McGeer [1] developed a simple bipedal robot with passive legs attached to the hip; this robot could walk stably without the need for any energy input other than gravity by descending a slope. The aforementioned study indicated that passive locomotion can play a significant role in gait. However, the dynamical mechanisms under such locomotion is complex to fully understand sorely from observation. To overcome the limitations of observational approach, several researchers have investigated gait mechanisms using simple walking models [2-7]. Bipedal locomotion consists of not only walking but also running. Therefore, some researchers further developed simple running models. For example, the spring-loaded inverted pendulum (SLIP) model, comprising a point mass and prismatic massless spring, can effectively reproduce the dynamics of running [8-12]. So far, to realize the running motion, we developed a bipedal robot that utilizes bouncing rod dynamics [13]. Recently, we developed a bipedal robot with actu-CONTACT Tomoya Kamimura.