Bear in mind it's the bow arm that really lets people down! It's much easier to stand upright and heave a cable/string back to 120# for instance, if you're able to use your whole body. Once you are holding the other end of the cable with the bow arm it becomes very, very hard.
Triceps and forearms need to be built up on the bow arm in order to withstand the massive compressive forces they have to deal with when you're shooting heavy bows. There's an interesting trick you can do with a friend, if you hold a heavy bow and as you're about to draw it back, get your friend to hold your wrist, arm or the bow itself just above the arrow pass. Get them to hold it firmly, and you'll suddenly be pulling back way further than you could normally.
If you wanted to isolate muscle groups deliberately to increase bow draw strength, work on the bow arm triceps and biceps and forearms with grip exercises as well as the usual curl type exercises for upper arms. If you couple those with shoulder, chest and upper arm exercises on the drawing arm, you'll get the correct bias for warbow-style weights. It's one of those funny, asymmetrical exercises that really you can only replicate properly by shooting heavy bows.
