From my experience in the game,(mostly high school, and a little college) shoulders are important! It's an explosive game when you get into it from the waist up. I was an Offensive Lineman, Defensive End and got some time at Fullback. They all had upper body strength demands. As an O-Lineman I found that I need strong shoulders the most during cup and other zone pass protections, because you have to stabilize your oponent's momentum into a box that you create. The action of extending your arms and controlling is very stressful on your shoulders, so it helps to beat them up and get them strong in the off season. Also Wide recievers need strong shoulders to push off and swim through and over defenders. IN fact I don't know a position on that field besides specialists that don't need strong shoulders! As fullback, you needed the muscle padding so you could make your shoulders level when intitiating the hit or opening the block. Carrying a ball, popping tacklers and driving stiffarms into people was very shoulder intrusive. However, as all football players would say. The base (waist down) has to be really strong in order to drive. Next is back and chest for the push ,and then everything else has to work in a support system to the majors!