RIGID BODY MOTION CALCULATED FROM SPATIAL COORDINATES OF MARKERS

Several methods have been developed for the kinematical analysis of bone movements in cadaveric specimens or living subjects. Methods based on X-ray or light photogrammetry of markers connected to bone are usually relatively accurate as compared to electro-goniometry. Spatial co-ordinates of markers in or on bone can be calculated from the co-ordinates of projections of these markers in two different directions on one or two planes. These spatial co-ordinates are used to determine kinematical parameters. The object in study is considered rigid and its movement between two subsequent positions is taken to be a screw motion. Such a motion can be described by a rotation matrix and a translation vector or by the position of the screw axis, the angle of rotation about this axis and the translation along this axis.