- 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 because ...
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 following four factors cause foot blisters. You need all four of them at the same time!
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.
More on the 4 requirements for blister-causing skin shear:
1. Type of skin
The skin of our feet is susceptible to blister formation, particularly the plantar (sole) surface. That’s because it's thicker and less mobile than other skin and most able to form and maintain a fluid-filled lesion. And every individual's skin has an inherent shear strength - mine's rather low and that makes me blister prone. Other people are blister resistant because the shear strength of their skin is high. We're all different in many ways, from blue eyes to big noses ... shear strength is just another of those differences.
2. High coefficient of friction (pressure and friction)
No other body part sustains pressure like the foot does - because we walk on them. And there are few other body parts where the micro-climate dictates there will be high friction by default (heat, moisture & little evaporation). This high coefficient of friction means the skin grips the sock, and the sock grips the shoe. These surfaces remain stuck together, for longer. The result is internal tissue layers slide further relative to each one another. When the connections that bind these layers are stretched too far, they tear (it's a mechanical fatigue - Comaish, 1973). Fluid fills the injury site and you have a blister.
[Insight: To see how important friction is, put a drop of oil on the back of your hand (minimise friction) and wobble again. Regardless of how hard you press, there is almost no shear. Seriously ... do it now! This is why minimising friction is key to blister prevention].
3. Moving bone
As you walk, run and play, the bones move around within your foot. As your foot plants, the bones skid forward to a stop, and then backwards as they push off into propulsion. This is normal and unavoidable. But the further the bones move, and in the presence of high friction, skin shear can become excessive. That is, the structural connections fatigue and fail. This is a blister-causing level of skin shear.
The more times soft tissue is subjected to these high shear distortions, the closer the structural connections between the tissue layers are to breaking and forming a blister. Everyone sits in a different position on the blister continuum and this explains why some people seem to be blister prone - they get blisters with relatively few repetitions. And others are blister resistant - they can run day after day in the most challenging of conditions and not suffer a single blister.