Understanding the Five Factors in Blister Formation
This is Part III in a multi-part series on blister formation. This is a subject that still confuses many athletes. In this post, we look at the five factors that are the leading causes of blister formation.
For years, we thought blister formation was caused by heat, moisture, and friction. So, ever since the first edition, the image used to show this was a triangle with heat, moisture, and friction at its three sides. Everyone thought the three factors combined to make the skin more susceptible to blisters. That’s what was promoted in articles, running forums, and general discussion. It seemed to make sense.
Then we discovered shear, as you read in the previous Part II post. Shear encircles everything. The image shows a series of two concentric circles where shear is outside as the base on which everything else rests. The next circle contains the five factors that contribute to shear: skin resilience, bone movement, pressure, friction, and moisture. In the next post, we’ll look at the two inside additional circles: the first with the five major components of blister prevention and the innermost circle with the remaining eight minor components, which I will explain later. For now, we’ll look at the five factors that contribute to shear.
1. Skin Resilience
Our skin is very resilient. However, repeated stressors to the skin, over time, can cause breakdown. The healing process can’t keep up with the ongoing trauma. This is why well-fitting shoes are important. Skin that is thin (like on the top of our feet) will abrade before it blisters, whereas thicker skin (like on the soles of our feet) is more likely to blister. We can make the skin on our feet more resilient by progressively increasing distance over time. This gradual increase of the frequency and magnitude of forces applied to the skin helps change the characteristics of the skin. Reducing calluses and using moisturizers also contributes to skin resiliency.
2. Bone Movement
As the bones in our feet move back and forth and up and down through the foot strike, and the overlaying skin remains stationary, the soft tissue layers in between stretch in a shearing motion. The more movement, relative to the skin surface, the more chance you’ll blister. When that happens, there is stretching and distortion between the inner tissues under the bone. Shoe fit is an important component in controlling bone movement. Changing your biomechanics is another way to work at reducing blister formation.
Pressure is vertical force exerted against an object or surface. In this case pressure is the normal force of the foot through the foot strike. The heel comes down, the foot rolls forward onto the forefoot and off the toes. At all of these transitions, there is pressure downward, forward, and most likely side-to-side. Optimizing the fit of your footwear, correcting any biomechanical issues with an orthotic, and adding cushioning or padding can reduce pressure.
Mention the word friction to athletes and they most commonly think of rubbing. For instance, “My heel rubbed inside my shoe and created a blister.” It may help to know that the dictionary gives two definitions for friction. The one most people think about is the action of two surfaces rubbing against each other. Rubbing can cause abrasions to one or both surfaces, but not a blister. Forget about this definition as we talk about blisters. Let’s look at the second definition.
Scientifically, friction is defined as the force that resists one surface sliding against another. It’s easiest to view friction in one of two ways. When two surfaces slide easily against each other, we have a slippery connection—and low friction. When two surfaces resist movement against each other, we have a sticky connection—and high friction. Friction is required for shear to reach traumatic levels.
Rebecca Rushton of BlisterPrevention.com.au says it well: “There is high friction in your shoe. Surfaces are resisting movement against each other. When your skin is moist, your skin grips your sock; your sock grips your shoe. All three surfaces grip together so your foot doesn’t move around in your shoe. But with every step you take, your foot bones are moving under the skin. And while the skin is stuck the bones are moving back and forth. Everything in between is pulled and stretched. This pulling and stretching is what causes blisters. We call it shear.” Note: Rebecca wrote the forward to the 6th edition of Fixing Your Feet.
There is good friction and bad friction. High friction, as Rebecca described, is when things grip together. From this comes traction, which we need for the mechanical efficiency of our gait. Most friction is good, but when high friction causes blisters, it’s bad friction, and has to be managed—but only in that specific location. ENGO patches are an example of targeted management of friction. Now let’s look at the forces affecting friction, specifically the coefficient of friction.
4.a The Coefficient of Friction
The coefficient of friction (COF) describes the relationship between the force of friction and the normal force between two objects at which sliding is initiated. It is a number that represents the slipperiness or stickiness between two surfaces and is generally below 1.0. Within the shoe, the COF between the foot, sock, and insole can range from 0.5 to 0.9. In contrast the COF between a sock and a polished floor is around 0.2. The lower the number, the better the effectiveness in preventing blister formation.
Here’s an example. A runner may have damp feet, creating a moist condition. The COF in his case might be a 0.7. By moving away from the moist condition to either very dry feet or very wet feet, he might reduce his COF to 0.5. If his blister-causing threshold is 0.6, getting to 0.5 will reduce his chance of blistering. Moist skin is higher friction than dry or very wet skin, meaning it’s more susceptible to blistering.
Understanding the COF of materials is beneficial to knowing how shear starts and what we can do to reduce blistering. Managing the moisture on the skin, using different socks systems, and using ENGO patches are the easiest ways to reduce the COF. The COF of an ENGO patch is about 0.16 equally against a dry or wet sock, effectively reducing friction by 80%. Compare that to the COF of moleskin against a dry sock of 0.6 and a wet sock of 0.86. Again, the lower the number, the better the result.
Moisture is the last factor. Increased moisture leads to an increase in friction. Beyond that, moisture does not cause blisters in any other way. Generally speaking, a lower COF is achieved with dry socks compared to wet. Some degree of moisture control can be gained with moisture-wicking socks, antiperspirants, powders, and some special lubricants. Our feet have approximately 250,000 sweat glands that can produce continuous moisture, without factoring in sweat produced though an increase in body temperature as we engage in physical activity. When active, most athletes have feet that are always damp from moisture. Moisture can also occur from stream crossings, rain, and puddles, as well as from pouring water over our heads to cool us down, which runs down our legs into our shoes.
There are three additional factors worth mentioning, but they are not important enough to warrant lengthy explanations or inclusion on the chart. The first is heat. Heat is not a factor in blister formation, but many athletes equate heat to causing blisters. Heat produces moisture as the feet sweat, leading to a higher COF and an increase in friction levels. But heat does not cause blisters in any other way. The second is repetition. We can minimize the chance of blisters by reducing repetitions. This can be done by shortening the length of your activity, reducing the level of intensity, and changing the frequency of your activity. But not many athletes will stop running or hiking—so there’s little point in talking about reducing repetitions.
The third factor is shear absorption. The less shear absorption in your footwear, the more pressure and friction your skin has to absorb—and that means more chance of blisters. Most of this is covered in cushioning components like socks and insoles as they work to reduce pressure and absorb shear.
The next blog post will look at the components of prevention.