The purpose of this research study is to compare post-operative knee function of persons who have undergone one of three types of knee arthroplasty for the treatment of knee osteoarthritis. A variety of functional performance tests will be assessed and compared using a group of evaluations including: functional activity performance tests and questionnaires; leg strength; knee range of motion; and forces at the knee during a series of activities (walking, bending, stepping etc.) via a motion analysis system. The study will compare the three surgical subject groups and one group of healthy, non-surgical subjects for a total of 100 subjects.