The Point Cluster Technique

The Point Cluster method provides a basis for determining the artifact associated with non-rigid body movement of points placed on the skin. It has been shown, by simulation, that the error due to non-rigid body movement could be substantially reduced by using the Point Cluster method. This method also allows the researcher to obtain data on knee rotation and translation that compares favorably to the results previously obtained from normal subjects with intra-cortical pins placed into the femur and tibia.

The marker placement used for the Point Cluster Technique is pictured below. Reflective markers are placed on the thigh, shank, and foot creating 2 cluster groups. Each point within the cluster is assigned a mass, and the center of mass and inertia tensor of the cluster of points is calculated. Eigenvalues and eigenvectors of the inertia tensor are used to (1) determine non-rigid body motion and (2) set up a coordinate system within the cluster. The eigenvalues of the inertia tensor remain invariant if the segment is behaving like a rigid body, which therefore provides a method for determining variations for nonrigid body motion.

For a more detailed description of the PCT, please refer to the following article: Andriacchi, T.P. "A Point Cluster Method for In Vivo Motion Analysis: Applied to a Study of Knee Kinematics", Journal of Biomechanical Engineering, Dec. 1998, Vol. 120, p. 743-749.