One of the best and simple "tools" I have used through the years to build up and keep in shape the muscles used in shooting a bow is simple rubber resistance bands/tubing. Just about any physical therapy center can hook you up with these. I tie a loop in both ends of the tubing to form the handles, Put both handles together, then tie a knot to be used to secure the bands. This knot can be placed in a door frame and close the door to secure. Simple and very effective.