I over corrected my OTTB's right lead to the point where he now prefers it. Side reins are good for helping build back muscle, but I wasn't thinking of it as a back issue. I took it from a stand point of the rear leg not being strong enough since race horses don't use it much. I used a lot of shoulder-fore and haunches in to strengthen his left rear hind leg. It took about 6 months before he was cantering as much on his right lead as he could on his left, bue eventually he balanced out. Now I can barely tell which lead he's on sometimes.