Hip extensor mechanics and the evolution of walking and climbing capabilities in humans, apes, and fossil hominins

Authors

Elaine E. Kozma, CUNY Graduate CenterFollow
Nicole M. Webb, CUNY Lehman College
William Harcourt-Smith, CUNY Lehman CollegeFollow
David A. Raichlen, University of Arizona
Kristiaan D’Août, University of Liverpool
Mary H. Brown, Lester E. Fisher Center for the Study and Conservation of Apes
Emma M. Finestone, CUNY Graduate Center
Stephen R. Ross, Lester E. Fisher Center for the Study and Conservation of Apes
Peter Aerts, Universiteit Antwerpen
Herman Pontzer, CUNY Graduate CenterFollow

Document Type

Article

Publication Date

4-2-2018

Abstract

The evolutionary emergence of humans’ remarkably economical walking gait remains a focus of research and debate, but experi- mentally validated approaches linking locomotor capability to postcranial anatomy are limited. In this study, we integrated 3D morphometrics of hominoid pelvic shape with experimental mea- surements of hip kinematics and kinetics during walking and climbing, hamstring activity, and passive range of hip extension in humans, apes, and other primates to assess arboreal–terrestrial trade-offs in ischium morphology among living taxa. We show that hamstring-powered hip extension during habitual walking and climbing in living apes and humans is strongly predicted, and likely constrained, by the relative length and orientation of the ischium. Ape pelves permit greater extensor moments at the hip, enhancing climbing capability, but limit their range of hip extension, resulting in a crouched gait. Human pelves reduce hip extensor moments but permit a greater degree of hip extension, which greatly improves walking economy (i.e., distance traveled/ energy consumed). Applying these results to fossil pelves suggests that early hominins differed from both humans and extant apes in having an economical walking gait without sacrificing climbing capability. Ardipithecus was capable of nearly human-like hip ex- tension during bipedal walking, but retained the capacity for powerful, ape-like hip extension during vertical climbing. Hip ex- tension capability was essentially human-like in Australopithecus afarensis and Australopithecus africanus, suggesting an economi- cal walking gait but reduced mechanical advantage for powered hip extension during climbing.

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This article was originally published in PNAS, available at DOI: 10.1073/pnas.1715120115.

This open access article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).