I would actually NOT recommend circle work until the muscles are strengthened! Circles are VERY hard on a horse and too much could do more harm than good. Lots of long and low work, hill work, trail rides. The more trails the better. 24×7 turnout if possible, ideally on a slight slope.
You could try an injectible like Adequan, depending on the actual issue it may benefit the horse.
I have an older TB (retired) who used to have this issue. With lots of turnout on slight hills he has not had a problem.
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