The human foot is unique among living and extinct non-human primates. Obligate bipedalism, facilitated partly by changes in foot structure and function, is considered one of the great transformations in human evolution. The way the foot deforms at contact and recoils during push-off is presumed to be unique and the result of alterations in bone shapes, the adducted great toe, as well as the presence of a substantial aponeurosis and a medial longitudinal arch. These structures are different, reduced, or absent in other extant apes and are theorized to be evolutionary adaptations that are the result of selective pressures for habitual walking and running economy. Ideas around the requirements of efficient bipedalism and how the foot has adapted to these requirements have shaped conceptions about what makes us human and subsequently influenced approaches to treat foot pathologies, as well as prosthetic, orthotic, and shoe design.
The foot helps maintain balance, absorb shock and assists in push-off during locomotor activities. There are many theories about how and why the foot has evolved the way it has and how it functions, yet very few of the proposed form-function relationships have been tested well enough to be accepted or refuted. The purpose of this talk is to discuss our research investigating foot form and function with new approaches using state-of-the-art approaches in imaging and biomechanical modeling. These approaches may allow us to revise our understanding of how the foot contributes to bipedal locomotion.
There are three proposed mechanisms which are thought to provide humans with an advantage for efficient bipedal locomotion across various substrates. The first is the arch-spring model that describes use of elastic tissues, such as tendons and ligaments, to create a spring-like foot that is capable of first absorbing energy and then returning this energy through deformation and recoil of the medial longitudinal arch. This theory is consistent with evidence of other mechanisms that take advantage of elastic storage in humans and other organisms. The second is known as the windlass mechanism. The windlass is facilitated by the plantar aponeurosis – a thick band of ligamentous tissue that inserts at the calcaneus (heel bone) and spans the bottom of the foot, wrapping around the metatarsal bones and inserting on the toes. In 1954, Hicks proposed that the plantar aponeurosis essentially functions as a rigid cable, winding the aponeurosis around the metatarsal heads as the toes extend, which shortens and stiffens the arch. Finally, the human mid-foot is thought to be stiffer than most other primate species (both quadrupedal and bipedal), which reduces mid-foot break and creates a more rigid lever for force production. Mid-foot stiffness is thought to be facilitated by the transverse tarsal locking mechanism, where the axes of rotation of bones in the mid-foot move from an aligned position facilitating mobility, to a crossed orientation, effectively stiffening the foot. Recently, all three mechanisms have been questioned. Finally, other important features, such as the transverse arch have been identified as important to foot function. It is our position that a unified framework is needed to understand how the foot mechanically functions. The goal of this proposal is to establish this framework.