Check the ElliptiGo. It gives you a similar feeling without the impact on the knees or hip. I know someone who was an avid long life runner. He substituted running for using the ElliptiGo for years and seemed very happy with it. Surprisingly, he was able to run again. I also have the ElliptiGo, but I don't have knee problems, I started using it because at one time I was having neck pain and couldn't run, bike or swim.