The thoracic spine or rib cage is a central player in shoulder mobility and health but it is also part of the entire spinal "engine" as described by Gracovetsky. One of the best descriptions I have found for thoracic function comes from a September 2007 Newsletter by Robert Burgess, BEd, PT, PhD, Feldenkrais Practitioner. He posed the question - "An immovable cage or a mobile spring?" In reference to the thorax or thoracic spine.
Think about it - with all of those ribs connecting to the spine and up front on the sternum how can there be much movement in the "rib cage"?
Well - let us examine this further...
Burgess in his newsletter cites Buchalter et al 1988 and Willems et al 1996 for data on movement in the thoracic spine. The Thoracic spine provides us with 50 degrees of rotation, 26 degrees of side bending, 25 degrees of extension and 30 degrees of flexion in sitting - "...in fact with each pair of ribs moving on the next- the thorax behaves like a hooped spring." "The pelvis and thorax rotate in opposing directions and together with side bending are the prime movers of human locomotion (Gracovetsky 1997)."
Breathing - if we are breathing correctly and using the diaphragm and intercostals for respiration there is a tremendous amount of movement in the thoracic spine. However, most people breath through their traps and upper chest which facilitates the development of kyphosis (rounding of the upper back) and a rigid rib cage.
Anatomy - But what about the ribs and the connections to the spine and sternum? Surely that means the thoracic spine is rigid - doesn't it? Not if you look at the actual anatomy of the area! The ribs connect up front on the sternum by way of a large series of cartilage joints - rather than being more solid "bone to bone" joints like the Sternoclavicular joint. These cartilage joints provide a more mobile situation and even the sternum has at least two semi-movable joints in its structure. Look up Costral Cartilages and Vertebrocostral Joints (New Atlas of Human Anatomy 1999, McCraken) if you would like to appreciate the actual anatomy of the rib cage and the reasons behind it's mobility. Think about breathing and CPR - you can compress the chest 2"+ during CPR - Where does that movement come from?
So - is the rib cage a cage or a spring?
From Dead anatomy to Living Function - Dead anatomy is a cadaver laying on a table. Living function is a miraculous interplay of the anatomical structures and the Neurological and Physiological systems of the human body. Dead anatomy is only a starting point. It cannot and will never match up to the myriad wonders of the whole integrated system. Mechanoreceptors, Golgi tendon organs, intrafusel muscle fibers, and many, many other neurologically based structures change the dead anatomy freeze frame picture into a flowing movie. Ten of the ribs have joints connecting them to the spine and have two more joints where the ribs meet the costral cartilages and then where that cartilage joins the sternum. All of these joints are alive with neurological pieces and parts that relay information to the brain. Instead of being a piece of dead rigid cage - the thoracic spine is alive with movement and function.
For the shoulder the thoracic spine is essential to the health and movement of the scapula and thereby the glenohumeral joint. If you lack shoulder movement check the thoracic spine.
What about the rest of the spine? Yes you need mobility and movement skill at the Lumbar and Cervical areas of the spine but don't forget about the Thoracic spine as an integral part of the "spinal engine".
Want to know more about the interplay of the Thoracic spine and shoulder? Please check out Secrets of the Shoulder by myself and Gray Cook - click the dragondoor.com link.