An algorithm for determining gravity line location from posturographic recordings

In posturographic recordings, the center of pressure (COP) displacement does not accurately reflect migration of the gravity line (GL), the vertical line passing through the body’s center of gravity, COG. Since the horizontal ground reaction force, Fx, is proportional to the horizontal acceleration of the COG its second integral does represent the horizontal position of the gravity line (GLP). However, the initial constants of integration, initial velocity and position, are not known. In this note, a technique for estimating these integration constants is suggested: zero-point-to-zero-point integration. This method is based on a postulation that when Fx=0, the GLP and COP coincide. By integrating Fx from one zero point to another zero point, both the instantaneous GLP and its velocity are determined. A validation of the suggested algorithm was performed using optical methods to determine the GLP during a one legged standing tasks. Cross-correlation values of GLP determined via videography and the suggested algorithm ranged from 0.79 to 0.96. These results suggest that the zero-point-to-zero-point-integration is an acceptable technique for determining GLP from posturographic recordings.