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.

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.

Patching Heel Counter Wear in Shoes

Holes in Heel Counter

Holes in Heel Counter

Occasionally I see runner’s shoes that have wear holes in the material in the heel counter. This picture from Dan shows his shoes with holes to the outside of center on both heels. I emailed the following suggestion.

ENGO Back of the Heel Patch

ENGO Back of the Heel Patch

I’d try an ENGO Back of Heel Patch. They are made to shape to the curve of the back of the heel counter. Put them in when the shoe is dry. Rub them a bit in a curving motion to help them form to the shape of the shoe. Then peel off the paper backing and apply from the center outwards to the sides. Rub well so they adhere to the shoe’s material. You could also try the large ovals if the hole is small. The blue ENGO patches are very slick and can take a lot of wear. Once the blue wears down to expose white, replace the patch. Sometimes the holes are on the sides of the heel.

Heel Patch in Shoe

Heel Patch in Shoe

Heel Bump

Heel Bump

Last summer at Western States, I had a runner whose shoe was rubbing her heel raw. You can see from the image that she has a prominent heel bump. This, of course, contributed to her problem. This is the kind of problem one needs to plan for before it becomes a major issue – especially at the event the magnitude of Western States.

Cut Shoe Heel

Cut Shoe Heel

We decided to cut a notch out of the outside of her shoe’s heel. This allowed the back of the shoe to spread apart for her large heel bump. Then I applied an ENGO Back of the Heel Patch to the inside of the shoe’s heel counter. This provided some protection to her heel bump.

The large ENGO ovals can also be used when there is a small area to be covered.

I have been a fan of ENGO Blister Prevention Patches since I first discovered them years ago. The ovals are perfect for the common areas at the side of the heel where blisters form at the junction of insole and heel counter. The large rectangles are great to put on insoles underneath the ball of the foot, another common problem area.

Check out the ENGO website for more information. ENGO products are available through Zombierunner. If you are in Australia or New Zealand, ENGO products are available through Rebecca Rushton’s Blister Prevention website.

Disclosure: Tamarack Habilitation Technologies supplies me with ENGO Patches as I need them for races.

ENGO Blister Prevention Patches

September 8, 2013 by · Leave a Comment
Filed under: blister care, Foot Care, Foot Care Products, Footwear, Sports 

In my last post, I talked about four ways to reduce shear and the likelihood of blisters. To recap, they were fit, cushioning, moisture management, and socks. Today’s post will cover a fifth way by using ENGO Blister Prevention Patches.

Tamarack Habilitation Technologies is well known for providing healthcare professionals and clients with innovative, value-added orthotic-prosthetic componentry and materials. Their ShearBan product is similar to the ENGO Blister Prevention Patches reviewed in this article. ShearBan is used in the orthopaedic and prosthetic industry on prostheses at amputation stump sites to reduce the incidence of skin breakdown.

ENGO in Footwear

ENGO in Footwear

Introduced in 2004, ENGO Blister Prevention Patches have radically redefined the way hot spots, blisters and calluses are treated. As a preventative measure, ENGO patches provide peace-of-mind that blisters won’t become a painful, debilitating problem. If a blister has already formed, applying patches to footwear, corresponding to the blistered area eliminates painful irritation and further skin damage, allowing continued activity. Friction forces are reduced by more than 50% when you apply an ENGO Patch to your footwear.

ENGO Applied

ENGO Applied

The patches are made from an ultra-thin Polytetrafluoroethylene (PTFE) film and are 0.015 inches total thickness – a very slippery surface. They are very durable, lasting four to six weeks in most applications. The aggressively sticky patch peels away from the backing and is applied to dry shoes or boots. The PTFE ENGO Patch reduces the ‘stickiness’ between the shoe and sock so they can glide over one another. The foot, inside the sock, glides over the patch shear distortion and friction are reduced, and blisters can be averted, in spite of pressure.

Avid runners, hikers and sports players rely on their feet to reach performance goals; from day hikes to ultra marathons. But quality footwear and socks alone don’t eliminate the skin trauma your feet can experience from repetitive rubbing — building friction forces to levels that cause hot spots, blisters and calluses. While I use these patches in runners’ footwear at races, they can also be used in ordinary every day shoes to reduce calluses.

Similar to Tamarack’s ShearBan material, ENGO patches are applied directly to footwear and equipment, not to the skin. Outcomes of this unique application include ease of use, long-lasting and guaranteed friction relief.

ENGO Patch in Shoe

ENGO Patch in Shoe

ENGO Patches are made in several sizes and types:

  • A large oval – 2 ¾ x 1 ¾
  • A small oval – 2 x 1 ½
  • A rectangle – 3 ¾ x 2 ¾
  • Back of the heel patch – 3 ¾ x 1 ¾
  • A cushion heel wrap – 3 ¾ x 1 ½

When I work a race I always have a bag with different sizes of ENGO patches. I have applied the ovals and rectangles and the back of the heel patches. The patches are applied to the shoes and insoles – not to your skin. This means wherever you are going to apply a patch has to be dry. My advice is to apply patches before your race when your shoes are dry. I have used them inside the shoes in the sides, in the heels, and on the insoles.

ENGO in a Shoe's Heel

ENGO in a Shoe’s Heel

Typical problem areas in footwear are under the heel and forefoot, and at the side of the heel. An oval patch can be applied to overlap the side of the heel counter and the insole as seen is the photo. I often use a rectangle or large oval under the ball of the foot or an oval under the heel – applied directly to the insole. The patches are useful over stitching or seams in footwear that are rubbing the wearer. If necessary, a patch can be cut to shape for where it will be applied.

The patches will reduce shear and friction; provide relief from hot spot and blister pain, and can be used in any type of insole or orthotic and footwear, from sandals to running shoes, and any type of hiking or ski boot.

I like ENGO patches because they work. The patch is thin and does not alter the fit of the shoe. When properly applied to dry footwear, they stick.

Rebecca Rushton, a podiatrist in Australia, strongly recommends ENGO Patches. She discovered the patches after getting blisters herself and now represents ENGO in Australia. She has written several free reports on blister prevention available on her website, Blister Prevention.

If you are unclear about shear and blister formation, here’s a link to my article An Introduction to Shear and Blister Formation.

The Technical Stuff

JM Carlson, in a 2009 report wrote, “The measurement of friction is the ‘coefficient of friction’. The coefficient of friction (COF) is a number that represents this slipperiness or stickiness between two surfaces and is generally below 1.0. Within the shoe, the COF between the foot, socks and insole can range from 0.5 – 0.9. In contrast the COF between a sock and a polished floor is around 0.2.” Tests have shown PTFE patches to reduce the coefficient of friction (COF) in the shoe by up to 80%. The COF is in approximately 0.16, which is significantly lower than all other in-shoe materials. Importantly, the low COF is maintained even in most and wet conditions inside the shoe.

Check out GoEngo.com for more information about ENGO Blister Prevention Patches. They also offer a money-back guarantee.

ENGO Patches can be purchased at the ENGO website, Zombierunner.com, and Rebecca’s Blister Prevention website.

Disclaimers: I support ENGO Patches and am supplied with whatever I need for the events I work. I am an affiliate of Zombierunner and make a bit of any sale made through the link above.

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