A calcaneal stress fracture develops when repetitive mechanical loading exceeds the heel bone's ability to repair itself. Unlike an acute fracture, which occurs after a single traumatic event, a stress fracture develops gradually over days or weeks as microscopic areas of bone damage accumulate faster than new bone can be formed. It is essentially an overuse injury of the skeleton.
Healthy bone is constantly undergoing a process known as bone remodelling. Tiny areas of microscopic damage occur naturally with everyday activities such as walking and running. Normally, specialised bone cells remove the damaged tissue while new bone is laid down to replace it. This process allows the skeleton to adapt to increasing physical demands. However, when loading increases too rapidly or recovery is inadequate, the balance shifts towards bone breakdown, eventually leading to a stress reaction and, if untreated, a stress fracture.
One of the most common causes is a sudden increase in physical activity. Increasing running distance too quickly, beginning military training, starting a new exercise programme or preparing for a sporting event without allowing adequate time for adaptation all place the calcaneus under repetitive stress. Even people who begin walking significantly more than usual may overload the heel bone if progression is too rapid.
Training errors are frequently involved. Running on consecutive days without adequate recovery, rapidly increasing training intensity, changing to hill running or switching to harder running surfaces all increase impact forces through the calcaneus. The bone requires time to adapt to these new demands, and insufficient recovery increases the likelihood of stress injury.
Foot biomechanics also influence how force is distributed through the heel. Excessive pronation, cavus feet, altered gait mechanics, reduced ankle mobility or leg length differences may concentrate stress within particular regions of the calcaneus. While these factors alone do not necessarily cause a stress fracture, they may increase susceptibility when combined with repetitive loading.
Footwear can also contribute. Worn running shoes gradually lose their shock-absorbing properties, increasing the forces transmitted to the heel with every step. Changing to minimalist footwear without an appropriate transition period may similarly expose the calcaneus to higher mechanical loads before the bone has adapted.
Bone health is another critical factor. Individuals with osteoporosis, osteopenia or vitamin D deficiency have reduced bone strength and are more susceptible to stress injuries. Women with menstrual irregularities related to low energy availability, athletes with inadequate nutritional intake and older adults with reduced bone density are particularly at risk. Long-term corticosteroid use and certain medical conditions affecting bone metabolism may also increase fracture risk.
Body weight influences the magnitude of force transmitted through the heel. Every additional kilogram increases the load passing through the calcaneus during walking and considerably more during running. However, it is important to remember that calcaneal stress fractures occur frequently in lean endurance athletes as well. The condition is ultimately caused by the relationship between bone strength and repetitive loading rather than body weight alone.
Calcaneal stress fractures may also develop in patients with longstanding plantar fasciosis or calcaneal bone marrow oedema. Chronic traction at the plantar fascia insertion increases stress within the heel bone, and untreated bone marrow oedema represents an earlier stage along the same continuum of bone stress injury. Recognising these related conditions allows intervention before a true fracture develops.
Age also influences risk. Younger athletes may develop stress fractures during periods of intense training, whereas older adults are more likely to sustain them because of age-related reductions in bone density and slower bone remodelling. Regardless of age, the common feature is that the mechanical demands placed on the calcaneus exceed its capacity to repair itself.
Understanding why the stress fracture occurred is just as important as diagnosing the fracture itself. Unless the underlying cause is identified and corrected, patients remain at risk of delayed healing, recurrence or developing stress fractures elsewhere in the skeleton. Successful treatment therefore involves both allowing the fracture to heal and addressing the mechanical, nutritional or medical factors that contributed to its development.