Strengthing Hind Leg Muscles?

My horse had injured the muscles in her right hind leg. The vet said she is sound but a horse massage women came out and worked on her and found many knots and sore spots. She is not done working on her...she is going to work on her 2 more times. But once she is done working on her and getting her muscles loosened up....what are some ways/exercises I can help her strengthen her hind end leg muscles?

Hand walking on gentle sloping hills helps a lot.

I agree. Just start her off gently and build up the workload gradually. You could lunge her on a slight incline too but don't overdo it. Walking in-hand over a few ground poles would be a good thing to build up to as well, as this would get her lifting her feet a bit more. Little and often should do nicely.