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Describes a new surface-marker imaging system designed to measure the rotational axis pathway (RAP) of a moving knee in the sagittal plane. Measurement of this parameter can provide important information about a knee's slipping and rolling action that can aid clinical assessment. Seated subjects are video recorded as they actively extend their legs. A series of stills is then captured and analyzed to extract the coordinates of markers plated on the subjects: upper and lower legs. These coordinates are then processed to deduce an instant center of rotation (ICR) for each still. These ICRs are then plotted to derive the joint's RAP. The system has been validated with a mechanical model and tested in a clinical study of ten patients with unilateral anterior cruciate ligament (ACL) ruptures. The study found that the system could consistently measure differences between a patients normal and injured knees. Leg extension caused the normal knees ICRs to displace anteriorly with a mean value of 17.4 mm, whereas the injured knees had a mean displacement of 7.5 mm. This loss of roll in the ACL-deficient knees is consistent with their abnormal biomechanical arrangement.