Black toenails in runners—often called runner’s toe or subungual hematoma—are commonly blamed on shoe issues or high training volume. While those factors certainly play a role, one of the most overlooked and underestimated causes is weak feet and ankles. When the muscles that support the foot and ankle begin to fatigue, the body looks for stability elsewhere. The toes respond by gripping or “clawing” the ground, which increases pressure from underneath the nail. Over time, this repeated internal pressure can cause trauma to the nail bed, leading to bruising and black toenails.
Main Causes of Runner’s Toe (with emphasis on strength and stability)
Weak Feet and Ankles (often missed): As the small stabilizing muscles in the feet and ankles tire, the toes curl and overwork in an attempt to create stability. This toe clawing increases upward pressure into the nail, which can be just as damaging as external impact from the shoe.
Repetitive Impact: Toes repeatedly striking the front of the shoe—particularly during downhill running, faster-paced sessions, or long runs—can irritate and damage the nail bed.
Poor Shoe Fit: Shoes that are too tight compress the toes, while shoes that are too loose allow the foot to slide forward with each step, increasing friction and trauma.
Long Toenails: Nails that are not kept trimmed are more likely to press against the shoe’s toe box and become bruised.
Foot Swelling: During longer efforts, feet naturally expand. Without extra room in the shoe, this added pressure can contribute to nail damage.
Running Mechanics: Certain running styles, such as a strong forefoot strike, can place greater demand on the toes and front of the foot.
Fungal Infections: These can cause discoloration that may be confused with bruising, or they may occur alongside nail trauma.
How to Prevent It
While shoe fit and nail care are important, the most effective long-term solution is often to build stronger, more resilient feet and ankles:
Strengthen the intrinsic muscles of the feet to reduce toe clawing
Improve ankle stability to support better overall running mechanics
Increase muscular endurance so the feet can handle longer runs without fatiguing
Reduce unnecessary load through the toes and nail bed
Of course, good habits still matter: choose properly fitted shoes with enough toe room, keep toenails trimmed, use secure lacing to prevent sliding, and wear quality socks to reduce friction. But without addressing foot and ankle strength, these strategies may only provide temporary relief.