Leg stiffness?

I am considering buying an 11 year old paint gelding. When he moves (mainly trot or canter) it seems as if his right hind hock is stiff. All of his other legs seem fine and he doesn't act like that leg bothers him. This stiffness is hardly noticeable. He does not limp or act in pain.

The owner said that he has done this since she bought him 9 years ago. He is a very good horse but I do not want to run the risk of having problems later in life if I choose to buy.

I only was able to see the horse for about an hour. The owner seems to be reliable but she could be lying just to try to sell the horse. She says that they have done Xrays and the vet found nothing.

Any thoughts on what is wrong?
Thanks.

Nope, but I wouldn't buy him. Could be arthritis. But at 11...he had to have had a pretty active career to have constant stiffness, or so I would think. Keep in mind, she could have him on bute and the "stiffness" you see, could actually be a lot worse. I would pass if it were me.