Reducing rigid-body error in a functional technique to determine ankle joint axes

Introduction Functional methods of finding joint axes of rotation involve tracking the movement of one bone at a joint relative to another and subsequent calculation of the joint axis position and orientation from the locations of skinmounted markers. van den Bogert et al. [1] proposed a functional method of deter-mining the non-weight bearing axes of the ankle joint complex. Their subjects performed a full range of motion at the ankle with markers on the shank and shoe. The tech-nique implemented an optimisation algorithm to fit a kinematic model of the subtalar and talocrural joints to the experimentally acquired motion data. Soft tissue movement and associated marker movement was re-ported to be a significant source of error. The aim of the present study was to investigate how this skin movement error might be reduced through a better selection of marker positions.