 The purpose of this preliminary substudy to the parent study Aerobic Exercise Intervention for Knee Osteoarthritis is to determine the repeatability and reproducibility of various measurement techniques. The study will enroll healthy volunteers. All participants will receive x-rays and MRI examinations of the knees. Walking characteristics will be evaluated in a Motion Analysis Lab; this will involve taping reflective markers to participants skin and having them walk on a level surface and up and down stairs. All participants will be evaluated two times, with a 1-week period between evaluations. 