In the research unit ARKI, internal motion mechanisms in human joints are investigated in particular. Examples are the study of patterns of coupled motion in the cervical spine and in the lumbar spine as well as the study of locking and unlocking mechanisms in the joints of the extremities. Hereby, special attention is devoted to the variability of intra-articular rotation and translation components in healthy subjects and in patients. Computerised electrogoniometry, electromagnetic tracking devices, video-based systems and medical imaging techniques are used to realise 3-D motion analysis. The group has been innovative in using modern medical imaging techniques (MRI and helical CT) for 3-D joint kinematics. Recently, a method for simultaneous analysis of 3-D motion components and contact areal displacements was developed, starting from data, obtained with helical CT. The implementation of additional software to control, to treat and to organise the output stream of kinematic results, collected with a commercial available magnetic tracking system, provides user-friendly registrations, both for clinical use and for research applications.