Lots of collection, walking and trotting. It is best to do this in a nice 6-8 inch deep arena footing or finding yourself a nice sandy wash. If your horse will allow you can drag logs. But the best is walking and trotting in nice deep footing. To build muscle you also need to make sure the horse is getting enought protien in their feed. Hallow muscles are a due to lack of enough protien.

Hope this helps