Understanding Shear

This is part II of a series on blister formation and prevention. If you’ve missed the first post, I encourage yo to go back and read it to get a foundation on blisters. Here the link: Blister Formation.

In this part II, we’ll look at shear. For years we didn’t understand the concept of shear and its effect on blister formation. So let’s start with a story.

In Fixing Your Feet I tell the story of a runner at Badwater whose feet I patched. In short, he had run 90 miles of the 135-mile race, on pavement, in extreme heat. He had Elastikon tape on both balls of the feet. His feet were hurting to the point of quitting. I replaced the Elastikon tape with smoother kinesiology tape, which allowed movement between the tape and his sock, greatly reducing the shear movement between the layers of skin and the bones of his feet. I also added a large ENGO Blister Prevention Patch on each insole under the ball of each foot. By reducing the shear level, the runner was able to finish the race with less pain. Try to picture the following: as your foot moves through its foot strike, the bones of the foot move against the layers of underlying skin—then you apply a tape that is not smooth to the skin, pull on a sock, and finally put your foot inside a shoe. The tape sticks to the skin. As you run, the foot naturally moves a bit inside your shoes.

However, the sock cannot move freely against the coarseness of the tape. The sock and tape move as one, which stresses the outer layer of skin against the inner layers. The only movement is the shearing effect between the layers of skin.

That experience was the first time I made the connection to shear, although I didn’t know it by that name. All I knew is the stickiness at the sock–shoe interface, the tape–sock interface, and the tape–skin interface—and one, or all three, had created this major problem for the runner. I never forgot the story. His skin was stuck to the Elastikon tape, the coarse tape didn’t move against his sock, and the sock didn’t move against the shoe’s insole. With the smoother kinesiology tape and the new slipperiness between the tape and the sock, the coefficient of friction was reduced and in turn shear was reduced.

Shear is a new concept for most athletes, especially as it relates to blisters. Shear is defined as a strain in the structure of a substance when its layers are laterally shifted in relation to each other. Applying the definition to the above example, shear happened between the layers of skin as the bones of the foot moved through the foot strike. The internal layers of skin were connected. But those connections can break under the stress of shear and the cavity fills with fluid—and you have a blister.

To understand shear, try this. Place the tip of your index finger against the skin on the back of your hand. Keep it stuck to the same bit of skin while you move it back and forth while. See how your skin stretches? The skin on your hand has moved against the underlying bones. That is shear that causes blisters.

Note that nothing has rubbed against the skin. Your finger did not rub the skin. J. Martin Carlson, the founder of Tamarack Habilitation Technologies, has championed shear as the cause of blisters. Tamarack has a long history of providing innovative orthotic-prosthetic componentry and materials. Their focus on friction management, especially for amputees, has won them many awards and much recognition. This knowledge in turn led to the creation of a new product that can be applied to footwear to reduce high friction levels and, in turn, the shear that leads to blister formation: ENGO Blister Prevention Patches.

Shear in Action

After watching a video on Tamarack’s website, I understood more about shear in action. The video showed a cutaway on the heel area of a shoe, showing the sock and foot inside moving through a foot strike motion. In one video, the cutaway showed a sock and foot on an insole where there were high levels of friction. The sock and foot were distorted as they were held against the insole. It was as if they were stuck together. In another video, the sock and foot were on an insole with an ENGO patch underneath. No distortion occurred as the sock and foot moved easily through the foot strike, over the slippery surface of the ENGO patch.

It’s important to grasp how shear happens. As described above, shear results in distortion occurring between the skin and soft tissues underneath. This shear distortion is what causes blisters. The bones in our feet move back and forth as they move through each foot strike. When the skin at the bottom of the foot is stuck by high friction (stickiness) to the sock and shoe, the middle tissues are distorted. When this is repeated over and over, traumatic levels are reached and a blister forms. This distortion can happen anywhere on the foot: in an up-and down motion in the heel, the sides of the foot, and between toes; in a side-to-side motion at the ball of the foot, under the heels, and at the bottom of the toes; and in rotation as the foot moves through its foot strike. As we walk, run, and pivot in our shoes, the surface of our skin incurs a shearing force.

Certain amounts of shear are normal, and our feet can deal with a lot. However, with repeated traumatic levels of shear, blisters will develop. How much is too much? It varies from person to person, and some people are simply more blister-prone than others.

Part III will look at the five factors of blister formation.

Blister Formation

October 14, 2017 by · Leave a Comment
Filed under: blister care, Foot Care, Footwear Products, Health, Sports 

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:

  1. 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.
  2. 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.
  3. 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.
  4. 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.

  • Subscription Form

    Enter your email address:

    Delivered by FeedBurner

  • Recent Posts

  • Categories

  • Recent Comments

  • Archives

  • Pages

  • Circulation

%d bloggers like this: