Hi Force 10, I'm going to make a few assumptions here and just fire a few possible reasons, so please tell me if I'm wrong. You mentioned the gym so I'm guessing you lift fairly often. How are your pecs? Tight pecs, particularly upper fibres which attached to the coracoid process (should blade) can protract and upwardly rotate the shoulder clade and therefore effect the scapulothoracic joint and affect the function of the muscles on your back which may result in pain and discomfort which you are feeling. Also I'll assume you have a forward head posture when looking from the side which will effect the cervical spine, thoracic sine and therefore the muscles in your back. Specifically muscles as you asked, the picture sort of outlines the attachments of trapezius, but when people talk about 'should blade pain' typically it can be the rhomboids, levator scapula (particularly in forward head posture) and the erector spinae in that region which if you have a slight curve in your mid back (kyphosis) will be perpetually lengthened and not working efficiently which will result in trigger points and pain. You mention breathing a few times so I think your thoracic spine is probably stiff and therefore can't expand to allow you to breathe properly so a deep breathe will cause discomfort. Treatment wise I could go for deep tissue release of your upper pecs, and shoulder blade muscles, mobilisation or manipulation of your thoracic spine and do some low level strengthening of your core. It sound slike your abs and big prime movers in the core are probably strong but you may need to gain control of the small muscles. Stuff like this youtube video Beginner Pilates - 4 Point Kneeling arm and leg extensions - YouTube Hope this helps! Ben