show off a battered toenail as though it were another finisher's medal.
But a nail turning dark after a run may mean bleeding underneath it. Repeated knocks against the front or side of a shoe can damage tissue, especially when your foot slides around or your toes are cramped.
Check three simple things: room for your toes, laces that hold your foot securely, and nails that aren't too long. Downhills push your foot farther forward, too.
And a dark nail isn't always caused by running. If the cause is unclear, the mark persists, or pain gets worse, see a doctor. Cleveland Clinic guide