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In legged terrestrial locomotion, the duration of stance phase, i.e., when limbs are in contact with the substrate, is positively correlated with limb length, and negatively correlated with the metabolic cost of transport. These relationships are well documented at the interspecific level, across a broad range of body sizes and travel speeds. However, such relationships are harder to evaluate within species (i.e., where natural selection operates), largely for practical reasons, including low population variance in limb length, and the presence of confounding factors such as body mass, or training. Here, we compared spatiotemporal kinematics of gait in Longshanks, a long-legged mouse line created through artificial selection, and in random-bred, mass-matched Control mice raised under identical conditions. We used a gait treadmill to test the hypothesis that Longshanks have longer stance phases and stride lengths, and decreased stride frequencies in both fore- and hind limbs, compared with Controls. Our results indicate that gait differs significantly between the two groups. Specifically, and as hypothesized, stance duration and stride length are 8–10% greater in Longshanks, while stride frequency is 8% lower than in Controls. However, there was no difference in the touch-down timing and sequence of the paws between the two lines. Taken together, these data suggest that, for a given speed, Longshanks mice take significantly fewer, longer steps to cover the same distance or running time compared to Controls, with important implications for other measures of variation among individuals in whole-organism performance, such as the metabolic cost of transport.

Human bipedal locomotion is characterized by a habitual heel-strike (HS) plantigrade gait, yet the significance of walking foot-posture is not well understood. To date, researchers have not fully investigated the costs of non-heel-strike (NHS) walking. Therefore, we examined walking speed, walk-to-run transition speed, estimated locomotor costs (lower limb muscle volume activated during walking), impact transient (rapid increase in ground force at touchdown) and effective limb length (ELL) in subjects (n=14) who walked at self-selected speeds using HS and NHS gaits. HS walking increases ELL compared with NHS walking since the center of pressure translates anteriorly from heel touchdown to toe-off. NHS gaits led to decreased absolutewalking speeds (P=0.012) and walk-to-run transition speeds (P=0.0025), and increased estimated locomotor energy costs (P<0.0001) compared with HS gaits. These differences lost significance after using the dynamic similarity hypothesis to account for the effects of foot landing posture on ELL. Thus, reduced locomotor costs and increased maximum walking speeds in HS gaits are linked to the increased ELL compared with NHS gaits. However, HS walking significantly increases impact transient values at all speeds (P<0.0001). These trade-offs may be key to understanding the functional benefits of HS walking. Given the current debate over the locomotor mechanics of early hominins and the range of foot landing postures used by nonhuman apes, we suggest the consistent use of HS gaits provides key locomotor advantages to striding bipeds and may have appeared early in hominin evolution.

When considering the array of terrestrial locomotor behaviors, bipedalism is a particularly rare way of moving about the landscape. In fact, humans are the only obligate terrestrial mammalian bipeds. Therefore, understanding both how and why it evolved is particularly intriguing. However, there is debate over why the evolution of bipedalism occurred and there is a large gap in knowledge for the mechanisms that underpin the evolution of these adaptive morphologies. One complicating factor for sorting out which models best explain how our hominin ancestors became bipedal is that they all rely on the same set of traits. Moreover, many of the traits that are thought to be diagnostic of bipedalism are only linked by association and have not been experimentally tested. That is, they do not appear in non-human primates and other quadrupeds. Therefore, addressing why the evolution of bipedalism occurred requires understanding the adaptive significance of traits linked with bipedalism. In this dissertation, I use an experimental approach employing both human and animal models to explore links between morphology and behavior and to tease apart the adaptive significance of particular traits. For the human portion of the dissertation, I use an inverse dynamics approach (estimating muscle forces from kinematic, kinetic, and anatomical data) to determine how modern human anatomy functions while walking using ape-like postures to clarify the links between morphology and energy costs in different mechanical regimes to determine the adaptive significance of postcranial anatomy. The results from this portion of the dissertation suggest that adopting different joint postures results in higher energy costs in humans due to an increase in active muscle volumes at the knee. These results lead to two conclusions important for understanding the evolution of human bipedalism. One is that human anatomy maintains low energy costs of walking in humans compared to chimpanzees regardless of lower limb postures. Second, the results suggest that erect trunk posture may be an important factor in reducing energy costs, therefore indicating that lumbar lordosis (the curvature of the lower spine) is important for reducing costs. For the animal portion of the dissertation, I use rats as a model for the quadrupedal-to-bipedal transition and experimentally induce bipedal posture and locomotion under a variety of loading conditions to determine if traits consistent with the evolution of bipedalism occur and under what conditions. This experimental design also has the ability to determine if there is a role for developmental plasticity in generating bipedal morphology to help answer the question how the evolution of bipedalism occurred. I find that inducing bipedal behaviors in a quadrupedal animal generates morphology consistent with human bipedal traits and that loading conditions have specific effects in different skeletal elements and at particular joints. I also find that there is a plausible role for developmental plasticity in generating adaptive bipedal morphology in the earliest hominins. Overall, the results from the experimental procedures in this dissertation were able to clarify links between behavior and bipedal morphology, demonstrate a plausible role for developmental plasticity in early adaptation to bipedal behavior in australopiths, determine the adaptive significance of human postcranial anatomy, and the ways in which postcranial anatomy reduces costs.