Date of Award
Doctor of Philosophy (PhD)
Mechanical Engineering & Materials Science (ENAS)
Human foot mechanics have been extensively studied over the last century to understand human evolution and treat foot pathologies.Much of this understanding has centered on the foot's longitudinal arch that runs from heel to toe, especially its role in making human feet stiff enough to withstand many times our bodyweight during walking and running. But numerous studies on foot biomechanics, orthopedic surgical reconstructions, and human evolution point to substantial gaps in the understanding based on the longitudinal arch alone. In this thesis, I present two new findings related to foot stiffness and how it produces the mechanical power needed for propulsion. First, I show that the transverse arch that runs across its width contributes more to foot stiffness than the longitudinal arch. Second, I show that a mechanism involving the plantar fascia and the longitudinal arch, called the windlass mechanism, has no effect on stiffness as previously assumed. Instead, it helps transmit power from muscles outside the foot to locations within the foot for efficient walking. These findings hinge upon a combination of mathematical models, mechanical foot models, mechanical tests on cadaveric feet, and measurements on live walking and running human volunteers. Together, these studies show how the foot's function emerges from the combined roles of the longitudinal and transverse arches, and how those features may have evolved to enable human walking and running. After an introduction in chapter 1 to the current understanding of foot biomechanics and evolution, chapter 2 will present a series of studies using mechanical foot mimics and three-point bending tests in cadaveric human feet to show how the curvature of the transverse arch contributes to sagittal plane stiffness.Chapter 3 extends this result to in vivo foot stiffness measurements in walking humans and shows that despite active muscle contraction changes during walking, the transverse arch continues to influence sagittal foot stiffness. The evidence indicates that the evolution of a transverse arch played a key role in human evolution by enabling a stiff foot for propulsion. Chapter 4 turns to the longitudinal arch with an experimental examination of the windlass mechanism in walking and running humans, and finds that the windlass does not increase foot stiffness but plays an important role in power transmission so that the foot can output mechanical power without the need for heavy muscles within the foot itself. This thesis advances the understanding of structure-function relationships in the human foot and impacts the fields of evolutionary biology, podiatry and robotics. The renewed understanding of the windlass mechanism, taken together with other studies that compare feet across many primate species, suggests that the power transmission function of the windlass is important for walking, and could have emerged well before the longitudinal arch evolved in humans. Future clinical studies have to consider the transverse arch in evaluating foot function and may find that to be a potential target for the design of foot reconstructive surgeries, design of orthotic implants, and evaluation and classification of flatfoot disorders. Finally, the mechanical foot mimics that were developed may inspire the design of lightweight robotic and prosthetic feet with tunable stiffness properties that exploit the curvature induced stiffening of the human foot.
Yawar, Ali, "Structural Origins of the Stiffness and Work Contributions of the Human Foot" (2022). Yale Graduate School of Arts and Sciences Dissertations. 682.