Static vs Dynamic Friction
shear peak

Reducing the shear peak

When it comes to blisters, the frictional forces in play are technically described by “dry friction”. This just means where two objects are in contact with one another. Dry friction is broken down into “static friction” and “dynamic friction”.

This concept is great for describing how shear reaches its maximum (peak shear). And how that maximum might be blister-causing. It even shows what you need to do to successfully prevent blisters.

When peak shear causes blisters

Blisters will form when shear is excessive and repetitive. Presumably, there is a threshold above which shear becomes blister-causing. That threshold is going to be different for each individual. We have no measure of it, though we do know there is such a  thing as being blister prone.

A state of static or kinetic friction?

Imagine a wobbly box sitting on a table (let's say it's made out of jelly). If you push the wobbly box to try to slide it across the table, a small amount of force will result in no motion across the table. That's because the force of friction between the bottom of the jelly box and the table is higher than the pushing force. This is a state of static friction. Push harder and the jelly box begins to slide across the table (yes, this is a very thick and sturdy jelly). This is a state of dynamic friction. These two states describe what is happening between the box and the table.

Now before the wobbly jelly box slides, the push causes it to distort. This distortion is shear and shear is what causes blisters. Shear increases more and more until the bottom of the box 'breaks free' and slides. What happens next has special relevance to blister prevention. You see as soon as the box "breaks free" and slides, research shows shear immediately gets less - see below.

Static friction vs Dynamic friction

Look at the static and dynamic regions of this graph. Now look where shear is highest. It’s at the end of the static region, just before there is sliding. The important point here is this: A state of static friction leads to the shear peak (Naylor, 1955). If static friction is prolonged, shear is more likely to peak above the blister threshold. Conversely, a state of dynamic friction reduces shear. The earlier the slide, the more likely shear will peak below the blister threshold. INTERESTING!

A prolonged state of static friction leads to a higher shear peak

A prolonged state of static friction leads to a higher shear peak

Blister prevention and shear peaks

Blister prevention is all about reducing the amplitude of shear peaks so that they fall below the blister threshold. One way to do this is by reducing friction levels. That is, to reduce the coefficient of friction (COF), making it more slippery. Going back to the wobbly box analogy, this means making it more slippery between the table and the box. When a push is applied, the box slides sooner and the shear peak is lower (see below).

Shear peaks below the blister threshold due to an ealier onset of dynamic friction.

Shear peaks below the blister threshold due to an ealier onset of dynamic friction.

The earlier the slide, the lower the shear peak. This is exactly the effect when you use lubricants, ENGO Patches, moisture-wicking socks and other friction-reducing blister prevention strategies.

By reducing friction you are encouraging an early slide

Let’s be clear in what we are hoping to achieve by reducing friction for blister prevention. By reducing the level of friction you are encouraging earlier sliding movement. That’s what you are hoping to achieve. This might be different to what you’ve previously imagined. But by reducing friction, you are encouraging an earlier slide between two surfaces.

Maintaining a state of static friction (keeping the foot still in the shoe) is not the overarching aim of blister prevention. And it is certainly not what you are trying to achieve by reducing friction. In fact the opposite is true. Shear injury is avoided by allowing an early slide. However, the nature of the slide is critical [the degree of the slide, location and area of the slide and the likelihood of added complications as a result of the slide].  This is what sets apart an average blister prevention strategy from and excellent one.

PS:  The concept of static and dynamic friction has been used to describe blister causation since Naylor's early blister research in 1955. It's nothing new!

Podiatrist, blister prone ex-hockey player, foot blister thought-leader, author and educator. Can’t cook. Loves test cricket.