The moment arm is an important parameter for calculating forces and pressures at a joint. Present techniques described in the literature are limited for joint studies in animals because of the invasive procedures required which destroy the joint integrity. This paper describes a new application of the instantaneous helical axis (IHA) for determination of the moment arm of the knee joint without compromising its integrity. Measurements based on the IHA technique are compared to an established technique using six cadaveric cat hind limbs. One experiment using the IHA technique was performed to demonstrate its application in situ. It was found that the moment arm determined by both techniques resulted in continuous estimations of similar magnitudes over the expected range of motion of the knee joint, although the IHA technique is sensitive to smoothing. A trend toward increasing moment arm as a function of increasing knee angle was observed in four of six specimens. The in situ experiment demonstrated the ability to use the IHA technique without compromising the integrity of the knee joint so that further histological, morphological, or topological information could be investigated.