The stiffness of the human foot is strongly influenced by an arch that spans its width, a new study suggests. An international research team, led by Madhusudhan Venkadesan at Yale University in the US, came to this conclusion by doing simulations and experiments of the physical mechanisms underlying the foot’s transverse tarsal arch (TTA). Their discovery could lead to new advances in medicine and biotechnology – and deliver new insights into how bipedalism first evolved in our distant ancestors.
Humans are unique among primates because the inherent stiffness of our feet enables us to efficiently push off the ground when walking and running (see video). The median longitudinal arch (MLA), which runs from the heel to the ball of the foot, is thought to play a critical role in this stiffness.
Stiffened by a bowstring-like arrangement of ligaments, the MLA not only keeps the foot rigid. It also stores and releases mechanical energy like a spring as we walk and run. Yet despite our detailed knowledge about the role of the MLA, the precise relation between midfoot curvature and stiffness is still widely debated among anatomists.
Venkadesan’s team believe that previous analyses of the foot had overlooked the stiffening influence of the TTA, which spans the width of the foot perpendicular to the MLA. To understand the role of the TTA, the team subjected uniform elastic shells to curvatures in both longitudinal and transverse directions; stiffening each curve with ligament-imitating springs.
Measurements on the shells – and computer simulations – have revealed that the transverse bending contributes more to the stiffness of the shell than the longitudinal bending. This is independent of other factors including shell size and thickness. The team also tested the importance of the TTA theory using cadaver feet. This showed that by cutting transverse ligaments, overall foot stiffness is reduced by 40%; compared with just 23% for longitudinal ligaments.
Venkadesan and colleagues are also exploring how and when the foot’s stiffness and curvature first appeared in the evolutionary history of our ancestors. They have studied a variety of fossils of extinct hominins – which were more closely related to humans than to chimpanzees. This analysis revealed that human-like TTAs predate our own genus, Homo, by over 1.5 million years. This suggests that both the MLA and TTA were critical for the emergence of human bipedalism.
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Future studies could also help us better understand the role of the MLA. For example, the MLAs of individual feet have a range of curvatures that is not reflected in the range of foot stiffnesses. It is possible, therefore that the TTA and MLA work together to create the optimum overall stiffness.
The researchers hope that their insights could lead to new treatments of flatfoot disorders, which can significantly reduce a person’s mobility. The research could also lead to more advanced artificial feet for prosthetic limbs and even robots that can walk and run like humans.
The research is described in Nature.