There are several muscles used in this procedure, but the main muscle used in drawing a bow by far is the trapezius muscle(traps) extending from the top of the shoulder all the way to the spine mid way down the back near the shoulder blade. Others include various small deltoid muscles, lats, forearms, triceps and biceps. Basically, if you can get an overall upper body workout you will work the muscles used in drawing a bow. I remember reading an article about working out your archery muscles with a bow in Instinctive Archery magazine in the sample issue if you click on the classifieds. Good info in there. Cheers, Matt