This is the first in a multi-part series on blister formation and prevention. Subsequent posts will focus on various part of the blister process. Too many people do not understand how blisters form, some even think they are a natural part of the running or hiking experience.
Blisters are the result of shear trauma to the skin. When running, every step is “repetitive loading” as the foot moves through the foot strike. The whole of the body’s weight is put on the skin and inner tissues, onto the bones and joints, up the leg, into the knees and hips, and into the pelvis and spine. For now, let’s focus on the foot. The skin can take a lot of repetitions without a problem, but once skin and tissue damage reaches a painful level, the trauma is escalating rapidly.
Repetitive loading has two components. The first is a vertical loading to the surface of the skin and by itself is relatively harmless. The second is a repetitive friction loading parallel to the surface of the skin. The two combine to create shear stresses within the skin. When these shear stresses exceed a certain level, micro tears begin to form within the third layer of tissue. As more repetition occurs, more micro tears appear and existing tears grow. These tears in the tissue form a cleft parallel to the skin’s surface. The cleft fills with serous fluid and a blister has formed—caused by shear stressors. This typically occurs in the stratum spinosum, the inner fourth layer of the epidermis that is the least resistant to shear.
If the blister is deep or traumatically stressed by continued running or hiking, the serous fluid may contain blood. When the serous fluid lifts the outer layer of epidermis, oxygen and nutrition to this layer is cut off and it becomes dead skin. This outer layer is easily burst. The fluid then drains and the skin loses its natural protective barrier. The underlying skin is raw and sensitive. At this point, the blister is most susceptible to infection.
Rebecca Rushton, an Australian podiatrist and the author of The Blister Prone Athlete’s Guide to Preventing Foot Blisters, has studied extensively about blister formation. She has identified four requirements for shear that cause foot blisters to form:
- Skin resilience: The skin on our feet is, by its very nature, susceptible to blister formation. Repeated gradual exposure to shear can improve the skin’s ability to withstand shear stress. Often times the skin is stressed by a sudden increase, or an increase over many days. Examples include increasing one’s mileage too quickly and doing more than normal in a multiday event.
- Moving bone: Bones within the foot move throughout the foot strike. The more they move relative to the skin’s surface, the higher the shear stress created, and the more likely you are to develop a blister. An example is the movement of the heel bone underneath the skin as it moves up and down in the shoe’s heel counter.
- Repetition: It’s the repetitions throughout the foot strike that increase the odds of a blister forming. An example is studies showing that runners in a multiday event blister more as the race progresses.
- Friction and normal force: The levels of friction and pressure determine the amount of shear that impacts the skin.
It’s worth talking about how blisters will deroof—when the stop layer of skin is torn off the top of the blister. Consider a back-of-the-heel blister. A blister has formed because the heel bone has moved up and down while the outer skin has remained fixed inside the sock and shoe. The shear stressors, as described previously, have caused the blister. With repeated levels of movement against the skin on the back of the heel, the skin can be rubbed over and over. This is what causes abrasions. When the forces that cause blisters combine with forces that cause abrasion, the skin is stretched past normal levels and is torn off.
In reality, some people are more blister-prone than others, irrespective of the amount of training and preparation. In spite of having done everything right, the shear strength of their skin is going to be lower than others—no matter what they do. They are simply blister-prone. For others, some of this has to do with the training that they do in preparation for races. It’s quite common at races to see lead runners come through aid stations without any blister problems. They typically have conditioned their feet through extensive training. But as the race progresses, the middle- and back-of-the-pack runners enter aid stations with a lot of blister problems. They typically have put fewer miles on their feet and, in some cases, have made poor choices about footwear and foot care.
This is the first in a multi-part series on blister formation and prevention. The material is extracted from Fixing Your Feet, 6th edition.