Understanding the evolution of the windlass mechanism of the human foot from comparative anatomy: Insights, obstacles, and future directions

Abstract

Humans stand alone from other primates in that we propel our bodies forward on a relatively stiff and arched foot and do so by employing an anatomical arrangement of bones and ligaments in the foot that can operate like a "windlass." This is a significant evolutionary innovation, but it is currently unknown when during hominin evolution this mechanism developed and within what genera or species it originated. The presence of recently discovered fossils along with novel research in the past two decades have improved our understanding of foot mechanics in humans and other apes, making it possible to consider this question more fully. Here we review the main elements thought to be involved in the production of an effective, modern human-like windlass mechanism. These elements are the triceps surae, plantar aponeurosis, medial longitudinal arch, and metatarsophalangeal joints. We discuss what is presently known about the evolution of these features and the challenges associated with identifying each of these specific components and/or their function in living and extinct primates for the purpose of predicting the presence of the windlass mechanism in our ancestors. In some cases we recommend alternative pathways for inferring foot mechanics and for testing the hypothesis that the windlass mechanism evolved to increase the speed and energetic efficiency of bipedal gait in hominins.

The influence of mobility strategy on the modern human talus

“…Hunter‐gatherers display an enlarged talar neck and more rounded, broader navicular facets. It is known that, during toe‐off, transmission of body weight shifts from the talar head to the first and second metatarsals (Griffin et al, ; Jashashvili, Dowdeswell, Lebrun, & Carlson, ). Trinkaus () showed that unshod Native Americans have more robusticity in hallucal proximal phalanges compared to habitually shod Inuit and modern Euroamericans.…”

“…However, different substrates may require different mechanisms (i.e., talar configurations) to achieve effective stability, for example navigating on uneven terrain versus asphalt roads may create different loading patterns. Moreover, the talus facilitates plantar‐ and dorsal flexion, as well as inversion‐eversion of the foot (Griffin, Miller, Schmitt, & D'Août, ; Huson, ). How these ankle motions could be influenced by the use of restrictive shoes is still an open question (D'Août et al, ).…”

Reevaluating the functional implications of Australopithecus afarensis navicular morphology

“…The presence of a longitudinal arch is reflected in the geometric relationships among the bones of the human foot and ankle (DeSilva and Throckmorton, 2010;Ward et al, 2011;Prang, 2015b), including the declination of the talar head relative to the plane of the talocrural joint (Day and Wood, 1968;Peeters et al, 2013) and the declination of the calcaneocuboid joint relative to its proximodistal axis (Aiello and Dean, 1990;Prang, 2015b). Humans also possess soft tissue specializations associated with the longitudinal arch, such as a well-developed plantar aponeurosis, calcaneonavicular ligament, and long plantar ligament (Gomberg, 1981(Gomberg, , 1985, which all contribute to midtarsal stabilization via the 'windlass mechanism' (Hicks, 1954; reviewed by Griffin et al, 2015). Other primates lack these hard and soft tissue specializations and instead have much more mobile feet that are probably adapted for varying degrees of arboreal and terrestrial locomotion (e.g., DeSilva, 2010).…”

“…333 naviculars are incompatible with the presence of a longitudinal arch, then they could belong to a species other than A. afarensis. Since significant hallucal abduction (e.g., as seen in extant African apes) is functionally incompatible with the 'windlass mechanism' associated with the longitudinal arch (Hicks, 1954;Griffin et al, 2015), and thus necessarily reflective of a flat foot, a candidate taxon for the A.L. 333 naviculars could be the one to which the BRT-VP-2/73 foot belongs.…”

Functional aspects of metatarsal head shape in humans, apes, and Old World monkeys

“…A more dorsally projecting metatarsal (MT) head is thought to increase the dorsiflexion range of motion (ROM) at the MTPJs DeSilva et al, 2012). This increased ROM serves to stabilize the forefoot and aids propulsion at the end of stance phase by tightening the plantar aponeurosis, which is a fibrous band that originates from the calcaneal tuberosity and inserts distally on the proximal phalanges (DeSilva, 2010;Griffin et al, 2015). This tightening of the plantar aponeurosis has been likened to a "windlass mechanism" (Hicks, 1954;Bojsen-Møller, 1979;Susman, 1983;Griffin and Richmond, 2010;Griffin et al, 2015) wherein it is analogous to a cable, the metatarsal head a drum, and the proximal phalangeal base a handle.…”