I have found that pulling a bow of considerable weight back constantly and consistently requires a very specific set of muscles, muscles that other day-to-day activites don't generally target. Those who exercise at the gym and some other jobs, hobbies, or repeatitive tasks can get an advantage but for the vast majority only constantly and consistently drawing a bow will condition the muscles. That's a very long-winded way to say "practice, practice, practice". haha Keep at it again, the muscle will come. If you don't drop in weight at least incorporate stretching and conditioning into your shooting. Back and shoulder muscles are quickly lamented when we injure them!