Black toenails from running is one of the more common issues amongst distance runners.

It’s also one of the most misinterpreted!

Black toenails from running will usually appear within a few days of a long run, race or high mileage weeks.

It starts as a sore or sensitive toenail. It gradually turns black, which can take a week or more.

Depending on the severity of the issue, you may lose the toenail (don’t worry, it’ll grow back).

And if you ask a friend or do a quick Google search, you’ll find that tight shoes are the most commonly quoted reason for black toenails from running.

But my shoes don’t feel tight. How could that cause pressure?

Although it seems logically that tight shoes are squishing your toes, surely we can’t all be in the wrong size shoe. Right?

Despite tight shoes being the most commonly quoted reason, it’s also the least common reason for black toenails.

What’s putting pressure on my toenails?

The answer is in the biomechanics of the foot.

More specifically, the way the foot behaves as the leg gets fatigued.

We’ve all heard of pronation, the movement of the foot as it strikes the ground and the arch flattens.

It’s part of the energy storage cycle and helps to absorb shock on landing. It’s a good thing!

But as we fatigue, the muscles that control pronation – ankle stabilisers and hip rotators – have a reduced ability to control the movement.

Other indicators of stabiliser fatigue include very sore hip muscles, inner ankle pain or blisters on your feet (click on the issue to read more about its causes and fixes).

So the brain kicks in and switches on other muscles to assist.

One of the muscle groups activated are toe flexors, the muscles that curl the toes.

By curling the toes down, the toe flexors can help brace the arch of the foot.

It’s a great design, having back up muscles to cover for tired muscles, but it has unintended consequences.

Curling the toes can place some toenails in a vertical position, exposing them to shearing forces on landing.

Toenails are actually quite good at coping with compressing forces (like something dropping on your toe) but they don’t like forces that slide the toenail across its base (ie. shearing forces).

As the toenail keeping impacting with the ground, the shearing damages the attachment between the nail and the skin underneath, causing it to turn black.

How can I stop black toenails from running?

Stopping black toenails can be challenging, depending on the cause.

Let’s assume you haven’t tried to squeeze in to shoes that are two sizes too small.

So shoe size isn’t a factor (as mentioned above, it’s a less common cause).

Toe clawing on landing happens due to increased toe flexor activity.

And that increased muscle load occurs for three main reasons:

Running too far or too fast

Your muscles adapt to the training load you expose them to.

So if you’re running regular 8k training runs as a steady pace, they’re ready to roll for 8k plus a little more.

If you suddenly step up to a 21k half marathon, it’s likely that your stabilisers won’t last the whole journey.

This means that your back up muscles, including toe flexors, will kick in and you’re at risk of black toenails from running.

Similarly, if you go and race a 10k at a pace that’s a lot faster than your training pace, you’ll reach fatigue sooner and the toe flexors will kick in.

The solution here is to training for the distance and pace.

Just remember to give yourself enough time to build up endurance and speed to avoid black toenails from running training.

Poor footwear options

Different to the size of your shoes, this is about how they perform.

Over time, shoes lose their structure and materials degrade.

(If you’re not sure what all these terms mean, like “stack height” and “heel-toe offset”, have a read of our post here explaining each part and function of the running shoe.)

This leads to less support, less cushioning, a looser fit and reduced grip.

  • With less support, your stabilisers will fatigue quicker.
  • Less cushioning leads to more muscle bracing on impact.
  • Looser fit can lead to toe gripping to hold the shoe in place.
  • Reduced grip can lead to toe gripping for traction.

All of these mechanisms lead to excessive toe flexor activity and the risk of black toenails from running.

But it’s easy enough to identify shoes as a contributing factor.

You can change shoes to another pair and see if it still happens.

You may need to change for a few weeks as black toenails from running can be a slow process.

You can also feel what’s happening to your toes when you’re running – often you’ll notice that your toes are clawing late in a run if you pay attention to then.

If you’ve confirmed that poor footwear is a contributor, just find a different pair of shoes.

They can be fresh versions of the same or a different model – that part is personal preference.

Lack of strength in stabilisers

A lack of strength on stabilisers is slightly different to inadequate training in point 1.

A loss of strength can be localised and is often due to an injury or pain in the area.

Although your training volume has been good, you might have a loss of output from your hip stabilisers due to a painful hip tendon issue.

That can cause other areas to work harder, such as the foot on the same side or the hip muscles on the other side.

The end result is more compensatory muscle activity and an increased risk of black toenails from running.

To fix this one, think of what other areas of your body are getting sore on a run and fix that injury.

You may also benefit from a running partner watching you run – they can usually spot an asymmetry or limp, although they often can’t identify the specific area.

Once they’ve identified an asymmetry, you’ll need to find and fix the cause to take the pressure off your toes and stop black toenails from running.