What Causes Foot Blisters
What do you think causes blisters?
- Heat, moisture and friction?
- Poorly fitting shoes?
Blisters are not caused by rubbing!
Let me tell you a quick story ...
One day back in 2008 I was on my morning walk. I was 8 minutes in and I started to get that familiar hotspot feeling at the back of both heels. I tightened my laces and kept going. Within a few more steps, I had that stinging pain of a blister. "What the ..." I was confused.
Not only was I 100% sure my foot wasn’t moving in my shoe, but just as I did every morning before going for my walk, I had taped my heels. So how on earth could anything be rubbing on that skin? It was fully covered with tape!
The fact is, there wasn’t anything rubbing my heel.
But because of the way my heel bone was moving inside my foot, my skin was stretching up and down with every step I took. And I could feel it!
When the skin stretches (shears) too far and for too long, the connections between skin cells fatigue and break. These tiny tears under the skin surface are the start of the blister injury. Fluid fills the injured area and that when it starts to look like a blister. [Interesting fact: It can take up to two hours for a blister to fully fill with fluid].
This is what skin shear feels like
Step 1 Place the tip of your right index finger on the back of your left hand.
Step 2 Wobble it back and forth but keep it stuck to the same bit of skin. Notice how your skin stretches? This is shear and this is what causes blisters. Keep wobbling as you read:
Shear might look like rubbing but it’s not. Notice how your finger tip has not moved relative to the skin of the back of your hand? But your skin has moved relative to the underlying bone. This is shear - a parallel sliding of connected tissue layers across one another. Shear happens internally, whereas rubbing happens to the surface of the skin. When skin shear is excessive and repetitive, blisters form.
The relevance of heat, moisture and friction!
The popular theory is that heat, moisture and friction cause blisters. While these factors are relevant, they represent a shallow and incomplete understanding of the blister process - and an unhelpful one at that. Here's why:
It gets hot in your shoe - that's unavoidable. This makes your foot sweat (moisture) - that's unavoidable. This increases friction levels.
Try keeping your feet cool and dry in your shoes when you're exercising - it's impossible!
[Friction has an unfortunate double meaning: one is rubbing, the other is the degree of grip or slipperiness. The latter definition is the one we need to use].
It's how high friction levels contribute to skin shear that causes blisters. High friction levels cause the skin, sock and shoe to stick together for a bit longer. And because the bones continue to move inside your foot, the skin is made to stretch.