Finite helical axis calculation for human tibiotalar joint motion based on Burmester theory

A large number of mechanical conventions and mathematic procedures have been proposed for the representation and calculation of the axes of rotation at the human joints [1,2]. If the relative motion between two rigid bodies is sufficiently near to a rotation about a fixed axis (not equal, not to fall in singularity), the Burmester Theory (BT) can be exploited in order to find this axis. In this case, indeed, the Burmester points on one of the two bodies are expected to be nearly aligned in the neighbourhood of an axis, which can be taken as the axis of the relative motion. Based on this observation, BT has been used to find the helical axis of the passive tibiotalar joint motion, which is know to be a pseudorotation about an almost fixed axis. Comparisons with other techniques are reported which show the potential of the approach.