Shoulders are one of the most complex joints in the body. With how much mobility the shoulder has, it are cursed with joint instability. This instability leads to muscle imbalances which can alter how the joint functions, which in turn causes a limited range of motion.
In our society, the biggest imbalances are caused by an over tightening in the pecs, which leads to an over lengthening of the muscles in the back, and eventually weakness in the lengthened muscles.
When we look at flexibility, it is important that we not only lengthen the over tightened tissues, to bring them to their proper range of motion, but to also strengthen the weakened tissues, to keep them the tissues from returning to their original state!