I think Peter may have hit the nail on the head, but I also wanted to add another thought.
I think that a lot of styles of AiKiDo stress movements which are very hard on the joints. I would include many of the more mainstream AiKiDo styles within this -- from what I've seen, ASU, AiKiKai and Yoshinkan all have this tendency. In truth, though, the elderly practitioners (the 'shihans') of these styles often revert to more natural and more comfortable movements. You wil be told that only by using exaggerated hanmi and super-powered hip action will you be able to understand how the more natural, smaller movements work. Perhaps that is true, but in the interests of your own body it may be worth exploring the possibility that easy, comfortable and natural movements may provide a framework for the same learning that the big powerful movements do.