i do alot of rear deltoid exercises and trap and lat exercises these are the musles you should be using to pull back and hold. they are much bigger and more powerful than the muscles in the upper front torso. unless you over train your front torso and then your back torso is much weaker than your front and you actually end up rounding your shoulders forward because your pecs are pulling on your shoulders more than your rear deltoids and other shoulder muscles can hold back. so start light and work your way up. shoulders are the weakest joint in your body so you need to use a little caution with them.