Pathomechanics of Friction Blisters on the Feet

Following the publication of our paper in the Journal of Athletic Training at the start of the year, Doug and I have been working on a video explaining the pathomechanics of friction blisters on the feet.

There are several misconceptions about the causes of friction blisters, the most significant being that they are caused by objects rubbing on the skin, such as socks or shoes. To comprehend how friction blisters occur, we need to explore the movement of the foot and shoe while on the ground, the material layers involved, and the forces at play.

Forces Involved in Friction Blisters

There are vertical forces from impact and gravity, which cause compression, and horizontal forces from momentum, muscle action, and friction, which create shear forces. These forces act 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, providing traction for efficient running. Similarly, the friction between your skin, sock, and shoe lining material helps resist sliding of your foot within the shoe.

Friction force, which prevents sliding, is determined by the compressive force pressing materials together and the surface properties of each material. High pressure under the foot results in high friction force, preventing sliding at various interfaces from the skin surface to the ground. This high friction force is essential for traction, helping resist slippage during heel strike and push-off, and aiding in efficient gait.

Shear Deformation and Its Impact

Shear deformation occurs within materials as they bend, stretch, and deform under compression and with a movement force applied. At heel strike, the forward momentum of the calcaneus (heel bone) is countered by friction, causing shear deformation within the midsole of the shoe and the soft tissues of the foot. This shear deformation, the stretching and deforming of the soft tissues sandwiched between skin and bone, is crucial for normal function but has limits beyond which skin injury occurs.

Foot Biomechanics During the Stance Phase of Gait

From heel strike to toe-off, the soft tissues of the foot experience shear forces.

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.

Mid-Stance

During mid-stance, 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.

Approaching 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.

Epidermal Shear Deformation and Blister Formation

Zooming in on the layers of soft tissue, particularly the epidermis, we see that shear deformation occurs within the lower section of the stratum spinosum. This is where friction blisters form. Repeated shear episodes break down the intercellular connections and desmosomes, creating small pockets that fill with fluid, eventually forming the blister bubble. The damage is caused by bone movement, not by objects rubbing on the skin surface. The risk of blister injury depends on the magnitude and frequency of shear deformation.

Intraepidermal Fatigue

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.

Coming Up...

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 understanding of friction blister prevention will be further explored in an upcoming video.

?? Watch the video here: https://youtu.be/LqLspBBj9as?si=XKCShx8mawzyQO1r

?? Read the paper here https://doi.org/10.4085/1062-6050-0309.22

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.