EXTRACTING GRAVITY LINE DISPLACEMENT FROM STABILOGRAPHIC RECORDINGS

Three algorithms for determining gravity line (GL) location from cente r of pressure (COP) and horizontal force (F-x) recordings are suggeste d. The algorithms are designed to study upright standing posture and a re 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 n ot 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 a nkle joint(s). Because the oscillations are small, the equations of mo vement can be linearized and the horizontal position of the gravity li ne (GLP) located. (2) Trend-eradication algorithm. The second integral of the horizontal force calculated and an initial integration constan t ((x) over dot, the initial horizontal GL velocity) is found from the trend of the displacement curve. (3) Zero-point-to-zero-point integra tion. When the horizontal force is zero, the horizontal position of th e gravity line (GLP) passes through the COP. The instantaneous GLP and its velocity are determined by integrating F-x from one zero point to another zero point. (C) 1997 Elsevier Science B.V.