Experimental determination of instantaneous screw axis in human motions. Error analysis

The location of the instantaneous screw axis (ISA) is essential in order to obtain useful kinematic models of the human body for applications such as prosthesis and orthoses design or even to help in disease diagnosis techniques. In this paper, dual vectors will be used to represent and operate with kinematic screws with the purpose of locating the instantaneous screw axes which characterize this instantaneous motion. A photogrammetry system based on markers will be used to obtain the experimental data from which the kinematic magnitudes will be obtained. A comprehensive analysis of the errors in the measurement of kinematic parameters has been developed, obtaining explicit expressions for them based on the number of markers and their distribution. Finally, the developed methodology has been applied to the experimental determination of the ISA during an alternative motion of flexion and extension of the back.