What [Really] Causes Blisters On Feet?
You might have heard that foot blisters are caused by heat, moisture and friction?
- Yes your foot gets hot in your shoe – that’s unavoidable.
- Yes, this makes your foot sweat – that’s unavoidable too.
- And yes, that moisture increases friction levels – that’s also unavoidable.
Try keeping your feet cool and dry in your shoes when you're exercising even on a cool day - it's impossible! But that doesn’t mean you have to get blisters - even if it’s really hot and humid, even if you’re on your feet all day, even if you’re really sweaty.
So let’s get specific about what causes blisters
Blisters are caused by the skin stretching too much – it’s called shear. 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 within 2 hours, you'll have a blister. Watch the video below to understand shear better.
Here’s what goes into causing the high skin shear distortions we see specific to the feet and foot blister formation:
- First, there needs to be a lot of repetitions – that is, a lot of steps. That’s why runners and long distance walkers get blisters. And why tennis players get blisters in their long matches, not their short ones.
- Next, you need to understand that the foot doesn’t function like a block of cement. Think of the skin on the outside, the bones on the inside, and lots of layers of biological tissues in between. These connective tissue layers don’t exist independent of each other – they’re all connected. When it comes to foot function, the foot bones move around a lot.
- There’s a lot of pressure on all parts of our feet from weightbearing, shoe contact pressure and from toes sitting so close together. Add the weight of your pack and walking over difficult terrain, it’s easy to see blisters can happen.
- Our feet are subject to high friction levels at the best of times. Not only is it inevitable that your feet will get warm and perspire when you’re exercising. Socks, insoles and interior shoe linings are rightfully made from materials with a relatively high friction level. They need to be to give the foot some traction within the shoe so it doesn’t slip and slide all over the place in there.
Think of a single step. In fact, let’s hone in on just the initial heel contact.
- The heel of the shoe hits the floor. It doesn’t slide forward – it remains in stationary contact with the floor.
- The impact force of heel strike PLUS the high friction levels between the internal shoe lining, sock and skin means all of these surfaces remain in stationary contact with one another. That, plus the pressure of a firm lacing technique means that the foot doesn’t slide forward in the shoe.
- In spite of the shoe and the skin being stationary, the bones slide forward relative to the skin surface.
- This is one single episode of skin shear.
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 and blister sooner with fewer shear repetitions or smaller shear distortions, others seem to be blister-resistant.
The fact is, the in-shoe environment is rather inhospitable with blister-causing forces.
So how do you prevent blisters?
Blister prevention strategies hone in on these causative factors, plus a couple of other shear-minimisation strategies. They include the following:
1. Friction Levels
When you reduce friction, most people think you're trying to stop rubbing. In reality, you're trying to make things more slippery. This slippery surface (either on your skin, between sock layers or on your shoe) will reduce the skin from stretching too much. Try it. Put some Vaseline on the back of your hand, repeat the movement in the video and see how much your skin stretches. Not much!
There is a lot of pressure on all parts of our feet from weightbearing, shoe contact pressure and from toes sitting so close together so it’s easy to see why blisters are so common on the feet, especially when running, jumping and walking over difficult terrains. Try it. Press really lightly and repeat the movement in the video and see how much your skin stretches. Not much!
3. Bone Movement
If you can reduce the amount the bones move relative to the skin surface, you’lla automatically be reducing the magnitude of skin shear distortions.
4. Shear absorption
If you can use materials that undergo shear within their layers, there will be less shear required within the soft tissues of the foot.
5. Spreading shear load
Peak shear occurs in very localised areas. It’s likely that if you can spread that shear load over a larger area, peak shear will be reduced per unit area. I believe this is how tapes work because it’s unlikely they work by any other mechanism (for example, they are unlikely to be made from low friction materials – read here for more about what we know about taping and what we don’t know).
6. Skin Resilience
Research has demonstrated that we can make our skin more resilient to shear distortions and therefore more resistant to blister formation by subjecting our skin to these forces.
The next step
My blister story
I did my laces up firmly to stop blisters
One day back in 2008 I was on my morning walk. I was 8 minutes in and I started to get that familiar hot-spot feeling at the back of both heels. I tightened my laces, which relieved things a bit, and kept going. But shortly after that, I felt that familiar stinging pain of a blister. What the ... I was confused. Not only was I 100% sure my foot wasn’t moving in my shoe, I had taped my heels, like I do every morning. Plus I had been walking for only 8 minutes - my feet weren't that sweaty yet. So where was the heat, moisture and friction coming from. How on earth could anything be rubbing on my skin - it was fully covered with tape!
This confusion was what kicked off my crusade to understand blister causation and prevention better. Because if I was a patient in my own podiatry clinic, I would have failed dismally to offer any helpful treatment or advice.