The ability of the quadriceps muscle to extend the knee depends on the patellar tendon moment arm (PTMA). In this study, the patellar tendon moment arm of ten subjects were measured in-vivo, 6 - 18 months after anterior cruciate ligament (ACL) reconstruction. PTMA may be found once the line of action of the patellar tendon force and the instantaneous screw axis of rotation of the femur with respect to the tibia are known. Mobile biplane x-ray imaging was used to find the three-dimensional position of the bones in-vivo as the subjects performed level ground walking and downhill walking. PTMA was compared between the ACL reconstructed knee and the contralateral knee. In this seminar, the results of this study and the effect of ACL reconstruction on the function of the knee joint will be discussed.