AN ALGORITHM FOR DETERMINING GRAVITY LINE LOCATION FROM POSTUROGRAPHIC RECORDINGS

In posturographic recordings, the center of pressure (COP) displacemen t 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, F-x, is proportional to the hor izontal acceleration of the COG its second integral does represent the horizontal position of the gravity line (GLP). However, the initial c onstants of integration, initial velocity and position, are not known. In this note, a technique for estimating these integration constants is suggested: zero-point-to-zeio-point integration. This method is bas ed on a postulation that when F-x = 0, the GLP and COP coincide. By in tegrating F-x from one zero point to another zero point, both the inst antaneous GLP and its velocity are determined. A validation of the sug gested algorithm was performed using optical methods to determine the GLP during a one legged standing tasks. Cross-correlation values of GL P determined via videography and the suggested algorithm ranged from 0 .79 to 0.96. These results suggest that the zero-point-to-zero-point-i ntegration is an acceptable technique for determining GLP from posturo graphic recordings. (C) 1998 Elsevier Science Ltd. All rights reserved .