The muscles of the upper back are easily the most neglected in the patient populations that physical therapists see. Why? Two reasons reasons come to mind.
1. Most people sit at a desk all day in a forward-pitched position. The muscles in the front of the body become over worked with reaching movements of the arms, such as with typing at a desk. The posterior muscles of the upper back are constantly in a stretched out position with deskwork and become comparatively weak.
Clinically, we will often see, as a result, shoulder blades that are pulled out to the side to accommodate the forward positions of the arms. The following photo illustrates this.
You can see from this photo the outline of the shoulder blades: In this individual, the resting position of the shoulder blades are too far away from the spine! The normal distance from the spine to the edge of the shoulder blades (depending on the size of the individual) should be about 2-4 finger widths.
2. Many people who work out, especially men, tend to enjoy strengthening the muscles that they can actually see while looking in the mirror. The Biceps, pectorals, and abdominal muscle groups are all cosmetically pleasing to see in magazine photos and at the beach. The muscles of the front and the muscles of the back, however, need to maintain proper balance in order to idealize bodily function.
I have found that using an exercise ball is a fantastic way to strengthen the muscles of the upper back. In particular, the middle trapezium, the lower trapezium, and the rhomboids can be trained effectively without using heavy weights. Take a look at the following demonstration.
The key to this exercise is persistence! Most patients (myself included) had to perform these three exercises 3-4 times weekly for several months to get a noticeable change in shoulder blade strength and control. I have progressed myself over many months to holding two-pound weights in my hands. There is no need to overload your arms with these exercises, as other muscles will try to kick in and substitute for the muscles that we are trying to train.
If you are having a particularly difficult time with these ball exercises, I would try lying on a foam roller for about 5 minutes in order to stretch out the frontal muscles and gently extend the upper back (as pre-work). The following article outlines this: Why Should I Stretch My Back on a Foam Roll?