What Causes Foot Blisters?
Heat, moisture and friction?
The answer is... none of the above.
Blisters are caused by the skin having to stretch too much (it's called skin shear).
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. 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. Here's why:
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 truth 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! This skin stretching is called skin shear.
When the skin stretches (shears) too far and for too long, the connections between skin cells fatigue and break. This series of tiny tears under the skin surface are the start of the blister injury. It’s not until fluid fills that injured area that it starts to look like a blister. It can take up to two hours for a blister to fully fill with fluid.
Rubbing gets the blame
Rubbing gets the blame for blisters because once weakened by the injury under the skin surface, it doesn’t take much more stretching (shear) for the blister roof to tear and even dislodge, leaving a red raw sore. It looks like the skin has been rubbed raw, when in fact, it was the excessive stretching that caused the weakness in the first place.
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.
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.
This is what it takes to produce blister-causing skin shear
The relevance of heat, moisture and friction!
The other 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.
"Shear is normal and it happens with every step you take. Thankfully, the feet are able to tolerate a lot of it. Blisters only occur when shear is excessive and repetitive. That threshold is different for everyone ... some people are blister prone, others seem to be blister-resistant."
If you want to stop getting blisters on your feet, you’ll need to start from this new understanding of what causes blisters.
Actually, we’ve known since 1955 this is what causes blisters. Somewhere / somehow, it all just got lost in the translation.
You’ll need to start thinking about how you can:
Reduce friction levels
Reduce peak pressures
Reduce bone movement
Alter your skin’s cellular make-up to increase its resistance to blister-causing shear
Everything you’ll read and see from me about preventing and treating foot blisters is focussing on these 4 factors.