This paper describes analysis of the 1–Joint Spring–Motor Coupling System (SMCS). A system which includes series elastic elements shows vibrational responses in the velocity space, and our concept is to use the velocity peak of the system for dynamic motions such as pitching motions. By tuning parameters, we can obtain momentarily higher velocity than a system without any spring. We constructed a mathematical model of the 1–Joint SMCS and confirmed the velocity increasing effect by numerical analyses and real experiments. We also derived existence of the most effective inertia balance and the optimum spring constant. We established design criteria to utilize the effect and confirmed the criteria by experiments.