It has recently been hypothesized that many novel aspects of human anatomy reflect adaptations for endurance running. But humans, as bipeds, have special problems counteracting the tendency of the head to pitch forward during running, especially when the heel strikes the ground (heelstrike). The hypothesis tested here is that humans have evolved a novel mechanism, partially convergent with that of quadrupedal running animals, for counteracting the tendency of the head to pitch during bouncy running gaits at moderate, endurance speeds. Therefore, the project will study how humans stabilize the head during walking and running, and compare some of these mechanisms with a quadrupedal mammal. The key component of this project is a biomechanical and anatomical model that incorporates differences between the pendular and mass-spring mechanics of walking versus running. Although humans have vertically-oriented necks with restricted ranges of head movement, the muscular connections between the head and the shoulder are mostly de-coupled in humans, with the primary exception of the a small portion of the trapezius muscle (the cliedocranial trapezius, CCT). Humans also have a nuchal ligament, a tendon-like structure absent in great apes but present in other mammals. The human nuchal ligament connects with the CCT and inserts on the back of the head. Thus when the CCT contracts, it is hypothesized to link the mass of the arm with the mass of the head. Specifically, as the arm and shoulder girdle fall at heelstrike, inertial forces of the arm may passively extend the head just when it tends to pitch forward. Hypotheses derived from the model will be tested experimentally in humans and sheep during walking and running on a treadmill and force plate at a variety of speeds and with different tasks while looking at a focal point. Various sensors will be used to collect data on positions, accelerations and displacements of the head, shoulder and arms. In addition, electromyography (EMG) data will be obtained from the major muscles involved in head extension. In terms of scientific merit, the results of these experiments will help improve our understanding of the how humans stabilize their heads during running, a poorly studied subject. While there has been much research on the functional morphology of walking, less is known about running. The results are especially relevant to framing and testing the hypothesis that endurance running played a key role in human evolution. In terms of broader impact, the experiments planned here will be published in anthropology and biology journals, helping to integrate experimental and evolutionary research in these related fields, and on the web via the Peabody Museum as part of a new exhibit on human evolution. The experiments will provide research opportunities and training for students, as well as for a post-doctoral position. The results will also be useful to the wider audience of people interested in running.