Well, theres a lot of ways to gain muscle. Even just lunging daily can really help build up back muscles and trip up that belly. Daily riding helps too, up hill trail rides help legs, back, chest and butt. Even simple backing in hand helps build butt muscles. A lunging device I have used works wonders! I don't remember the exact name...I just called it the 'neck streacher'. All it is, is a surfcingle with two elastic cables that clip onto your horses bit, and run between the front legs and then connect the the girth on the surfcingle. Not only does it build lots of muscle it also encourages low head carriage. If you do use one or something similar, be careful and start out slowly. It takes a LOT of muscle and balance to move in a lower head carriage.