A blister is a fluid-filled sac that forms on the skin, typically in response to damage. This bubble protects the underlying layers from further injury. The fluid inside cushions the damaged area, providing an environment for new skin growth. Understanding why this separation occurs specifically on the heel involves looking beyond simple rubbing to the internal mechanics of the foot within the shoe.
The Biomechanics of Blister Formation
The primary cause of a heel blister is not surface-level friction but repetitive internal stretching, known as shear stress. This stress occurs because the foot’s bones move during walking, while high friction between the skin and the shoe causes the outer skin layers to remain stationary. The resulting mechanical fatigue causes the layers of the skin to tear apart beneath the surface.
Separation generally happens within the stratum spinosum, a layer of the epidermis. As the structural integrity of this layer fails, fluid from the surrounding tissue leaks into the newly created void, forming the blister. Heat and moisture contribute to this process by increasing the coefficient of friction between the skin and the sock or shoe lining. Greater friction means the outer skin layer is held more firmly, increasing the magnitude of internal shear stress.
Shoe Fit and Material Factors
The shoe is a major factor because it dictates the forces acting on the heel. Improper sizing, whether too loose or too tight, causes the heel to either slide or be compressed, both of which increase shear stress. A shoe that is too large allows the heel to piston up and down, while a shoe that is too small compresses the heel tightly, generating excessive strain on the skin.
The stiffness of shoe components plays a role, particularly the heel counter—the rigid structure at the back that supports the heel. If this counter or the surrounding collar is made of inflexible material, it creates a stiff edge that concentrates pressure on the skin. Internal stitching or rough seams within the heel area can also act as high-pressure points, initiating shear deformation. New shoes are notorious blister generators because they lack the pliability of worn footwear, requiring a gradual break-in period to soften the materials and conform to the unique contours of the foot.
Immediate Prevention Tactics
Addressing moisture and friction reduces the risk of blister formation. Choosing the correct sock material is the first step, as cotton absorbs moisture and holds it against the skin, raising the friction coefficient. Instead, synthetic fibers like polyester or acrylic, or natural fibers like wool, wick moisture away from the skin.
Seamless or double-layer socks reduce pressure points and allow movement between the two sock layers rather than against the skin. For areas already prone to blisters, applying a protective barrier before activity is effective. These barriers include specialized tapes, moleskin, or hydrocolloid patches that act as a second skin to absorb shear forces.
Lubricants, such as anti-friction balms or sticks, are another tactic. These products create a slick surface, lowering the friction between the skin and the sock. However, some lubricants, like petroleum jelly, may initially reduce friction but can increase it after an extended period by hydrating and softening the skin. These measures aim to either reduce friction or distribute the shear load over a wider area.
Caring for an Existing Blister
When a blister forms, the best course of action for a small, non-painful bubble is to leave it intact. The blister roof is the body’s sterile, natural dressing, protecting the underlying new skin and minimizing the risk of infection. Simply cover the area with a soft, protective bandage to prevent further friction.
If the blister is large, tense, or causing pain, safe drainage may be necessary to relieve pressure. Sterilization is required: wash your hands and the area, then sterilize a clean needle with rubbing alcohol. Puncture the edge of the blister in two or three spots, allowing the fluid to drain naturally while leaving the skin roof in place. Once drained, apply an antiseptic ointment and cover the area with a non-stick dressing or hydrocolloid bandage. Watch for signs of infection, such as increased redness, warmth, pus, or streaks extending from the blister, and consult a healthcare professional if these symptoms appear.