A callus is a localized patch of skin that has become hardened and thickened, representing a common biological response to protect the underlying tissue. This formation is a defense mechanism developed by the body to guard against repeated friction, pressure, or irritation. While calluses can occur on hands, they are most frequently found on the feet, particularly on the soles, heels, and the ball of the foot, where the most force is applied during standing and walking. Understanding the various factors that trigger this protective reaction is the first step toward preventing their buildup and maintaining foot comfort.
Understanding How Calluses Form
The underlying process responsible for callus formation is called hyperkeratosis, which is an accelerated production of skin cells in response to external stress. When the skin experiences persistent pressure or rubbing, the cells in the deepest layer of the epidermis, the stratum basale, begin to proliferate more rapidly. These cells, known as keratinocytes, start to accumulate, creating a noticeable thickening of the skin’s outer layer.
The newly formed keratinocytes do not differentiate or flatten completely as they move toward the surface, resulting in a dense, hard layer of tissue. This buildup of dead, resistant cells provides a protective shield that helps to distribute the pressure and minimize damage to the sensitive tissues beneath. The result is a broad, diffuse area of thickened skin with indistinct borders, which is the definition of a callus.
It is helpful to distinguish a callus from a corn, which is also a form of hyperkeratosis but presents differently. A callus is generally a large, superficial area of thickening that rarely causes pain unless it becomes excessively large. A corn, by contrast, is a smaller, focused, cone-shaped plug of hardened skin that penetrates deeper into the tissue, often over a bony prominence, and can be painful when pressed. Corns can be hard, usually found on the tops or sides of the toes, or soft, which tend to develop between the toes where moisture is present.
External Causes Related to Footwear and Activity
The most common causes of callus buildup are external factors that increase friction and pressure on the foot’s surface. Ill-fitting footwear is a frequent culprit, as shoes that are either too tight or too loose can create excessive rubbing against the skin. Tight shoes compress the foot, intensifying pressure on specific areas like the toes and metatarsal heads. Loose shoes allow the foot to slide around, generating continuous friction.
High-heeled shoes shift the body’s weight forward, concentrating pressure onto the ball of the foot, leading to rapid callus development in that area. Walking without socks, or wearing socks made of materials that do not properly wick away moisture, can also increase friction. The constant irritation from skin rubbing directly against the shoe lining triggers the protective hyperkeratosis response.
Certain activities and occupations that involve prolonged standing or repetitive high-impact motion also contribute to callus formation. Runners, construction workers, and those who spend long periods standing on hard surfaces subject their feet to continuous force. This sustained pressure, especially when combined with movement, leads to localized skin irritation. While friction causes surface irritation, the constant grinding pressure prompts the deeper thickening response in the skin.
Internal Causes Related to Foot Structure and Gait
In some cases, calluses form not because of external irritants, but due to internal biomechanical issues that create uneven pressure distribution. Foot deformities, such as bunions, hammertoes, or bone spurs, physically alter the foot’s structure. A bunion, for example, causes a bony prominence on the side of the big toe, which then becomes a new, abnormal pressure point against footwear, resulting in localized callus formation. Because these internal causes relate to the underlying structure and movement of the foot, a professional assessment is often necessary to address the root of the problem.
An individual’s unique gait, or walking style, can also be a significant factor in callus development. Abnormal gaits, such as excessive pronation (the foot rolling inward) or supination (the foot rolling outward), cause uneven weight distribution across the sole. This imbalance forces certain parts of the foot to bear a disproportionate amount of weight during each step, leading to predictable patterns of callus formation. Structural issues like flat feet or very high arches distribute weight differently than a neutral foot, creating concentrated pressure points that the body attempts to shield.
High arches may lead to calluses under the heel and the ball of the foot, where the weight is primarily borne. Furthermore, a natural loss of the foot’s protective fatty padding, which occurs with age, can reduce the natural cushioning and cause bones to press closer to the skin, thus increasing the likelihood of callus buildup.