Analysis by phase planes of the sit-to-stand movement from a chair

This paper presents a phase plane analysis of the motion of the CG (center of gravity) of the human body during the sit-to-stand movement. The time history of the joint angles of the ankle, knee and hip were measured by optical position sensors during the motion. The experimental data when combined with human morphology, such as mass and length of each part of the body, allows the tracking of the CG position. Experiments were done on four healthy young men. Analysis shows that the motion of the CG can be clearly recognized as three motion modes: an acceleration mode and two convergent modes to an origin of the phase plane and to an equilibrium state of the standing state. Furthermore, it is suggested that the elderly usually use a spiral trajectory to the equilibrium state in the sit-to-stand movement, which is observed when a system is close to an unstable state.