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Chapter 3: Why we get blisters on our feet
In this chapter
- Skin anatomy
- Blister formation
- Why do we get blisters on our feet?
- The blister threshold
- A hypothetical scenario
Skin is the barrier between the internal and external world. It has three layers: the epidermis, the dermis and the subcutis. A layer of fascia under that connects the skin to underlying tissues including the bones. Here are a few interesting things to know:
- Blisters occur in the outer skin layer called the epidermis
- The epidermis is made up of several layers itself and blisters occur within the stratum spinosum (prickle layer)² ³ ⁶ ¹¹
- The soles and palms are made of skin with no hair and a higher proportion of sweat glands (glabrous skin). Other areas of skin contain hair and less sweat glands¹¹⁴
- Bones are indirectly connected to the skin surface and therefore bone movement has an impact on the tension in more superficial skin and soft tissue⁹⁴
- the formation of an intra-epidermal split
- the filling of this split with fluid
Blisters form when the shear stress exceeds the shear strength of the skin, resulting in a separation within the prickle layer of the epidermis.¹² Presumably this is the layer of cells with the least resistance to shear. As shear becomes excessive, the structural connections between these cells stretch too far, they fatigue and fail. These microscopic tears are the initial blister injury. The higher the shear magnitude, the less repetitions required to cause a blister.² Knapik¹¹⁹ summarises the observable changes of rubbing the skin (below). Most recently, Hashmi¹⁰⁹ found the same changes in their experimental blister study on the back of the heels.
“A relatively uniform series of events are involved in blister formation. At first there is a slight exfoliation of the stratum corneum, and erythroderma [redness] is noted around the zone of the rubbing. With continued rubbing the subject may suddenly experience a stinging or burning sensation and a pale, narrow area forms around the reddened region. This pale area enlarges inward to occupy the entire zone where the rubbing is applied. The pale area becomes elevated over the underlying skin as it fills with fluid. Histological studies indicate that the pale area is a separation of cells at the level of the stratum spinosum, presumably due to mechanical fatigue.”¹¹⁹
A blister does not fill with fluid immediately after the epidermal split but it is fully filled within 2 hours.³ ⁶ ¹¹
WHY DO WE GET BLISTERS ON OUR FEET?
In Chapter 2 you saw what shear looks and feels like on the back of your hand. Here’s an extension to that which shows you why the skin of the feet are susceptible.
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 far your skin can stretch (shear) back and forth.
Step 3: Now do the same thing on the palm of your left hand. Notice how your skin doesn’t move as much? The skin on the back of the hand is thinner – you can pinch it and move it around easily. Most skin on your body is like this. On the palm it feels thicker and less mobile. It's just like this on the sole of the foot, even more-so because of weight-bearing. This is the type of skin that blisters form on most readily, as explained below.
- The skin is relatively immobile as it is adhered firmly to underlying structures. This is a functional requirement of the foot. But it means shear reaches a peak sooner.³⁷ Thankfully, the soft tissues of our feet are able to deal with a lot of shear … to a point.
- The very outer layer of skin (the stratum corneum) is very thick on the feet, particularly on the soles. A thick corneum makes the skin surface able to withstand shear and rubbing without abrading, and able to withstand the underlying pressure of a fluid-filled lesion (blister). Conversely, the thinner the skin, the quicker it tends to abrade (either before or as soon as the skin blisters).³ ¹⁰ ³⁷ ⁴⁸
There are other reasons why blisters are common on the feet. And these have to do with the in-shoe microclimate.⁸³ ⁸⁹ Conditions within the shoe are hot and humid at the best of times⁸³ and consider the unique structure and function of the foot:
- There are more sweat glands on the sole of the foot compared to other areas of the body. These sweat glands secrete a certain amount of perspiration all the time, and even more-so when you exercise and when the outside temperature is hot (and some people perspire more than others). Blister and skin friction studies consistently show higher friction levels on moist sweaty skin (compared to very dry or very wet skin) and on the soles and palms compared to other parts of the body.³ ¹¹ ¹⁷ ²⁵ ⁷⁹ ⁸² ⁸⁶ ⁸⁹
- Few other parts of the body are wrapped up like the feet are in socks and shoes. Air circulation is poor so cooling and evaporation are compromised.⁸³
- No other body part sustains weightbearing pressures like the feet do
- The nature of gait is repetitive and force repetitions are a requisite for blister formation.
THE BLISTER THRESHOLD
Shear is a normal consequence of transferring weight from one foot to the other and it happens with every step you take. It is present at well-tolerated levels most of the time as we walk, run and play sport. But shear can become excessive. Excessive shear is the last little bit of shear – where it reaches its maximum. We call this the shear peak. To put it simply, when the shear peak exceeds a certain limit blisters will form. You can think of this limit as your blister threshold and it’s represented by the dotted line. In essence, the position of the blister threshold (the height along the y axis) defines where an individual sits on the continuum between 'blister prone' and 'blister resistant'.
Blister prevention and shear peaks
The aim of blister prevention is to get this shear peak below the blister threshold. So let’s look at how reducing the horizontal forces (friction and bone movement) can get the shear peak to somewhere below the blister threshold so blisters are avoided.
By reducing friction, you make the environment (an area large or small) more slippery. An earlier slide, of the skin relative to the shoe, means the skin moves more in sync with the bone, with less shear the result. This slide can be between the skin and sock, the shoe and sock, or both!
Examples of friction-reducing blister prevention strategies include lubricants like Vaseline, ENGO Patches, moisture-wicking socks, powders and antiperspirants.
REDUCING BONE MOVEMENT
Rather than dealing with friction, if bone movement is reduced, quite simply, the shear peak will be lower, even in the presence of high friction.
Skeletal movement can be reduced by altering biomechanics or form, or by altering the intensity, duration or frequency of the activity. But these strategies can only be taken so far before they impact negatively on sporting performance.
A HYPOTHETICAL SCENARIO
Let’s apply our knowledge of friction and shear peaks to a hypothetical blister scenario.
Let’s say you’re a runner and you’re having a bad time with blisters, which is unusual for you. It’s a hot summer, you've been running a bit later in the mornings and you think the heat might have something to do with it. Let's say we could measure your blister threshold and we knew your blister-causing coefficient of friction (COF) was 0.6. You wear good runners, cheap socks from the sport shop and you run three times a week for between 5-8kms on a mainly flat terrain. Your form and biomechanics are good and your feet are in good condition otherwise. So it seems you can’t do much about bone movement, but you can definitely have an impact on friction, because you think the extra sweat is causing higher friction levels. If we measured the COF between your foot and the sock at the end of a run, we might get a value of 0.8 (which is above your blister threshold). So your aim would be to make your skin stay drier for longer (let’s say you used an antiperspirant and moisture-wicking socks) so you got the COF down to 0.5. At this level, you’ll have a successful blister prevention strategy.
If you still got blisters but they only developed on your 8km runs, it’s an indication the antiperspirant and socks keep the COF below 0.6 for longer, but not long enough. Presumably the antiperspirant loses effect and the moisture-wicking socks exceed their absorptive and wicking capacity. The next step could be to reduce the repetitions by keeping your running distance to 5kms. But you’re understandably not happy with that. So you try an ENGO Patch on your insole in an effort to reduce friction between it and the sock. Thankfully, even on your big runs you remain blister-free. This is an indication the COF remained below 0.6 for the duration of the run, in spite of the environmental and in-shoe conditions.
Notice how we’ve just addressed the four influencing factors of blister-causing shear?
- Thick and immobile skin – actually not much you could do about this one
- A high coefficient of friction – minimised with antiperspirant, moisture-wicking socks and ENGO patch
- Moving bone – we know your mechanics and shoes are good and the terrain is flat
- Repetition – you could have stuck to 5km runs but it would have been as a last resort
- The thickness and stiffness of the skin on the feet (particularly the soles) is most likely to form blisters.
- The in-shoe microclimate is suited to blister formation.
- The blister threshold explains why some people are more blister-prone than others.
- The blister threshold helps explain the aim of blister prevention strategies.