My vet recommended this exercise for stifles. Set a muck tub upside down and place 4 poles on top so they’re slanted like one-sided cavaletti. Kind of like this picture but with the poles spread out evenly at 12:00, 3:00, 6:00, and 9:00.
You start out by walking around the edges of the poles where they touch the ground, and slowly spiral in so the horse has to lift his inside hind leg higher to avoid hitting the pole. It’s supposed to be good at helping strengthen the inside hind leg without putting too much stress on anything else.
Hope this helps!