An Introduction to Shear and Blister Formation

June 17, 2013 by
Filed under: blister care, Foot Care, Health, Sports 

This is a guest post by podiatrist Rebecca Rushton from Australia. She has looked at blisters, how they are formed, what causes them, and how to prevent them. For years, the common thinking about blister causes has been friction, heat, and moisture. Rebecca’s research has led her to identify shear as a leading cause of blisters. Read the article and then check out her website. Over the next months, Rebecca and I will take an in-depth look into blisters, their formation, and treatments. Her website Blister Prevention has a lot of valuable information on blisters (more on that at the end of this post). Here’s Rebecca’s article.

An Introduction to Shear and Blister Formation

Foot blisters continue to wreak havoc with endurance athletes’ feet in spite of their best preventative efforts.

Poor blister prevention outcomes are due in no small part to the misunderstanding of the cause of this obstinate injury.

The force that causes ‘friction’ blisters is not friction. And it’s not rubbing. It’s shear. But if you ask 100 people the question “what causes blisters”, nobody would answer “shear”.

Shear is the sliding of layers across one another – internal layers that are structurally connected. Those connections can break and when fluid fills that cavity, you have a blister!

What Does Shear Look Like?  Try this …

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 your skin stretches? This is shear and this is what causes blisters.

Shear might look like rubbing but it’s not. Notice how your finger tip has not moved relative to the skin of the back of your hand? But your hand skin has moved relative to the underlying bone. This is shear. Your skin doesn’t need anything to rub over it for blisters to form. It just needs shear (this stretching of the internal tissue layers) to be excessive and repetitive.

Rubbing Causes Abrasions

Most of us use the term rubbing to mean two surfaces moving across one another – like when you rub your hands together.  The type of skin injury that rubbing causes is abrasions. An abrasion is where the top layers of skin are rubbed off – you end up with a red raw sore. Blisters (from shear) and abrasions (from rubbing) are completely different entities – they have different mechanisms of injury and affect different layers of the skin. Here’s a video on blisters versus abrasions on the feet.

Is the distinction important? Yes it is. The lack of understanding of blister causation is at the heart of why foot blisters continue to plague athletes.

Achieving True Blister Prevention Success

There are 3 factors that influence shear. Impacting on these is how we can minimise shear and prevent blisters.

1) Type of skin

Thinner and more mobile skin (like we saw earlier on the back of the hand) will abrade before it blisters.  In contrast, thicker and less mobile skin (like on the palm of your hand) is the type of skin more likely to form and maintain a blister.  Do the same experiment we did before but with your index finger on your palm – the skin is noticeably thicker and less mobile in comparison. (This is why blisters are most common on the soles and palms)!

Apart from the thickness and mobility characteristics which determine the ability to blister (and which you can’t do an awful lot about), shear is influenced by two other factors: friction and bone movement. You need both, not just one, to create skin shear. The good news is that these things we really can change! Change one or more of these, in one of many ways, and you can successfully prevent foot blisters.

The cause and influencing factors of foot blisters

The cause and influencing factors of foot blisters

2) Friction

Friction is the force that resists the movement of one surface against another. It’s the degree of slip or grip between surfaces. Low friction (slippery) is when two surfaces glide easily against each other. High friction (sticky) is when the two surfaces tend to grip together.

The moist in-shoe environment during exercise causes high friction levels between the shoe, sock and skin. This causes these materials to stick together … yes the shoe sticks to the sock and the sock sticks to the skin … for longer.  They all stick together for longer because of high friction.

3) Bone Movement

Meanwhile, as we run the bones move back and forth. With the skin remaining stationary (for longer) and the bones moving back and forth as far as they can go, the soft tissue in between stretches – that’s what shear is.

This concept of friction and bone movement leading to shear is depicted in the diagram below and in this video demonstrating shear.

Orange = shoe | White = sock | Brown = skin | Purple = section of soft tissue undergoing shear
Orange = shoe | White = sock | Brown = skin | Purple = section of soft tissue undergoing shear

The purple area is a section soft tissue between the skin surface and its underlying bone. Although the heel itself has not lifted within the shoe due to high friction levels, the bone has moved up relative to the skin surface causing shear to the soft tissues in between.

About Rebecca

Rebecca’s website Blister Prevention has a lot of valuable tips and techniques, and information on blisters. Take some time and explore the site, subscribe to updates and receive a copy of her ebook, Blister Prevention for Active People. Rebecca is a podiatrist in Australia.

Next Up? More on Shear and Blister Treatments

Over the next month or two, we will talk more about shear and common blister treatments – including what works and doesn’t work. Make sure you are a subscriber to this blog to receive each post. You can do that at the box at the upper right side of this page.

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6 Comments on An Introduction to Shear and Blister Formation

    […] Each interface has a certain level of friction. This is measured by the coefficient of friction (COF) which is a number usually less than 1. In-shoe friction levels generally range from 0.40 – 0.90. Read this to see how high friction keeps the shoe, sock and skin all stuck together, for longer. And why this causes …. […]

  1. johnvonhof on Mon, 8th Jul 2013 12:23 pm
  2. Joe Lugiano emailed with this comment:
    Thanks for a great post. I think she is spot on when it comes to shear. In my early days of running marathons to 100 milers, blisters seemed to be a given. In early cases I could track it back to thick acrylic socks. Later, as I switched to the Wigwam Ironman Triathlon socks and very little problem with blisters, I realized that any problems were created by steep or canted surfaces which resulted in the shearing affect. But even then it wasn’t as big an issue if I properly prepared my feet with Body Glide and my shoes fit properly. Thanks for addressing one of the key causes of blisters.

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