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For clinical assessment of kinematic foot function, it is of value to decouple ankle joint movement from subtalar joint movement. This cannot be done by simple gait analysis and requires a more complex mathematical analysis of the motion of the ankle-foot complex. Assuming the joints to be simple hinges it is possible to describe the parameters for the axes of these joints as part of a series of geometrical transformations from the shank via the ankle and the subtalar joints to the foot. Motion data for the shank and posterior foot were collected from a Vicon 510 8-camera gait analysis system. An optimization method was applied to obtain the best fit of the data to the model. Results show that a reproducibility of axes locations to within 2 mm and axes angles of about 2°. The system is being evaluated more fully in a series of clinical studies.