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.

Making Overlapping Toe Separators – Part 1

January 3, 2016 by · 2 Comments
Filed under: blister care, Foot Care, Foot Care Products, Footwear, Health 

This is part one of a two-part blog post.

Over the past few years, I have seen many athletes with a common toe problem – overlapping toes. Some people may call then underlapping toes or call them some other name. When a pinky toe goes under the 4th toe, both toes can be negatively affected. Skin is pinched. Hot spots and then blisters form. Often callus develops as the skin is constantly under pressure from the overlapping toe.

While most common to the 4th toe and pinky toe, overlapping toes can affect any two toes. This is not necessarily a problem limited to running shoes or hiking footwear. It can happen in everyday footwear too. The cause of over-lapping is unknown. Many experts suspect that they are caused by an imbalance in the small muscles of the foot.

There are some easy solutions, which may or may not help, because toes are different. You can switch to Injinji toe socks, giving each toe it’s own little sock and some degree of protection. You can cut out a portion of the insole under the toe that goes under the other toe, giving the toe some extra space. Another option is to tape around the toe or toes to give some protection too.

This is an idea to help runners, adventure racers, and hikers with the problem of overlapping toes. You will need Injinji toe socks, ENGO Blister Prevention Patches (large ovals), and removable insoles. There are two types of separators you can make. This post will cover the first of the two.

Toe Separator Number 1

I use an ENGO Blister Prevention Patch as the toe separators. They make a small and large oval, but I like the large because of its size.

The first toe separator is easy to make and use – and it uses one large ENGO patch. Take a scissors and cut a long oval into a strip, about ¾ inch wide and 1¾ inches long. If you are cutting this for a middle toe or for large toes, it may have to be 1 to 1 ¼ inches wide and a bit longer. Round all corners. Cut one of the remaining sections into a small strip, ¼ inch wide and 1¼ inch long. Take the large oval and remove half the backing from one end. Wearing Injinji socks, put the large oval between the two affected toes. Put the end of the large oval with the exposed adhesive over the toe next to the toe that goes under it. The blue side will go from the top of one toe, run between the toes, and under the toe that normally goes under the other one. What you have is an S shaped patch from the top of one toe, between them, and then under the next toe. Take the small strip and remove the backing, and put one end of the adhesive on the white backing that is underneath the toe at the bottom of the S. The other end of the strip can be stuck onto the top of that toes sock. The small strip is needed to hold the bottom of the S under the toe when you put your foot in your shoe. The S shaped patch will keep the toes apart. Obviously, these are single use. If the patch seems too weak, use two strips to make the S patch stronger.

Toe Separator #1 - top view

Toe Separator #1 – top view

Toe Separator # 1 - bottom view

Toe Separator # 1 – bottom view

Injinji socks and Engo patches can be purchased at Zombierunner.com. The patches can also be purchased at the Engo website.

Typical Heel Blister Problems

January 12, 2015 by · 4 Comments
Filed under: blister care, Foot Care, Footwear, Footwear Products 

Heel blisters are quite common – although they shouldn’t be.

Feet in the Jungle Marathon

Heels in the Jungle Marathon

Today’s post shows one participant’s feet at the 2014 Amazon Jungle Marathon.

If you look closely at this picture, you’ll see two heel blisters, both on the outside of the runner’s feet. The right foot blister is large but is not blood-filled. The blister on the left heel, however, is very large and filled with a large amount of blood.

It’s easy to think these are normal blisters – but their size makes they abnormal.

In my experience, heel blisters are caused by the constant shear when either 1) the heel is moving up and down inside the shoes’ heel, or 2) by the constant movement at the place where the shoe’s insole touches the inside of the shoe. Over the years, the majority of heel blisters have been the latter. One of the characteristics of this “insole/shoe junction” blister is that they often are flat across the bottom. The blister starts at the point where the insole’s edge at the side of the heel touches the inside of the shoe. That’s what makes the flat line at the bottom. Then the blister forms upward as the fluid forms and it grows. Given enough time and movement, you’ll get blood inside.

Patching

These are relatively simple to patch. The skin must be cleaned with alcohol wipes, and then the blister can be lanced and drained. Depending on the size of the blister, you’ll need to apply some type of blister patch. The bottom line is that you need to have something over the blister to protect the skin and prevent the top layer of skin from tearing off. For these, I would use strips of kinesiology tape (my preference is either StrengthTape or RockTape H2O) with antibiotic ointment over the blister to keep the tape from sticking to the skin. The larger the blister, the harder these are to patch but it can be done.

Prevention

You are better off to prevent these blisters in the first place.

Start with the fit. Make sure your shoes hold your heels in place with just a little movement.

Check your shoes and insoles for rough and/or thick edges at the inside and outside of each heel. Side blisters are much more common than the back of the heel. If the insole has a large thick edge, replace them. If the shoe’s fabric is worn into a hole, you are due for new shoes. Under the fabric is generally a plastic edge of the shoe’s heel counter – the plastic that curves around the heel from side to side.

Engo Blister Prevention Patches are perfect for to help prevent these types of blisters. These patches are super slick. Either the small or large oval can be applied to the inside of the shoe and cover the offending edge of the insole/shoe junction. Clean the inside of the shoe and insole first. I work the patch with my fingers to form a curve to fit with area I need to cover. Then remove the backing and apply the center of the patch first and then push the top and bottom of the patch into place. Rub it a bit to assure adherence.

Reduce Friction to Reduce Shear

December 21, 2014 by · Leave a Comment
Filed under: blister care, Foot Care, Foot Care Products, Sports 

I have often mentioned the website BlisterPrevention.com.au as a great source of information on blister care. Rebecca Rushton, manages the website and is on top of developments in the prevention and care of blisters. She’s a podiatrist in Australia – and a friend.

Through research and looking at the mechanism of how blisters form, Rebecca has changed some old theories of what causes blisters. Heat, moisture, and friction were always considered the three contributors of blisters. Further consideration has found that shear is a major factor. Shear and friction combine to cause blister formation.

You must reduce friction to  blister-causing shear.

You must reduce friction to blister-causing shear.

I want to quote a blog post by Rebecca about Healing Foot Blisters Faster to help you understand more about friction and shear.

“You know friction is responsible for friction blisters. But I bet you think friction is rubbing. It isn’t. Friction is about grip. High friction means two surfaces grip together. Low friction means they don’t … they’re slippery.

“Here’s how friction is responsible for foot blisters … There is high friction in your shoe. There just is. This means your skin grips your sock; and your sock grips your shoe. All three surfaces grip together so your foot doesn’t slide around in your shoe.

But with every step you take, your bones are moving around under the skin. And while the skin is stuck and 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. And it needs high friction to get anywhere near blister-causing.”

With this opening, Rebecca starts to explain the effect of shear and friction on blister formation. She talks about cutting friction levels, especially when a blister develops, and gives examples of six friction reducers. Some of these are better than others.

We have always tried to reduce friction in both preventing blisters and when treating blisters. As Rebecca says, and I support, “Otherwise all that stretching (shear) continues at the blister base while it’s trying to heal. Making it hurt more. And taking longer heal.”

So take a moment and click on the Blister Prevention link and read Rebecca’s full blog post. While you are there, I encourage you to subscribe to her email list.

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