Role of the calcaneal inclination in the energy storage capacity of the human foot--a biomechanical model.

The energy storage capacity of the human foot as a passive structure depends on both its geometry and elastic properties. The present study examines theoretically the relationship between the energy stored in the longitudinal arch and the inclination of the calcaneus, when a vertical load is applied. A simple two-dimensional model was used, consisting of two inclined rigid elements, hinged at the apex of the arch and connected by a horizontal tension spring at the bottom of the arch. The spring serves as the energy-storing element of the model, representing the foot plantar ligaments. The behaviour of the model was examined using both a linear and an exponential spring. It demonstrated that the stored energy depends strongly on the calcaneal inclination. For a given vertical load, the energy storage capacity is low at both large and small inclinations and rises markedly at an intermediate value. Assuming that the incidence of stress fractures during locomotion is affected by insufficient energy attenuation of the foot, this model provides an explanation for the higher prevalence of tibial and femoral stress fractures in subjects with high or low arches.