The adoption of habitual bipedal locomotion required major alterations in the pelvic shape of hominins, changes which are clearly evident in the fossil record by at least 3.4 million years ago. However, variation in pelvic width both between extinct hominin species, and in the form of sexual dimorphism among modern humans, has led to the hypothesis that wider pelves result in less efficient or less effective locomotion. This hypothesis is based on biomechanical models which suggest that wider pelves diminish the mechanical advantage of the hip stabilizer muscles (hip abductors) thereby increasing muscle force production and increasing the metabolic cost of walking and running. However, this model has never been tested during locomotion. This study will test whether skeletal measures of pelvic width are correlated with relevant mechanical dimensions during locomotion, and determine how the hip abductors are specifically related to locomotor cost. Kinematics, kinetics, and oxygen consumption data will be collected on 30 individuals along with magnetic resonance imaging to determine the mechanical advantage, muscle force, and energetic cost associated with activation of the hip abductors. Once these relationships are established, they can be used to address the energetic consequences of variation in pelvic shape between living humans and make inferences about locomotor efficiency in extinct hominin taxa. Accurate reconstructions of locomotor cost in early hominins will aid in testing hypotheses about the selective pressures responsible for the evolution of bipedalism. Furthermore, quantifying the relationship between skeletal morphology and locomotor cost is relevant to understanding the energetic accommodations required by the major changes in brain and body size, and alterations in ranging and dietary patterns seen with the evolution of the genus Homo. The locomotor consequences of changes in parturition related to these increases in brain and body size in Homo can also be addressed. Additionally, establishing a new empirically tested biomechanical model of the hip during locomotion has important applications in orthopedics, where determining proper treatment for conditions such as hip dysplasia and osteoarthritis require accurate assessment of hip joint stresses, and ground and muscle forces.
