1Evolutionary Anthropology, Duke University, 2Department of Biology, James Madison University
Saturday Morning, Alexander's
Studies designed to infer limb posture during bipedalism of early hominins must often rely on a “bracketing” approach using multiple models including humans and nonhuman primates adopting natural and induced gaits. We use such an approach to address a relatively novel issue in hominin locomotion: the biomechanics of swing phase. Most previous studies have focused on support phase where loads and muscular effort are the highest. But animals also use muscles to control the swinging limb, which has an impact on energetic costs and may constrain animal velocity. Previous studies have suggested that, with increasing velocity, support time decreases while swing time remains invariant. However, it is not known if this is true for all bipeds. Nor is it known how swing time varies by species or, since flexion of the swinging limb changes its pendulum length, how swing time changes with limb posture. We examined swing time in bipedally walking chimps and humans walking normally and with a bent-hip, bent-knee gait. Chimps and humans both showed significant decreases in support time with speed and no decreases in swing time, suggesting that swing phase time is constrained and that swing phase mechanics is an important factor influencing limb design. There were no significant differences in swing time between normal and bent-hip, bent-knee walking in humans. Compared to humans, chimpanzees devoted a relatively shorter time to swing phase. Our animal models suggest that attention should be paid to swing phase mechanics when reconstructing locomotor behavior and energetic costs in fossil animals.