This same mechanism is hypothesized to close-pack the great ape MTPJs in plantarflexion for increased stability during pedal grasping. In the close-packed position the appositional surfaces of the MT head and proximal phalangeal base are maximized and the tightening of collateral ligaments limits joint motion outside of the sagittal plane, increasing joint stability when maximum joint congruency is achieved 23, 28. Dorsal mediolateral expansion of the MT heads also allows for close-packing of the MTPJs by tightening the collateral MTP ligaments when the joints are in dorsiflexed postures 27, 28, 29. The mediolaterally expanded dorsal MT head in humans has been argued to enhance joint stability under increased loads during push off from highly dorsiflexed MTPJs 8, 23, 25, 26, 27. Additionally, the more plantarly oriented MT head in great apes is hypothesized to increase plantarflexion ROM for arboreal pedal grasping 8, 18, 23, 24 at the expense of dorsiflexion ROM during push off on terrestrial substrates 18, 19, 24.

The plantar aponeurosis is only weakly developed in great apes and consequently these species are believed to lack a windlass mechanism 1, 11. Tightening of this structure acts as a “windlass mechanism” 20, 21 that raises the longitudinal arch, providing midfoot stability and facilitating propulsive force production at push off 22. In humans, dorsiflexion at the MTPJs causes tightening of the plantar aponeurosis, which is a thick band of ligamentous tissue that originates from the calcaneal tuberosity and inserts distally on the proximal phalangeal bases. The dorsally oriented MT heads in the human forefoot are thought to increase the dorsiflexion range of motion (ROM) at the MTPJs 18, 19 by providing more dorsal articular surface area on which the proximal phalangeal base can slide. The interfaces of the metatarsal heads and proximal phalangeal bases, the ellipsoidal metatarsophalangeal joints (MTPJs), allow plantarflexion for pedal grasping and dorsiflexion during push off. The phalanges of modern humans are short and straight and the proximal phalangeal bases demonstrate dorsal canting 2, 14, 15, 16, 17. In contrast, the human forefoot is characterized by a permanently adducted hallux that is incapable of opposing the other digits and more dorsally oriented and mediolaterally broad metatarsal head articular surfaces. Additionally, the phalanges are long and curved, enabling them to resist the bending strains incurred during pedal grasping 13. The extant great ape forefoot is characterized by an abducent opposable hallux and metatarsal (MT) heads that are plantarly oriented in sagittal profile and dorsally narrow 1, 9, 10, 11, 12. Among the features affected by these changes, those of the forefoot region have received much attention in comparative morphology studies due to their role in the push-off phase of bipedal walking 1, 2, 3, 4, 5, 6, 7, 8.

Analyses of Australopithecus afarensis metatarsals reveal morphology intermediate between humans and chimpanzees, suggesting that this species used different bipedal push-off kinematics than modern humans, perhaps resulting in a less efficient form of bipedalism.ĭuring the evolution of bipedalism the hominin foot underwent a number of dramatic changes that converted it from a prehensile grasping organ to a strong propulsive lever. This kinematic pattern is mirrored in the morphometric analyses of metatarsal head shape.

Among MTPJs 2–5, both species exhibit decreasing peak angles from medial to lateral. Using three-dimensional kinematic and morphometric analyses, we show that humans push off with greater peak dorsiflexion angles at all MTPJs than do chimpanzees during bipedal and quadrupedal walking, with the greatest disparity occurring at MTPJ 1. The morphological affinity of the metatarsal heads has been used to reconstruct locomotor behavior in fossil hominins, but few studies have provided detailed empirical data to validate the assumed link between morphology and function at the MTPJs. In contrast, chimpanzees are believed to possess MTPJ morphology that precludes high dorsiflexion excursions during terrestrial locomotion. Particular features of metatarsal head morphology such as “dorsal doming” are thought to facilitate this stiffening mechanism. During bipedal walking, modern humans dorsiflex their forefoot at the metatarsophalangeal joints (MTPJs) prior to push off, which tightens the plantar soft tissues to convert the foot into a stiff propulsive lever.

Chimpanzee feet