It took me over 20 years to injure my shoulders. My right shoulder is hurt now. I still shoot 70lbs, but don't shoot 500 arrows a week any more.
The trad bows seem to work easier on the shoulders. I hear folks say they would like to shoot trad but have to shoot wheels due to hurt shoulders. But if they tried they would find that with a wheel bow you are holding more wieght half way thru the draw than any tradbow. And this is a critical place to hurt the shoulders. By the time I get to full draw all the weight is on my back with a longbow.