A technique for measurement and description of three-dimensional six degree-of-freedom motion of a body joint with an application to the human spine

Abstract

A new experimental technique for the measurement of three-dimensional six degrees-of-freedom motion of a body joint and data reduction is presented. It utilizes three spheres that are rigidly attached to the moving body. Six of the nine components of three translation vectors of centers of the spheres are measured by six linear variable differential transformers (LVDT). Data is recorded in real-time by a minicomputer Equations are derived to compute the three Euler rotation angles and the three-dimensional translation vector at a given point of the rigid body. Further, this six-dimensional motion vector is transformed into six parameters that describe the instantaneous helical axis of motion of the joint. Practical considerations in developing a program to compute these helical axis parameters are presented. The technique has been successfully employed to study kinematics of the human spine segments. Examples for the L3-4 lumbar spine segment, subjected to different loads are included. This technique, although especially developed for the in vitro study of the three-dimensional flexibility measurements of the spine segments, may be advantageously utilized in other situations where small three-dimensional motion needs to be studied with high accuracy and in real-time.