Extracting gravity line displacement from stabilographic recordings

Three algorithms for determining gravity line (GL) location from center of pressure (COP) and horizontal force (Fx) recordings are suggested. The algorithms are designed to study upright standing posture and are based on the following premises: (a) the foot(feet) is a solid body and does not move, (b) the axis of rotation of the ankle joint does not displace, and (c) the forces and moments acting at the ankle joint can be reduced to a resultant force through the axis of rotation and a force couple. The algorithms are: (1) Single-pendulum algorithm. The human body is modeled as an inverted pendulum oscillating around the ankle joint(s). Because the oscillations are small, the equations of movement can be linearized and the horizontal position of the gravity line (GLP) located. (2) Trend-eradication algorithm. The second integral of the horizontal force calculated and an initial integration constant ( , the initial horizontal GL velocity) is found from the trend of the displacement curve. (3) Zero-point-to-zero-point integration. When the horizontal force is zero, the horizontal position of the gravity line (GLP) passes through the COP. The instantaneous GLP and its velocity are determined by integrating Fx from one zero point to another zero point.