Longitudinal arch load-sharing system of the foot

The longitudinal arch of the human foot is a complex mechanical structure that must be compliant on uneven surfaces and also have sufficient stiffness to allow the foot to be an efficient propulsive organ during walking and running gait. To serve these functions, the longitudinal arch has a unique four-layer load-sharing system consisting of the plantar fascia, plantar intrinsic muscles, plantar arch extrinsic muscles and plantar ligaments. These four layers of tension load-bearing elements, working together with the osseous elements which serve as the framework of the longitudinal arch, work synergistically to increase longitudinal arch stiffness during weightbearing activities. The passive tension load-bearing elements of this load-sharing system, the plantar fascia and plantar ligaments, are not under direct central nervous system control and thus serve to stiffen the longitudinal arch with an automatic stiffening mechanism that is based on Achilles tendon tension and plantar forefoot loading. The active tension load-bearing elements, the plantar intrinsic and plantar extrinsic muscles, are under direct central nervous system control and serve to increase or decrease the stiffness of the medial and lateral longitudinal arches depending on the type and intensity of the prevailing weightbearing activity of the individual. Together, the elements of the longitudinal arch load-sharing system ensure that proper weightbearing function of the longitudinal arch, and the foot and lower extremity, can still occur even when a failure of one of these tension load-bearing elements occurs due to injury.