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 hi story of the joint angles of the ankle, knee and hip were measured by optic al position sensors during the motion. The experimental data when combined with human morphology, such as mass and length of each part of the body, al lows 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 recogni zed 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 s tate. Furthermore, it is suggested that the elderly usually use a spiral tr ajectory to the equilibrium state in the sit-to-stand movement, which is ob served when a system is close to an unstable state.