Ontogeny of bipedalism : pedal mechanics and trabecular bone morphology
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A unique pattern of pedal loading from heel-strike at touchdown to hallucal propulsion at toe-off is a distinct feature of mature human bipedalism, however, its first appearance in the fossil record is debated. The main goal of this dissertation is to identify anatomical correlates to a modern human heel-strike, rigid foot, and propulsive hallucal toe-off. First, a biomechanical analysis of toddler walking is used as a 'natural experiment' to investigate the influence of non heel-strike (NHS, n = 11) and immature heel-strike (IHS, n = 7) on the location of the center of pressure and orientation of the ground reaction force resultant in relation to specific foot bones during stance phase. With an expanded knowledge of foot bone loading in toddlers, a microarchitectural approach is used to test the influence of a heel-strike, rigid foot, and propulsive hallucal toe-off on trabecular bone fabric properties in an ontogenetic series of human and African ape (chimpanzee, bonobo, and gorilla) calcanei, tali, first metatarsal heads and hallucal distal phalanges. This dissertation presents the first ontogenetic analysis of pedal trabecular bone in primates. Heel-strike and toe-off are developmentally independent from one another. Although most toddlers lack a hallucal toe-off, NHS and IHS apply equally high propulsive forces when the entire width of their forefoot is in contact with the ground. Biomechanical and fossil evidence suggest that a generalized active propulsion may have preceded the evolution of a propulsive hallucal toe-off. Although pedal trabecular fabric properties are more complex than predicted, trabecular correlates to heel-strike and hallucal toe-off are identified within adult human foot bones. Compared to toddlers and African apes, adult humans have a unique combination of relatively thick trabecular struts and an anteroplantar to posterodorsal primary trabecular orientation in the plantar aspect of the calcaneal tuber. In the calcaneal tendon volume of interest, adult humans have a unique anteroplantar to posterodorsal primary trabecular orientation associated with a propulsive hallucal toe-off. This dissertation provides the comparative context necessary to begin assessing the evolution and developmental timing of foot function and specific bipedal gait events in juvenile and adult fossil hominins.