Friction blisters on the feet are among the most common injuries sustained in running, walking and many sports. Unfortunately, there are several misconceptions about what causes friction blisters. The most significant of these is that they are caused by objects rubbing on the skin, presumably by the sock or shoe. To understand the pathomechanics of friction blisters on the feet, we need to look at how the foot and shoe move while on the ground, we need to appreciate all the material layers involved, and we have to look at forces.
Forces involved in the friction blister injury on the feet
There are vertical forces and horizonal forces. Vertical forces from impact and gravity cause compression. Horizontal forces from momentum, from muscles pulling on bones, and from friction, create shear forces which make materials bend and stretch. Vertical and horizontal forces exert their effects between pairs of materials at their interface, and within materials that deform.
For example, friction between your shoe and the ground prevents your shoe from slipping out from underneath you. Consider a runner on the road or an athletic track. The materials of the sole of their shoe and the running surface itself, plus the compression underfoot, all help provide traction so every running stride is maximally efficient. There are similar forces that resist sliding of your foot in the shoe, between your skin, sock and the shoe lining material. In a way, this is by design. Footwear manufacturers construct outsoles that promote traction rather than slipping. Sock manufacturers use materials that are much less grippy so you can pull the sock on to your foot easily, and slip your foot into your shoe easily, yet still help provide some level of traction for your foot in the shoe. Plus, the moisture from perspiration further increases traction. The opposite of these traction scenarios would be running on a slippery floor, or running with Vaseline on your feet – there is a lack of traction, which will be particularly noticeable at heel strike and push off. These are all examples of how two materials interact with one another at their interface to either slide or resist sliding.
Vertical and horizontal forces also act within materials. These materials bend, stretch and deform, which is described as shear deformation. At heel strike, your body’s forward momentum is countered by friction which keeps your shoe held stationary to the ground. The result is not only compression of the midsole but also shear deformation within it. The same thing happens within the soft tissues of your foot at heel strike – there is forward momentum of your calcaneus, but this momentum is resisted at every interface external to the skin surface by the force of friction, giving your foot traction within the shoe. The result is shear deformation – stretching and deforming of the soft tissues sandwiched between skin surface and bone.
The role of friction in friction blisters on the feet
It is worth noting at this point something very important about friction in the pathomechanics of friction blisters. Friction force is the mechanical property which prevents sliding. Friction force provides traction for efficient gait. Friction is not rubbing. Friction force actually prevents rubbing or sliding of the shoe across the ground, of your sock against the lining material of the shoe, of the sock against your skin, and at any other material interface that might exist in your shoe. Interfaces slide when there is low friction force, and stick together when there is high friction force.
Friction force is determined by two things: the compressive force pressing them together; and the surface properties of each material – each pair of materials has its own tendency to either grip together, or slide over one another. Pressure increases friction force and prevents sliding across material interfaces. There is generally high pressure under the foot, resulting in high friction force. Material selection in the manufacture of footwear and socks, plus perspiration, tend to prevent sliding. This all contributes to high friction force at every material interfaces external to the skin. All in all, high friction force dictates a tendency for stationary contact between all the interfaces from skin surface to ground surface.
So, high friction force produces better traction. Traction helps to resist forward slippage during heel strike, and resist backward slippage during push off. Friction provides the traction which is needed to resist momentum of the bones pushing against the skin at heel strike, and muscle action pulling on tendons which move the bones against the skin during heel rise and push off.
The pathomechanics of friction blisters
Moving bones in the presence of high friction force dictate a significant shear event within the soft tissues of the foot with each step, which is all part of normal function. The foot is designed to endure this shear deformation. However, as we will soon see, there are limits to what the foot can tolerate before skin injury occurs.
Now let’s focus on how the soft tissue components of the foot compress and deform during a single stride, from heel strike to toe off. We’re going to assume there is good traction of the foot in the shoe, so there is little to no sliding between the skin and the sock, or the sock and the interior shoe lining, or of the shoe on the ground. We recognise that shear deformation occurs within the components of the shoe itself. However, we are going to focus solely on the shear deformation that occurs within the soft tissues of the foot, with a focus on the plantar surface and the posterior heel. Of course, there is shear deformation that may become blister-causing at other foot surfaces, dependent on individual structural and functional characteristics. This includes the dorsum of the toes, the digital apices, between the toes, and the medioplantar and lateral plantar aspect of the rearfoot, midfoot and forefoot.
Heel strike
At heel strike, the foot approaches the ground at an angle which is not purely vertical or horizontal, much like an aeroplane landing on a runway. The vertical direction of touchdown causes compression of all shoe and foot “materials” under the heel bone, and to a lesser extent at the back of the heel. The horizontal direction of touchdown creates shear forces in these compressed soft tissues. Essentially, the calcaneus moves down and forward, but the skin surface doesn’t follow, because friction is holding these in-shoe interfaces stationary. The soft tissues sandwiched between skin and bone bend and stretch as a result – they undergo shear deformation.
Foot Flat
At foot flat, pressure is exerted to the metatarsal heads as it reduces under the calcaneus. The primary force acting on the foot comes from gravity, and forward momentum continues. Shear deformation reduces at the heel and increases under the forefoot.
Midstance
During midstance, as the tibia moves forward over the foot, tensile force builds in the Achilles tendon, pressure increases under the forefoot and the arch flattens a little. Essentially, the metatarsal heads push down and move forward. Friction force is increasing under the ball of the foot, holding the skin surface and sock stationary within the shoe as the metatarsal heads move forward relative to the skin. This creates increasing shear deformation in the soft tissues under the forefoot.
Heel rise
At heel rise, pressure is removed from the plantar calcaneus as elastic recoil of the Achilles and plantar ligaments lift the calcaneus and increase pressure and friction force under the forefoot and toes. Contraction of the flexor muscles of the leg pull the phalanges and the metatarsals down and backwards to generate forward propulsion. This heralds a reversal in the direction of shear deformation within the soft tissue under the forefoot and toes. There is also a less significant shear event at the posterior heel, in the opposite direction to that experienced at heel strike, as now the calcaneus is being pulled upwards.
Towards toe off
With increasing heel lift during the final moments of stance, the metatarsophalangeal joints are further dorsiflexed on the fixed phalanges as the metatarsals are being pulled further downwards and backwards. The centre of mass moves ahead of the foot while the bones of the forefoot are pushing backwards relative to the skin surface, creating a significant shear event in these tissues. Friction force is high under the forefoot and toes to enable efficient toe off.
As the foot leaves the ground and enters into the swing phase of gait, plantar shear is all but eliminated, until the next touchdown.
The skin-to-bone soft tissue sandwich
Let’s zoom in now and look at the layers of soft tissue which are sandwiched between skin and bone. The concept shown in this section is vital to understanding the pathomechanics of friction blisters. The bone is at the bottom of screen now with the skin above, so you could imagine this image as viewing the dorsal surface of a clawed toe. Remember, it doesn’t matter what part of the foot this is depicting, it is the bone structure that initiates movement. You could envision turning this image upside down and it would then depict the relationship of a metatarsal head pressing down on the ground. Or turn it to the side to depict the posterior calcaneus.
Intraepidermal fatigue
Now let’s zoom in further and look at the very outer layer of skin where blisters occur, the epidermis.
The base of the epidermis is being pulled back and forth as movement is transmitted from the moving bone which is now off screen. This movement is able to be transmitted from deep to superficial because every layer of soft tissue is structurally connected to the layer above and below. You’ll also notice that the shoe, sock and very surface of the skin is not moving. So, the bone is moving in one direction and the skin-sock-shoe interfaces are being held stationary. This “out of sync” movement creates shear deformation within the layers of the epidermis.
The zone of least resistance to repetitive shear deformation is the lower section of the stratum spinosum. This is where the blister injury occurs.
Repeated shear episodes lead to breakdown of the intercellular connections and desmosomes which hold the cells of the stratum spinosum together, and the cells themselves are damaged. The remaining layers of the epidermis remain intact during the formation of the friction blister, forming the blister roof.
As the desmosomes fail and skin cells die, small pockets or voids are created which fill with fluid. You won’t see the typical blister bubble on the surface of the skin immediately, but the damage is done and a blister it on its way, even if you cease the blister-causing activity.
As more and more pockets of fluid coalesce, the volume of fluid at the injured site causes the outer skin to bubble up. It can take up to 2 hours for a blister to become fully filled with fluid after the initial tear has occurred.
Pathomechanics of friction blisters
The breakdown of the cells of the stratum spinosum is the result of bone movement, not objects rubbing on the skin surface! It is the lack of synchronous movement of all the layers above the bone which causes structural failure within the stratum spinosum. The risk of blister injury is dependent upon both the magnitude and frequency of this shear deformation experienced by the soft tissue. With high friction and no sliding at an interface external to the skin, a larger magnitude of shear deformation occurs in the soft tissue, increasing the likelihood of mechanical fatigue within the stratum spinosum.
Take home message
The take home message is this – the motion which creates the friction blister occurs from INSIDE THE FOOT, NOT ON THE OUTSIDE! Nothing need rub against the skin surface. On the contrary, when the skin is held stationary, the magnitude of shear deformation is larger, bringing with it a greater potential for structural failure within the epidermis.
If the top layer of the epidermis is allowed to slide freely or “in synch” with the underlying bone, shear distortion will be minimized within the soft tissues sandwiched between skin and bone.
Reduce friction force at one of these interfaces, and the skin surface will be able to slip and move in synch with the bone at an earlier moment, reducing the magnitude of each shear deformation. This is how many blister prevention strategies work – they encourage a little bit of slippage at an interface.
This will be the topic of an upcoming video – how blister preventions actually work.
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Thanks for watching.
References
Rebecca Rushton, Douglas Richie; Friction Blisters of the Feet: A New Paradigm to Explain Causation. J Athl Train 8 January 2024; 59 (1): 1–7. doi: https://doi.org/10.4085/1062-6050-0309.22
Video link: https://youtu.be/LqLspBBj9as?si=XKCShx8mawzyQO1r
