Simultaneous measurement of body center of pressure and center of gravity during upright stance. Part I: Methods

Clinical and experimental assessments of balance use measures of center of pressure (COP) excursion to quantify postural stability during standing. This assumes that the greater the COP excursions, the greater the imbalance. However, it is the position of the body center of gravity (COG), specifically in relation to the base of support, that determines static stability during upright standing. The reason the COG is frequently ignored is that it cannot be measured directly, whereas the COP is readily obtained from a force platform. We report here on a method for simultaneous measurement of the COP and COG displacements. To compute the COG displacements, an optoelectric imaging system operating in synchrony with a force platform was used to measure the three-dimensional positions of the body joints and body segment endpoints. The COG displacements were then computed from the segment kinematics using subject-specific anthropometric measurements. In Part II of this paper, we compare summary measures of the COP and COG excursions obtained from six subjects during three different upright stances.