Anatomy and biomechanics of the hindfoot.

Hindfoot function involves eccentric loading of the subtalar joint and repetitive strain of the calcaneal soft tissues. Both are induced by the serial pattern of foot support. The subtalar joint experiences rapid eversion following heel strike and subsequent inversion during terminal stance. Although these actions reduce the rotatory strain on the ankle joint, they also challenge the local soft tissues and controlling muscles. Compression and traction of the soft tissues about the heel are normal events during each walking cycle. Loading the limb at the onset of stance causes heel pad compression. Conversely, the plantar fascia and tendo Achillis are subjected to significant traction as body weight is transferred onto the forefoot during the latter half of the single limb support period. The intensity of hindfoot stress increases with the vigor of activity. Running creates symptoms that do not arise with ordinary walking. Microtrauma is cumulative. Also, the aging process reduces the ability of tissue to accommodate repetitive force.