This paper proposes a design method of robots with high specific stiffness for dynamic jumping motions. In the proposed method, we regard drive-trains as part of structures supporting frame loads. Particularly in the case of rotary joints driven by linear actuators, we resolve joint moments into actuator's tensile forces and frame's compressive forces and reduce loads exerted on a frame. Though this effect is already known, our method is novel in terms of utilizing this effect for designing robot structures systematically and for saving robot weights. In addition, we introduce a set of the wrenches which would be exerted on a frame (the frame load region) and evaluate the lightness of several robots' structures by using the frame load region. As a resultant of the proposed method, we developed a new life-size humanoid robot prototype JAXON3-P. Then we demonstrate the high motion performance of JAXON3-P and the effectiveness of the proposed design method through jumping motion experiments.
