1Institute of Human Movement Sciences, University of Groningen, and Laboratory of Human Movement Analysis, University Hospital, PO BOX 196, AD Groningen 9700, The Netherlands
The well-known condition for standing stability in static situations is that the vertical projection of the centre of mass (CoM) should be within the base of support (BoS). On the basis of a simple inverted pendulum model, an extension of this rule is proposed for dynamical situations: the position of (the vertical projection of) the CoM plus its velocity times a factor 1/g should be within the BoS, l being leg length and g the acceleration of gravity. It is proposed to name this vector quantity ‘extrapolated centre of mass position’ (XcoM). The definition suggests as a measure of stability the ‘margin of stability’ b , the minimum distance from XcoM to the boundaries of the BoS. An alternative measure is the temporal stability margin τ, the time in which the boundary of the BoS would be reached without intervention.
Some experimental data of subjects standing on one or two feet, flatfoot and tiptoe, are presented to give an idea of the usual ranges of these margins of stability. Example data on walking are also presented.