Accurate estimation of the position and orientation (pose) of a rigid bone from the motion of skin markers is limited mostly by the relative motion between the skin markers and the bone, which is known as the Soft Tissue Artifact (STA). This work proposes a new method, based on continuum mechanics, to describe the non-rigid kinematics of the markers, which is aimed at reducing bone pose estimation errors. In this method, the cluster of markers is characterized by Triangular Cosserat Point Elements (TCPEs) defined by all combinations of three markers. The kinematics of each TCPE is evaluated in terms of its translation vector and deformation gradient tensor. Subsequently, three instantaneous scalar parameters are defined to characterize the magnitude of strain in each TCPE, and the relative rotation and translation between pairs of TCPEs. The method was evaluated using previously collected ex-vivo data on the motion of the thighs of three cadavers. Femur pose estimates and the three parameters were calculated from the trajectories of 12 skin markers using the TCPE method, while the reference motion was measured using bone pin markers. The correlation coefficients between the bone pose errors obtained by the different TCPEs and the three parameters were analysed. The potential for predicting the accuracy of the bone pose estimation of different TCPEs was demonstrated for some of the parameters. As a result, the described parameters may be used to select quality subset groups of TCPEs which are more likely to accurately estimate the underlying bone pose.
