Determination of 3D spinal kinematics without defining a local vertebral coordinate system

In this paper a method is presented to calculate Euler's angles of rotation of a body segment during locomotion without a priori defining the location of the center of rotation, and without defining a local vertebral coordina te system. The method was applied to in vivo spinal kinematics. In this met hod, the orientation of each segment is identified by a set of three marker s. The orientation of the axes of rotation is calculated based on the avera ge position of the markers during one stride cycle. Some restrictions and a ssumptions should be made. The approach is viable only when the average ori entation of the anatomical axes of rotation of each spinal segment during a stride cycle coincides with the three axes of the laboratory coordinate sy stem. Furthermore, the rotations should be symmetrical with respect to both sides of the plane of symmetry of the spinal segment, and the subject shou ld move parallel to one axis of the laboratory coordinate system. Since in experimental conditions these assumptions will only be met approximately, e rrors will be introduced in the calculated angles of rotation. The magnitud e of the introduced errors was investigated in a computer simulation experi ment. Since the maximal errors did not exceed 0.7 degrees in a range of mis alignments up to 10 degrees between the two coordinate systems, the approac h proved to be a valid method for the estimation of spinal kinematics. (C) 1999 Elsevier Science Ltd. All rights reserved.