so the shoulders rest on top of the ribs, big deal….
Yes it is a big deal because the ribs are supposed to be buoyant. I refer to the rib cage as a rib slinky. Between each set of ribs there are three sets of muscles diagonal left, diagonal right, and vertical, when fully stretched, functional and strong they can create a powerful bellows effect for the lungs (which have no muscles themselves) . Ribs should move up and down, in and out, side to side. They allow for rotation with one side compressing while the other side expands, turns, bends.
Your thigh has one bone that is longer than the whole of the thoracic unit which contains more than thirty bones. Why so many bones? just as the foot and hand have so many bones because of the movement demands on them, the ribs were intended to be flexible and very mobile. they compress and expand. the upper ribs are connected front and back but your lower ribs are only attached in back, this allows them to move out of the way when you bend forward to tie your shoes and to really open out lake a gate when you take a large expansive breath. Your diaphragm muscle attaches to the underside of the ribs, thoracic vertebrae and the sternum providing for a canopy like expansion to take place during singing and breathing.
Eyes must rest on a horizon, that’s a neurological rule; if the neck is off center because of damage or poor posture, the ribs may compensate allowing the shoulders to remain relatively level. the later dysfunction in the shoulders as the ribs become less mobile, because Wolfe, Davis and Starling’s laws can work against our conscious goals.
Want to try something? hold your ribcage still and swing your arms… Hold your arms still and rotate your rib cage. Notice you can turn your ribs using your ankles, knees or hips and keep your shoulders almost perfectly still. not move them together, hold your ribcage at the furthest possible point of rotation, and add shoulder retraction, pulse this and find you move further and further still because at that end range, your rib cages is again in a fixed position and the shoulder blade can move on it once again. try doing this with your arm at different levels and different positions. I am always amazed at what tightnesses I find and can relieve in my own body when I do this as an exploratory exercise.
Freeing up the rib cage allows greater freedom of the arm. Especially if you lengthen the latissimus dorsi to decrease its compressive forces on the rib cage
Change your rib cage change your shoulders