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Protector or antagonist: managing blisters in athletes
A blister is a small pocket of fluid that forms between the skin layers. It’s like a cushion that helps protect the skin from further damage. However, they can be painful, increase injury risk, and affect performance. Marianke van der Merwe investigates the pathology and provides recommendations on blister prevention and management.
London Marathon - Netherlands’ Sifan Hassan reacts as she crosses the finish line to win the elite women’s race REUTERS/Andrew Boyers.
Blisters on the feet are among the most common medical issues encountered by individuals who participate in weight-bearing activities, such as long-distance hikers, marathon runners, triathletes, and military personnel(1,2). Perspiration, friction, and shear forces cause the separation of the epidermal layers, causing blisters(2,3). These blisters impair the individual’s concentration and cause gait alterations, leading to musculoskeletal injuries and decreased activity enjoyment(1,2).
Friction blister formation
Mechanical separation of the epidermal cell layers occurs due to the shearing, scraping, and compression forces between the sock and foot(2). The shearing forces result from repetitive tangential forces applied to the skin (see figure 1)(1).
The shear forces lead to cellular distortion, cell disruption, and necrosis in the stratum spinosum layer of the epidermis. Serum-like fluid or blood fills the cellular disruption (see figure 2)(1,2).
Etiology
The four main components that cause friction blisters are high friction force, moving bone, skin characteristics, and repetitive shear events(1,4). These four components are all interlinked. Friction is the most apparent factor affecting the formation of blisters(1). Friction is the tangential force resisting the relative motion of two surfaces against one another(1). The four underlining factors that help explain how friction causes blisters are skin moisture, particulate matter, skin lubrication, and normal force at the skin (see figure 3).
Coefficient of friction
When two objects are in contact, and one tries to slide or move over the other, friction comes into play. The coefficient of friction (COF) measures how much force is needed to move one object over another. The COF is at its lowest when the skin is dry or wet(1). Conversely, the most significant risk for blister formation is when the skin is moist, increasing the COF. Furthermore, it is best to keep skin dry as prolonged exposure to moisture breaks down and softens the skin, known as maceration(1).
Athletes may use lubricants to reduce the COF, specifically at the skin-sock interface(4). Powders are commonly used to reduce friction as they absorb moisture, thus enabling the skin to be drier, and secondly, it works as a dry lubricant(4). However, friction increases when the powder becomes wet due to sweat or rain, and the material can clump, making it abrasive(4).
Lubricants
Dry lubricants are typically powders or films that remain solid at room temperature. They reduce friction between two surfaces by forming a protective barrier or coating on the surface.
On the other hand, wet lubricants are liquids that are applied to the surface of two objects to reduce friction. Wet lubricants can be oils, greases, or even water-based solutions. The lubricant stays liquid, creating a film between two surfaces to reduce friction.
Structural prominences affect the force on the skin. For example, orthopedic foot deformities such as having a high arch or flat foot influence the distribution of forces, resulting in higher peak load at the plantar surface and high friction forces(2). Moreover, external forces, such as carrying heavy backpacks, increase load, resulting in higher friction between the footwear and the foot(2). Combined with high-loading cycles (i.e., long hikes or runs), the foot produces heat and water vapor, which increases friction and results in blisters(2).
The final factor affecting COF is particulate matter. Environment debris such as sand and rubber may cause blisters(4). Larger debris, such as pebbles, may increase the focal pressure, which is more likely to cause superficial-to-deep abrasion injuries(4).
Mechanical stress
Increased mechanical stress and moisture due to increased perspiration make the skin less resistant to shear forces, predisposing it to blister formation. In contrast, the extent of skin shear that leads to blister formation is influenced by friction, skin characteristics, biomechanics, and the shear modulus of footwear.
Skin characteristics
Skin characteristics influence the formation of blisters. Callouses, skin temperature, tissue hydration, skin adaptation, and epidermal health status all affect blister formation risk (see figure 4)(1).
- Repetitive shear forces cause skin adaptations that increase resistance to shear forces. The skin adaptations prevent blister formation by enlarging collagen fibrils, increasing basal membrane cell density and size, and thickening the corneum layer(1).
- Callus formation is a protective mechanism by thickening the stratum corneum layer and increasing the epidermal volume to distribute shear forces. However, a physical callus increases the risk of friction blister formation(4).
- Elevated skin temperature accelerates suction blister formation. Furthermore, inadequately hydrated skin and nicotine negatively affect epidermal health, increasing the risk(1).
Biomechanics
Footwear
Table 1: Different sock types and their characteristics(4)
| Sock Type | Fiber Type | Moisture Absorption | Drying Time |
| Cotton | Hydrophilic | Three times that of synthetic acrylic fibers | Ten-fold greater than synthetic fibers |
| Synthetic (acrylic, polypropylene, polyester) | Hydrophobic | Transport moisture along fiber = superior moisture management | |
| Coolmax (specialized polyester) | Scallop oval cross-sectional geometry | A 20% increase in surface area = facilitates moisture transport |
15% faster than acrylic fibers
|
Management
Prevention strategies
Table 2: Preventative strategies for each risk factor(1)
| Etiologic factor | Preventative strategies | |
| Coefficient of friction | Skin moisture |
Avoid getting the feet wet Moisture-wicking socks Fast-draining socks Waterproof socks/shoes |
| Particulate |
Remain vigilant when exercising in areas with high particulate matter. Change or clean socks regularly if needed for more prolonged activities. |
|
| Skin lubrication |
Quality lubrication. Frequent lubrication application. |
|
| Normal force on the skin |
Soft insoles. Proper fitting orthotics. Show modification to relieve pressure points. Foot arch training. Load carriage reduction. |
|
| Skin Characteristics | Callouses | Callous removal |
| Skin temperature |
Proper shoes and socks that allow heat loss. Vigilance when participating on hot surfaces. |
|
| Skin adaptation |
Proper training. Regular skin monitoring for pressure adaptations. |
|
| Tissue adaptation | Maintain adequate systemic hydration. | |
| Epidermal health | Avoid tobacco use. | |
| Bony movement | Characteristics of activity | Vigilance on steep and canted angles. |
| Biomechanics and form |
Proper gait pattern. Proper training to limit gait changes with fatigue. |
|
| Shoe fit |
Proper fitting orthotics The shoe fit is not too loose or tight. |
|
| Shear modulus of footwear | Insole characteristics | Low shear modulus insoles. |
| Shoe midsole characteristics | Low shear modulus midsoles. |
Conclusion
References
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