How To Manage Friction Blisters
Friction blisters are fairly common among athletes, hikers and the military. They can lead to pain and infection, and complications such as cellulitis and sepsis if they are not managed in a timely, appropriate manner. Accordingly, this author offers a thorough review of the literature and provides insights on the pathomechanics and treatment of this condition.
The most common foot injury in sport remains poorly understood and treatment of this condition still follows the tradition established over 30 years ago. Yet the incidence and disability from this seemingly benign injury continues at rates higher than any other condition affecting the human foot.
Every year, over 400,000 people participate in a marathon distance running event in the United States. It has been estimated that up to 39 percent of marathon runners experience a blister during the race.1 In military training, friction blisters will affect over 40 percent of soldiers while over 50 percent of active backpackers and hikers will be hampered by this condition.2,3 Friction blisters can lead to significant disability from either pain or infection or both. Complications include cellulitis, sepsis and even toxic shock syndrome.4,5
With such a high incidence and potential for disability, one would think that the prevention of friction blisters would be better understood. Yet few research studies have been published on this subject in the past 20 years. As a result, many myths continue to be propagated regarding the prevention and treatment of friction blisters.
Understanding The Pathomechanics Of Friction Blisters
A blister occurs when a tear or cleavage occurs between the top three layers of the epidermis (stratum corneum, stratum lucidum and stratum granulosum) and the underlying stratum spinosum. The lower layers, including the basal cells and dermis, stay intact. A cleft forms and subsequently fills with fluid from the capillaries due to hydrostatic pressure. Blister fluid is similar to plasma but has a lower protein level.
The tearing of skin at the level of the stratum spinosum is due to repeated shear stresses caused by frictional forces applied to the skin. This frictional force develops when the skin of the foot is in contact with an object such as a sock, an insole, a shoe or the ground itself. Frictional force resists movement of the skin when an external force is acting on the foot to push it in a forward-backward or medial-lateral direction. These forces are known as shear forces. These forces cause the skeletal segments of the foot to move out of synch with the overlying soft tissue and components of the shoe.
Shear forces are applied to the human foot during walking and running because of the mechanics of foot alignment during contact and propulsion. The foot approaches the ground at a tangential angle (not a purely vertical angle) and then pushes off in a similar tangential direction. The foot must skid to a stop and then push into the ground to propel forward. The skidding will occur in both an anterior-posterior and medial-lateral direction, depending on the activity and demands of the sport. Researchers have shown that athletes with a history of friction blisters have greater plantar pressure and shear stress magnitudes than a control group.6
A certain amount of frictional force is necessary on the plantar surface of the foot in order to develop traction and stability for propulsion. The integument of the human foot can withstand a certain amount of frictional force and vertical force for a limited number of repetitions. There is evidence that the foot can slowly adapt to these stresses and withstand a larger number of stress repetitions before blistering occurs. However, there will be a threshold where frictional force combined with vertical pressure applied in high frequency will cause the skin to tear at the upper levels of the epidermis.
One can increase frictional force by applying moisture to the skin surface.7 This is why people are observed to “spit on the hands” before gripping a baseball bat. Increasing levels of moisture on the surface of the feet will increase the likelihood of blistering.