The instantaneous axis of rotation of the foot and ankle based on kinematic analysis of a two-segment model

Several mathematical methods for locating the instantaneous center of rotation (ICR) have been reported. These methods, however, are planar in nature and not well suited to the 3D kinematic problem. In this paper, a method and algorithm for determining the dynamic, mean, 3D, instantaneous axis of rotation (IAOR) is presented. The method employs kinematic marker data to describe the instantaneous axis of rotation of the calcaneus with respect to the tibia. Results from a validation procedure using a pendulum apparatus demonstrate the applicability of the method. Experimental IAOR data acquired from a normal adult subject during ambulation indicate coronal plane variation of approximately 20 mm medio-laterally and vertically