Participation in sports and recreational activities offers numerous benefits, but it also comes with the risk of injuries. One of the most common injuries in running, walking, and various sports is friction blisters on the feet.
[0:00] Misconceptions Around Blister Cause
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.
[0:30] 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.
[2:27] 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.
[5:30] Foot Dynamics During a Stride
Throughout a stride, from heel strike to toe-off, the foot experiences shear forces
[6:06] Heel Strike
The foot lands at an angle, causing compression and shear forces under the heel. The calcaneus moves down and forward while the skin remains stationary due to friction, resulting in shear deformation of the soft tissues.
[6:48] Foot Flat
Pressure shifts to the metatarsal heads, reducing under the heel and heralding the start of shear deformation under the forefoot.
[7:06] Mid-Stance
The tibia moves forward over the foot, increasing pressure under the forefoot and creating shear deformation as the metatarsal heads move forward relative to the skin.
[7:38] Heel Rise
Pressure is removed from the heel, and increased pressure and friction force under the forefoot and toes generate forward propulsion. This reverses the direction of shear deformation under the forefoot.
[8:20] Toe-Off
The metatarsophalangeal joints dorsiflex further, creating significant shear deformation as the bones push backward relative to the skin.
[8:57] 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.
[8:52] Take Home Message
The key takeaway is that friction blisters result from internal shear deformation, not external rubbing. Allowing the top layer of the epidermis to slide freely with the underlying bone reduces shear deformation within the soft tissues. Blister prevention strategies work by reducing friction force at one of the interfaces, enabling the skin surface to move in sync with the bone, thereby minimizing shear deformation.
[12:16] Preventing Friction Blisters
This understanding of friction blisters will be further explored in an upcoming video on blister prevention strategies. Subscribe to the channel to be notified when the new video is posted. Thank you for watching.
👉 Learn more at doi: doi.org/10.4085/1062-6050-030...
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.