The purpose of this study was to obtain lines of action and moment arms in the sagittal plane of the major force-carrying structures crossing the knee joint. The muscles and ligaments studied were the quadriceps, biceps femoris, semimembranosus, and semitendinosus muscles and the anterior and posterior cruciate and medial and lateral collateral ligaments. All lines of action and moment arms of the structures of interest were determined as a function of knee joint angles and were expressed using polynomial regression equations. This representation of the results allows for easy application of the findings to musculoskeletal models of the human knee joint.