Biomechanical analysis of the instant axis or rotation (IAOR) of the hindfoot during normal ambulation

A method has been developed to noninvasively determine the instantaneous axis of rotation (IAOR) of the hindfoot during human ambulation. The algorithm uses 3-D external marker trajectories to describe the IAOR of the calcaneus with respect to the tibia. Optimization techniques were used to fit a plane and a sphere to the marker trajectories such that the sphere center lie on the plane. The IAOR was defined as perpendicular to the plane and passed through the center of the sphere. Validation results demonstrate applicability of the method to the Milwaukee foot model (MFM). Experimental IAOR data demonstrate application during normal adult gait. This self-determining system may benefit rehabilitation applications in pedorthic, prosthetic and orthotic treatment.