More on Shear and Blister Treatments

August 26, 2013 by · Leave a Comment
Filed under: blister care, Foot Care, Footwear 

Back on June 17 I introduced the concept of shear with a post by podiatrist Rebecca Rushton from Australia who has studied blisters and identified shear as a major factor in blisters.

It’s best to start by refreshing our memories about what was shared on the previous article. Here’s the link in case you want to see the full post: An Introduction to Shear and Blister Formation. Otherwise, here’s a short piece from that post:

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.

That’s shear.

Managing shear is key to managing blisters. Let’s look at several ways to reduce shear.

The first way is to make sure your footwear fits. Many people buy shoes that seem comfortable in the store but don’t make sure they feel ok by wearing them around the house for a few days. Make sure they have enough room in the toe box both in height and length. Make sure there is not undue pressure on soft tissues over any bony spots (sides of the forefoot, ball of the foot, sides and back of the heel, over the instep, etc.). Make sure they are not too loose, allowing too much movement leading to skin abrasions, hot spots, and then blisters.

Using a cushioning product is a second way to reduce shear. This might be a gel pad under the ball of the foot or under the heel bone, or a replacement insole meant to pad and cushion.

Applying power during an adventure race

Applying power during an adventure race

A third method is to manage skin moisture. This can include skin-drying strategies and skin lubrication. Studies have shown that you can reduce the incidence of blisters by keeping the skin either very dry or very wet. Drying the skin can be done with powder, benzoin, alcohol wipes, and antiperspirants. Lubricants can include SportSlick, BodyGlide, BlisterShield, and other popular products. Zinc oxide is also effective at controlling moisture. The method of having runners train with wet feet has been successfully used by Shirley Thompson and Vicky Kypta of the Jungle Marathon Amazon. They have found that the feet of their race participants have been better with this suggestion given to runners before the race.

The fourth method of controlling shear is with socks. This may include double layer socks or wearing two pairs of socks – a thin liner and usually, a thicker second sock. This allows movement between the two sock layers. Injinji toe socks are great for those with toe blister problems.

Next time, we’ll talk about  a fifth way to reduce shear – Engo Blister Prevention Patches.

In the mean time, check out ZombieRunner for many products that can help with cushioning, skin-drying and lubricants, and socks.

An Introduction to Shear and Blister Formation

June 17, 2013 by · 2 Comments
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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