We have developed a walking model that safely and reproducibly induces pain in subjects with knee OA in a manner that permits the comparison of the effect of different therapies on pain control. In a previous study, we demonstrated that a self-paced 20 minute walk followed by a one-hour rest and a second coordinator-paced 20-minute walk was well tolerated and generated moderate to severe pain in all participants within the time frame of the walk. We will now use this model, but with a crossover design to test the efficacy of celecoxib compared to placebo for the control of knee pain and improvement of walking function.