Muscle training for bow shooting

Hello everybody, I'm totally new on this site, but what i already have seen...it is great. Now my question: I am starting in shooting bows, now i want to increase the weight of my bow and because of that my muscles of my back. Now i don't know which muscels are really important for that. Is it the Dorsal or the Trapezius?...or both? My arms have an angle of 90° to my body and then I must pull my shoulders together. thanks in advance Vincenzo