A stricter condition for standing balance after unexpected perturbations.

In order to account for the dynamic nature of balance, the concept of the 'extrapolated centre of mass' XcoM has been introduced (Hof et al., 2005). The law for standing balance was then formulated as: the XcoM should remain within the Base of Support (BoS). This law, however, does not take into account that the centre of pressure (CoP) needs time to displace due to various neural and mechanical delays. The theory is extended to include the finite reaction- and displacement time of the CoP. Experimental results on humans standing on two feet undergoing sudden postural perturbations are presented. In this case it turns out that the area of the effective BoS is only a fraction, some 30%, of the area of the static BoS.