The Clavicles, or Collar Bones, are a pair of thin and slightly curved, strut-like bones on either side of the sternum (breastbone) to which they articulate at the sternoclavicular joint. The clavicles also articulate with the shoulder blades, the scapulae, thus forming the pectoral girdle (shoulder girdle) from which the arms hang.
The collar bones also act as shock absorbers that can transfer forces from the arms through into the axial, or central skeleton, for instance when we fall down. The collar bones tend to be easily broken but rarely become dislocated.
The sternoclavicular joints are the pectoral girdle's only link with the main part of our skeleton, the axial skeleton and they allow each clavicle some mobility in response to arm and shoulder movements. The existence of this joint comes as a surprise to many people when they have Alexander lessons and begin to learn about how to use their arms more freely and to bring about changes to their posture.
The clavicles help to support the arms and they push each shoulder blade outwards and into position, thus forming the shoulders. Therefore, when someone is learning to stop a habit such as being round shouldered, they need to be aware of their movable clavicles and to free up the muscles that have been habitually contracting and pulling their sternum and clavicles - and therefore their shoulders - downwards and inwards. No amount of forcing the shoulder blades back will work for long, if habits such as this are not stopped / inhibited.
When we stop contracting forwards and down, then the shoulders have a chance to drop back into their natural place and to widen out which allows us to become more poised and balanced once again.