This paper deals with a phase plane analysis of motions of the C.G. (center of gravity) of the human body in the sit-to-stand movement. The motion is calculated by both the time history data of the three joint angles of ankle, knee and hip, which are measured by a positioning sensor, and human morphology such as mass and length of each part of the body. Experiments were done on four young healthy men. This analysis shows that the motion of the C. G. can be clearly recognized as three motion modes: acceleration mode and two convergent modes to an origin of the plane and to an equilibrium state of the standing state. Furthermore, the trajectory to the equilibrium state in the sit to-stand motion, which the elderly use usually, is a spiral, which is observed when the systern is close to an unstable state.
