Calcaneocuboid joint and stability of the longitudinal arch of the foot at high and low gear push off.

The calcaneocuboid joint was studied in ligamentous specimens of ten human feet, and in skeletons of two gorillas (Gorilla gorilla beringei), six chimpanzees (Pan troglodytes), three orangutans (Pongo pygmaeus) and 25 human feet. The movement of the transverse tarsal joint was further studied in a living foot on a walk-way with a glass plate inserted, and with an underlying mirror. In man the joint is shaped as an asymmetrical sector of one end of an hour-glass shaped surface of revolution with its main axis oriented longitudinally in the foot. The calcaneocuboid joint becomes close packed by a pronation of the forefoot in relation to the hind foot because of a congruency between the joint surfaces obtained in this position and because the calcaneus overhangs the cuboid dorsally and stops the movement. At low gear push off the foot is inverted and the calcaneocuboid joint loose packed. The stresses are absorbed across the fibular, postaxial border of the foot. At high high gear push off there is a functional pronation of the forefoot with a stabilization of the transverse tarsal joint and a more effective tightening of the plantar aponeurosis. The foot becomes a rigid lever for propulsion. In contrast to the human condition, the anthropoid calcaneus has an anteromedial extension associated with symmetry of the calcaneocuboid joint. The calcaneus does not overhang the cuboid and there appears to be no close packed position. Correspondingly, the anthropoid foot has a mid-tarsal break at each push off in addition to the metatarsophalangeal break.