Hey folks, I had an interesting discussion with my physical therapist last year when I was recovering from my third shoulder surgery (my shoulder just seems prone to bone spurs, and about every two years I have to get the joint shaved down before the spurs cut into the rotator cuff too much). After testing my muscle strength, she said that I had a huge imbalance between the development of my upper traps and my middle and lower traps. In other words, my upper traps were "abnormally strong" (her words, not mine), but as a result, they were doing all the work and my lower traps had atrophied and were very weak. This led to unequal tension on the shoulders during upper-body movements and added to my rotator cuff problems.
It was immediately apparent to me that the cause of the imbalance was the fact that I shot trad archery, which relies heavily on the upper traps and rhomboids to draw and hold the bow. For my recovery, she stressed several exercises to develop the lower traps and correct the imbalance. I share this here because I suspect a lot of us have this imbalance and may not know it. After I recovered from the surgery and resumed shooting, I noticed that the increased strength in my lower traps helped even out the tension in my back and allowed me to hold the bow at full draw longer without any trembling. I was able to hold my back straighter and firmer, and my posture (both while shooting and when just sitting around) is noticeably straighter.
If you think you may have an imbalance, try this simple exercise. Lay on your stomach and stretch your arms above your head with your hands clasped together. Squeeze your shoulder blades together, and lift your arms about an inch off the ground (or bed, or whatever you're laying on). Hold for ten seconds and then slowly lower your arms. Do this fifteen more times. The first time my PT had me do this, I couldn't do more than four reps, and I was shocked at how sore I was afterwards.
Give it a try and see what you think. I do this exercise (along with a general strengthening routine) three times a week, and it's definitely made me a stronger shooter and helped my form.