CLASSIFICATION OF ERRORS IN LOCATING A RIGID-BODY

This paper discusses the manner in which random Gaussian errors affect the determination of body segment kinematics. For the process of mode lling rigid body (RB) motion, three types of kinematic errors, input, measured and theoretical, are identified. These correspond to errors i n: the determination of three-dimensional observed points, the RB fit of those points, and the estimation of true RB positions, respectively . Of these, the theoretical error is most critical and most pivotal. A ccuracy is provided when the theoretical error is minimised, yet only the measured error can be minimised by RB modelling algorithms. In com puter simulations one may determine the effect that such manipulations have on theoretical error, yet in most experimental conditions this v alue may not even be calculated. Fortunately, computer simulations can be performed to determine the inter-relationships between the types o f RB modelling errors. Such simulations can also be used to investigat e the effects of RB shape. In this paper, Monte Carlo simulations were performed on three unit radius RBs; a triangle, a square and a tetrah edron. Although use of the triangle provided the lowest measured error , this also coincided with the greatest theoretical error. The use of redundant points was found to yield superior theoretical accuracies. A slight advantage was gained with use of the non-planar point arrangem ent on the tetrahedron, both the measured and theoretical errors were reduced. Finally, the superiority of RB modelling over individual poin t tracking was reflected in all of the results; between 33 and 50% of the input error was eliminated with the use of RB modelling. Copyright (C) 1996 Elsevier Science Ltd.