Human running motion is a complex motion that not only based on the movement of legs but also needs the entire body to participate in. Therefore, we focused on the upper body, designed a trunk and pelvic rotation assist suit to apply an external force on the chest and pelvis for assisting the running motion, changing the energy flow between the upper and lower body to improve the running efficiency. However, human motion is very complicated and difficult to analyze, it is hard to explore the specific effect of the external force which is provided by the rotation assist suit on human motion. In this paper, we proposed an advanced spring-loaded inverted pendulum model for simulating the human running motion in natural running status and expanded running status in which the motion is expanded by wearing the trunk and pelvic rotation assist suit, parametrically represent the changes of human due to the intervention of the external force, and discuss the effect of the external assistant force on the human running motion.