SYSTEMATIC ERRORS IN DETERMINING THE CENTER OF PRESSURE (COP) WITH FORCE PLATES DEPEND ON THE LOAD DISTRIBUTION

  • Heinz-Bodo Schiedmayer
Keywords: Force plate, center of pressure, systematic errors

Abstract

Introduction: Systematic errors in determining the center of pressure (COP) with force plates have been reported in the literature [1]. Correction formulas given in the literature are based on a single force applied to the force plate [3]. From analytical models it is known that the errors are due to the statically over determined configuration [2]. Methods: Although the measured coordinate of the point of application of a single force is computed from the forces in load cells, the systematic error of a single force is dependent only on the actual point of force application and therefore can be written in the form of the nonlinear functions [...]. It is now assumed that the problem can be described within the theory of linear elasticity, where all deformations are linear functions of the applied forces, and a general solution can be found as a linear combination of single load cases. The measured COP of an arbitrary pressure distribution can therefore be written in the form [...] Results: By correcting the measured COP with the correction function g, corrected value x_COP in general differs from [...]. When correcting systemic errors in the determination of the COP, one must be very careful about the distribution of the loads on the force plate. Accurate corrections can only be made for forces distributed over a very small area. To avoid these errors the force plate should be statically determined. [I apologise for any inconvenience but formulas cannot be written down in this abstract. (Editor)] References: [1] Bobbert, MF, Schamhardt, HC. (1990). Accuracy of Determining the Point of Force Application with Piezoelectric Force Plates. J.Biomechanics 23, 705-710. [2] Schmiedmayer, H-B, Kastner, J. (1997). Explanation for Systematic Errors in Determining the Point of Force Application with Force Plates. Proceedings of the XVIth I.S.B.Congress, Tokyo, 369. [3] Sommer, R, Kohler, D, Calame, C. (1997). Center of Pressure (COP) Enhancement in Piezoelectric Force Plates. Proceedings of the XVIth I.S.B.Congress Tokyo, 18.