Most of us understand that foot blisters result from some combination of heat, moisture, and friction. While all three factors are relevant, this generalisation grossly over-simplifies the actual process. In fact, the perception one is left with is an inaccurate one. It should come as no surprise then that without an adequate understanding of what causes blisters on feet, obstacles to effective management are almost inevitable.
The old “what causes foot blisters” understanding
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.
This article highlights the holes in the old heat-moisture-friction blister paradigm and offers a new paradigm for what causes blisters. One that is accurate, easy for the lay population to understand, repeatable and points us towards the array of opportunities for effective management practices.
Heat, moisture and friction
Heat is relevant to blister formation. While friction blisters are not a thermal burn,1–9 we know it is easier to get blisters in hot conditions compared to cold,5,10 that there is an increase in skin temperature preceding blister formation,8 and there is a prolonged increase in temperature after the blister forms.8 What’s more, higher temperatures cause more perspiration3 and unsurprisingly, faster walking speeds increase in-shoe temperatures11,12 and increased sweating.13 This perspiration that provides the ‘moist’ environment known to increase friction levels 2,14–16 and blister formation.3,17
The heat – moisture – friction paradigm makes intuitive sense. Of course, heat means sweat, sweat means moisture, and moisture means more friction – things rub more on the foot making blisters more likely. It’s a simple one to repeat and for the lay population to understand. One sentence covers it! However, a closer look at the word “friction” is required. It is here that the old paradigm fails the lay-language test and is destined for misinterpretation and inaccuracy.
The contradictory definitions of friction is responsible!
Friction is a fundamental factor to the cause of foot blisters, but is more counterintuitive than one might expect. Why? Because friction has two meanings. It means both rubbing, and the resistance to movement of one surface over another18 – two contradictory definitions.
In the case of foot blisters, it’s the first definition that the majority of us see in our mind’s eye when we think heat-moisture-friction. However, it’s the second definition of friction that we should be thinking of – the resistance to movement. The second definition means things rub less – there is more resistance to rubbing so it takes longer for movement to occur.
So, let’s examine both a low friction scenario and a high friction scenario and determine their relevance to the cause of foot blisters.
Low friction scenario:
Let’s take the greasy lubricant, Vaseline. It is often used on the feet to prevent blisters. Vaseline reduces the resistance to movement. It makes things more slippery. It reduces the friction level. Vaseline reduces the friction level and allows things to slip or slide or rub or move across one another more easily. A low friction scenario is one where there is less resistance to movement so that things rub more. This is how lubricants (and other friction-reducing strategies) prevent blisters. Rubbing does not cause foot blisters. In fact, promoting rubbing by way of reduced friction levels is how you can prevent blisters.
High friction scenario:
High friction levels make things stick together for longer – an increased resistance to movement. For example, perspiration causes the skin to become clammy or tacky or sticky. It increases the friction level. Sweaty feet increase friction levels and make it more difficult for things to slip or slide or rub or move across one another. If movement does happen, it happens much later. This is the increased friction levels that are unequivocally linked to the cause of blisters on feet.
But how, exactly?
Soft tissue shear distortions cause foot blisters via a mechanical fatigue
It has to do with soft tissue shear distortions. Higher friction levels make the skin, sock and shoe surfaces remain in stationary contact for longer and this results in larger shear distortions. As the bones continue to move within the foot with each step, the skin does not follow. And so the soft tissues in between undergo shear. Picture the layers of skin and soft tissue gliding parallel over one another to allow for this skin-bone mismatch. The longer the skin remains stationary, the higher the shear distortion. These shear distortions, when repetitive, are known to cause blisters.1-4, 8,14,16,19–24
This is the crux of shear distortion
Doug Richie20 described it best when he mentioned the skin and bones moving out of sync with one another:
In my opinion, Richie’s is the best descriptive attempt to explain what shear distortions are – the skin and bones moving out of sync and everything in between undergoes shear distortion. You can see it happening in your mind’s eye. Now, ten years on, “skin and bone moving out of sync” should form the basis of the new blister causation paradigm.
In addition, there needs to be a break in the habit of thinking blisters as a superficial-to-deep wear injury due to rubbing. Blisters start as a tear under the skin surface, within the epidermis at the level of the stratum spinosum.1–4,8,14,20,21,23,24 Comaish1 in particular investigated potential causes of this and determined the mechanism of injury as an “epidermal fatigue” due to repetitive shear forces. The tear is often referred to as a “cleft”. In the spirit of keeping things simple, I believe tear is better word as it’s in the common vernacular and widely understood. Cleft is not.
The new “what causes foot blisters” paradigm
With the above in mind, a new paradigm is proposed for foot blister causation. As you can see, this is not new information; the existing body of research is unequivocal on what causes friction blisters on the feet. It’s simply an improvement on the current heat-moisture-friction paradigm by being:
Brief and descriptive
Easily understood by the lay population
Less likely to be misinterpreted
The order of the following points is intentional, each build on the previous, depending on the brevity required.
The first sentence encapsulates cause and effect and can be used as the briefest answer to the question “what causes blisters”:
Blisters are a tear under the skin surface caused by the skin and bone moving out of sync.
The second sentence introduces and defines shear distortion, the logical next point to make:
As the bones move within the foot with each step, the skin does not immediately follow. And so the soft tissues in between are made to stretch and distort. This is called a shear distortion.
The third sentence describes the injury that is so familiar to us all:
If there is too much shear, a tear develops under the skin surface which later fills with fluid to become what we recognise as a blister.
A segue to how blister prevention works
The previous sentence begs the questions how much shear is too much. It provides a nice segue to a high-level overview of the opportunities for blister prevention if the conversation requires it.
The point at which the tear occurs under the skin surface is determined by, a) the number of shear distortions, b) the intrinsic resilience of the skin to shear distortion and, c) the magnitude of shear distortions.
To explain this last point a little further:
Number of shear distortions: Simply, the fewer shear cycles imparted to the skin, the less likely blister formation is. This alludes to why blisters are more common in activities where you simply take more steps, like long-distance running and hiking. Training protocols can be altered in this regard.
Intrinsic resilience of the skin to shear distortion: This speaks to the large individual variation in time-to-blister shown in experimental blister studies, plus the important preventive strategy of skin adaption.
Magnitude of shear distortions: This is the very heart of blister prevention – to allow an earlier glide of that area of skin more in sync with the bone. The aim is to either: a) Reduce friction levels:This refers to strategies such as lubricants, Engo Patches, powders, double-socks, moisture-wicking socks, other moisture-management strategies. b) Reduce pressure: This refers to strategies such as cushioned insoles, gel toe protectors, toe-props, donut pads and other pressure-deflective paddings, shoe-fit and lacing techniques. c) Reduce the bony excursions: This refers to strategies such as stretches, customized orthoses, gait modifications. d) Spread shear load: This refers to taping and moleskin. e) Use of shear absorbing materials: This refers to cushioning materials such as Spenco, Poron and gel materials in the form of insoles and toe protectors.
A more verbose explanation of “what causes foot blisters” might go something like this:
The foot doesn’t function like a block of wood – the bones move around with every step we take. At the same time, the skin, sock and shoe tend to stay stuck in stationary contact with one another. Essentially, the skin and bones move out of sync. When a bone moves, it pulls on the soft tissue it’s attached to. Therefore, the problem of blisters starts when you consider that every layer of soft tissue between bone and skin are structurally connected.
With the bone moving and the skin not moving, all the soft tissue in between stretches and distorts. This is called a shear distortion. It comes from one tissue layer sliding over another. Understandably, there is a finite amount that can occur before there is a tear in these connections, and this point will be different for everyone. Once the tear occurs, it slowly fills with fluid to resemble what we know and love as a blister. Blister prevention is all about either increasing the skin’s resilience to shear distortion, reducing the magnitude of shear distortions or reducing the number of shear distortions, or combinations of these.
Conclusion – The new blister paradigm
Blisters are a tear under the skin surface caused by the skin and bone moving out of sync. As the bones move within the foot with each step, the skin does not immediately follow. And so the soft tissues in between are made to stretch and distort. This is called a shear distortion. If there is too much shear, a tear develops under the skin surface which later fills with fluid to become what we recognise as a blister. The point at which the tear occurs under the skin surface is determined by, a) the number of shear distortions, b) the intrinsic resilience of the skin to shear distortion and, c) the magnitude of shear distortions.
Please, when describing blisters (what they are, what causes them) avoid any reference to heat, moisture, friction or rubbing. If you can do this, you’ll be helping to progress the way we understand how to effectively prevent and treat them.
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