Pressure doesn’t cause blisters! But it is part of the problem. We need friction too. So why do we call them friction blisters, not a pressure blister? Do this:
- STEP 1: Press the tip of your right index finger firmly 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.
Here’s the relevance of pressure to blisters
- STEP 3: Wobble back and forth again but this time press softly with your finger tip. Notice how there is less shear. Low pressure allows your fingertip to slide across the skin before shear becomes excessive. High pressure sees your fingertip remain stuck to your hand for longer which causes a lot more shear.
Significance: Pressure enables shear to reach blister-causing levels (Carlson, 2006).
Pressure contributes to the coefficient of friction
But wait … there’s more
- STEP 4: Put a drop of oil on the back of your hand (reduce friction) and wobble your fingertip back and forth again – with really firm pressure. I’m doing it now and I can compress the tissues like crazy yet only a tiny bit of shear is produced! You have to do this to believe it.
So it’s not just about pressure. Nor is it just about friction. It’s the combination of the force of contact (aka pressure) and friction that leads to the friction conditions (known as the coefficient of friction) that will determine the degree of shear.
BUT, it seems friction is the bigger factor. Carlson in 2006 explains how Naylor’s research in 1955 found that when friction loads were doubled, damage occurred three times as fast – this was with no change in pressure.
It’s a common error to implicate pressure as THE cause of blisters. Here’s a word from the late Paul W Brand, an expert on shear and tissue damage, that provides some perspective:
“There are two types of force which occur on the sole of the foot, one is vertical force at right angles to the foot, which causes direct pressure on the tissues. The other is horizontal force, or shear stress, which is parallel to the surface of the foot and occurs in association with acceleration and deceleration. Of the two forces shear stress is more damaging than pressure” (Paul W Brand MD, 1983).
Take home message
Blisters are not just about pressure. Excessive pressure can not cause a blister in the absence of friction. Pressure and friction are mutually dependent in producing shear. We often concentrate exclusively on pressure reduction. We shouldn’t. Because it can be easier and more effective to reduce friction!