Black toenails in runners: causes and fixes

Black toenails. A common problem among runners of all distances and terrains. Our Physio team often see runners who get them in every race, despite bigger shoes and new socks.

Before you buy another half size up in shoes, you need to find the cause.

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Why do black toenails happen?

There are many causes of black toenails, but they’re not what you’d think. The most common causes in runners are:

  • Poor foot stability
  • Tight footwear
  • Long toenails

Poor foot stability will need some explanation, but it’s actually the most common reason for runner’s black toenails. Tight footwear and long toenails are as simple as sustained pressure on the toenail causing damage underneath the nail, causing it to turn black and separate from the nailbed.

Poor foot stability occurs when the muscles and connective tissue in the leg are unable to control the rate of movement generated on landing, primarily internal rotation at the hip and pronation (arch lowering) in the foot. This leads to compensatory changes in the way the foot attempts to stabilise itself.

One of these compensations involves the long toe flexors bending the toes down in an attempt to make the arch rigid (think of a cupped hand on a bench and the fingers digging into the bench to brace it). It can add additional stability for a period of time but it may also place the toenails in a vertical position where they sustain a shearing force with every foot impact (again, think of your braced hand and the shearing on your fingernails as you hit it on to the bench). This repeated force eventually causes damage under the toenail, leading to black toenails.

Only on some toes or on one foot?

When stability is challenged and toes brace, the nail doesn’t always end up in a vertical position. Some toes curl with the toenail ending up in a vertical position and at risk, whereas other toes flatten with the nail horizontal and are unlikely to be affected.

You can see the variation in your feet by doing a few barefoot single leg squats and watch your toes – as you’re leaning forward to look at them, they’ll usually curl to brace so you can see if the nails end up vertical or horizontal.

The risk to toenails is increased with longer toenails and for a longer 2nd toe.

Where the black toenail/s only occur on one foot, it’s likely that the stabilising muscles are fatiguing earlier on that side. This may be due to weakness, an asymmetrical gait pattern or the shape of the flexed toes as mentioned above.

Can black toenails be a warning sign?

Yes and no. If you’ve always had black toenails and you’re not currently increasing your distance or returning from a break, it’s probably nothing to worry about.

But if you’re returning from injury or absence, or if you’re trying to increase your long run distance, black toenails may indicate that you’re running past the point of muscle fatigue and are at greater risk of injury.

If you have been running for a while and suddenly start getting black toenails, it can also be a warning sign of failing stability and impending problems.

So how do I fix them?

The fix relates to the cause, so a bit of detective work is required here.

Step 1 is to cut your toenails. Simple and sometimes a solution in itself. Other times it just minimises the effect.

If you’ve recently changed shoes, don’t automatically assume that they’re too tight. If the new shoes feel tight around the toes while walking downhills, it may be a sizing issue.

But if the shoe offers less stability or is too big or loose-fitting, it may be causing early fatigue and excessive toe muscle activity. Loosening your laces can help a little with tight shoes but often makes stability issues worse.

Try it and if you’re not successful, think about having your stability assessed properly.

Written by

Pete Colagiuri
Sports Physiotherapist

Pete has over 20 years experience as a Physiotherapist and specialises in running biomechanics and complex injuries. He believes that you must identify and fix the underlying cause of an injury, to recover faster, prevent recurrences and improve performance.

Pete Colagiuri - Sports Physio