The biomechanics and evolution of impact resistance in human walking and running

Abstract

How do humans generate and resist repetitive impact forces beneath the heel during walking and heel strike running? Due to the evolution of long day ranges and larger body sizes in the hominin lineage modern human hunter-gatherers must resist millions of high magnitude impact forces per foot per year. As such, impact forces may have been a selective pressure on many aspects of human morphology, including skeletal structure. This thesis therefore examines how humans generate impact forces under a variety of conditions and how variation in skeletal structure influences impact resistance. This thesis includes four studies that can be separated into two parts. In the first part, I test two models of how variation in the stiffness and height of footwear affect the generation of impact peaks during walking and heel strike running. The first model predicts that variation in the stiffness of footwear introduces tradeoffs between three crucial impact force related variables: impact loading rate, vertical impulse and effective mass. The prediction of the second model is that higher heels have the same effects on impact forces as do footwear of lower stiffness. These hypotheses were tested using 3D motion data and force data in human walkers and runners wearing a variety of footwear. Experimental results show that soft footwear introduces tradeoffs between impact loading rate, vertical impulse and effective mass, and that high heeled shoes influence impact duration, loading rate and vertical impulse in predictable ways. In the second part of this thesis, I document variation in hominoid skeletal structure and experimentally test how this variation affects function during impact forces. In particular, I examine trabecular bone volume fraction in the calcaneus of gorillas, chimpanzees and several H. sapiens populations that vary widely in geologic age and subsistence strategy. I then develop and test a model of how variation in trabecular bone volume fraction affects several mechanical properties of trabecular bone tissue, including the stiffness, strength and energy dissipation. The comparative data indicates that trabecular bone volume fraction in the human calcaneus has declined after the Pleistocene. The experimental data shows that larger trabecular bone volume fraction results in increased stiffness and strength but reduced energy dissipation of trabecular bone tissue. A final examination of the comparative data relative to the experimental data suggests that the human calcaneus resists impacts by being stiff strong rather than by dissipating mechanical energy. The results of this thesis suggest that way in which impacts are both generated and resisted has changed in recent human history, as modern footwear alters impact loading rate and vertical impulse and decline in trabecular bone volume fraction negatively influence trabecular bone strength. These results also have implications for how bones evolve to resist impacts, suggesting that bone structures than favor stiffness and strength are favored to cope with impacts. Finally, the results of this thesis are important for understanding the etiology of osteoarthritis, and musculoskeletal disease that has been linked to both repetitive impact forces during human locomotion and to variation in trabecular bone volume fraction.