Equilibrium and Stability of the Upright Human Body

Abstract

We propose a new parameterization for describing forward and backward leanings of the straight human body with the feet fixed on the ground. During this motion stability of static equilibrium of the upright human body depends on the basic parameters such as coordinates XCG and YCG of the center of gravity of the body in the upright position and the radius R of the base of support. We introduce two coefficients k1= YCG/R and k2= XCG/R which are enough to calculate critical values of the angle α between the vertical axis OY and the axis of the cylinder the body is fitted into. We have calculated critical values αcr′ and αcr′′ when the body becomes unstable during leanings back and forward correspondingly. It is shown that stability of the upright body may be characterized by the angle αcr= |αcr′ |+|αcr′′| which strongly depends on k1 and weakly depends on k2 and αopt – the value of α for the most stable position when the body’s center of gravity is exactly above the geometrical center of the base of support. Stable equilibrium will be achieved for larger αcr and smaller | αopt|.