Walking and running animals are inherently energy economic, agile and versatile. Natural locomotion with its impressive dynamics relies on structural leg compliance for the physical interaction with the environment . The SpringMass template model is the only reduced-order model that reproduces the basic dynamics of human and animal walking and running, including ground reaction forces and energy cycles. We claim that designing the desired spring-mass passive dynamics into a machine enables the versatile and agile behaviors observed in animals. In this paper, we demonstrate some basic theoretically-predicted gaits on ATRIAS (Assume The Robot Is A Sphere), a spring-mass, human-scale, bipedal robot. The complexity of natural locomotion neuromechanics gave rise to reductionist models to identify basic locomotion principles. Attempts were made to capture natural dynamics of walking with models almost as simple as a rolling wheel . The spring mass template originally
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