wake boarding for back and shoulder muscles,, the bull riders swear by this!
seriously though find a super cheap bow 10# more than your hunting bow and that is a excellent work out tool, inside the house 10 reps of 10 very slow with extra long hold.
your hunting bow will feel like a kids bow in a few weeks.
an old friend made a cool bow workout tool after a trip to a cheap hardware store.
find a tree with a decent overhead branch,, ti rope around branch with a pulley hanging down.
now screw an eye hook into a cut log that you think is heavy enough,,,,now run rope thru pulley and down to eye hook in log.
stand back where the rope is now tight and pull it like your drawing which lifts up the log,,,, trim log with chain saw to get the weight right,,,, I'm sure you get the idea but it works great and this guy shot a 72# bow.