Possible Loose Stifle?

We started my OTTB working in dressage. My trainer is riding him 3 days a week. She works him for 1 hour. She has been really working his hind end. Today she noticed a slight slip in his stifle. I am worried. She feels he is out of shape. He has never used his hind end before and doing well. She thinks he is sore. We have been working on backing him several times each ride. He is ridden everyday and gets one or two days off per week. Has anyone experienced this before.

I plan to work him light tommorrow without backing. I have also been rubbing him down with a linament after his workout. If he does not improve I will have my vet look at him. Any words of experience is appreciated. Thanks!

It is very hard to tell just from a description. But especially with an OTTB, I would not wait to have a vet come out. He could have sustained an injury from his racing days that is bothering him now. It will do you no good to have a sore horse in training, as it will eventually sour him because of pain. I would personally rule out anything and postpone the training until you are sure he is sound.

Stifle issues can be tricky to deal with, depending on what the issue is. My horse had an issue with his back leg, I thought stifle injury. Turns out it was the hock, he has arthritis and needs injections and he's only 7. Just goes to show you never really do know unless you get a vet out to tell you exactly what is wrong.