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Quantifying the energy expended by modern humans during ambulation has been a long-standing priority among exercise physiologists and some anthropologists, but difficulty in reconciling theoretical predictions with empirical observations abounds. The metabolic energy used in bipedal walking in modern humans has generally been empirically shown to vary with velocity and body mass, while mechanical analyses predict that the energy used to walk should be dependent on, among other things, the relative proportions of the body segments. Increasing one such proportion, leg length, has been shown to theoretically increase the energy needed to walk. Empirical research has not, however, found such a correlation. I recently proposed that one possible reason for this apparent discontinuity between theory and empiricism might be that the empirical reports do not eliminate enough of the known individual variation in such measures as resting metabolic rate to allow the effect of leg length to be discerned. To test this proposal, I combined data from the literature with some recently developed in our lab. I found that when individual variation in resting metabolic rate was accounted, a model which uses velocity, leg length and, possibly, total body mass as independent variables can predict approximately 90% of the variation in energy expenditure. Velocity remains the strongest and most significant predictor of energy expenditure, but longer legs are associated with higher energy expenditure than shorter legs are at natural walking velocities. This result confirms the prediction from mechanical analyses and points to the need to explain the increased leg length seen in Homo relative to Australopithecus as an accommodation based on selective pressures other than increased mechanical efficacy.

Robust australopithecines (Australopithecus robustus and A. boisei) have been characterized as having anteroposteriorally compressed femoral necks when compared to primates in general, and to other hominids in particular. The degree of this anteroposterior compression, however, has never been quantified. To remedy this deficit, we determined the cross-sectional aspect ratios of the femoral necks of fossil hominids, as well as those of eight extant human femora. Cross-sectional properties (such as area moment of inertia) are also evaluated and used with estimates of body mass in order to examine the loading environments to which these hominids were exposed. Cross-sectional properties can inform about the loading environment that the individual experienced, as well as the typical environment seen by the species, because bones respond to applied load (by increasing bone concentration in areas of high load) and ate shaped through natural selection. Our data show two trends in hominid evolution: substantial anteroposterior compression of the femoral neck and less intense loading environments in the robust australopithecine lineage, and limited anteroposterior compression of the femoral neck and more intense loading environments in A. afarensis and Homo. The results may indicate that the morphology of the robust australopithecine femoral neck is a character with functional significance, and that additional study of the proximal femur of robust australopithecines is necessary to understand the locomotor behavior of these species.