The shoulder girdle is four bones, two collar bones (clavicle) and two shoulder blades (scapula). These four bones make up the shoulder girdle as well as the shoulder socket. The shoulder girdle ideally hangs from the head suspended over the rib cage without either the collar bones or scapula touching the ribs.
The shoulder girdle has only one bony connection to the trunk at the front of the chest— the sterno clavicular joint where the collar bones meets the sternum. At the back of the body the shoulder blades are connected by the rhomboid muscles that stretch from the outer edge of the shoulder blade to the spine. As a result the shoulder girdle is two pair of bony units- right scapula and clavicle and left scapula and clavicle— as opposed to the seemingly solid pelvis.
The shoulder girdle is structurally similar to the pelvis but functionally different. The shoulder girdles mobility in the arms vs. the weight support function of the pelvis blurs their similarities. Because it is lighter and bound in only one place without firm support the shoulder girdle is more vulnerable to deformation then the pelvis. And being farther from the ground it offers gravity greater leverage for destruction.
The shoulder is made up of three bones—the collar bone, the shoulder blade, and the humerus, the arm bone. The direction and quality of arm movement is determined by muscles that form a bridge between shoulder and humerus. If pectoralis major is too tight the head of the humerus will be rotated in. If the latissimus dorsi is tight the humerus will be rotated externally. Teres major also has a role in determining the arms movement potential.
The shoulder girdle is designed to allow for very free movement of the arm and shoulder because in standing the upper arm is no longer involved in weight bearing. As a result its stability has been sacrificed for a greater range of motion. While the hips socket is a solid cup for the head of the femur to sit into, the shoulder socket is only a loose structure formed by the acromial and coracoid processes of the shoulder blade.
The arm and shoulder are so loose that a strange thing happens to allow for all of the available range of motion. Tendons of certain muscles take on the role of ligaments that would be too strong and tight to enable to ultimate freedom that the shoulder allows. This section of the arm and shoulder muscles is known as the rotator cuff which I’ll explain in a later post.
To a certain degree the shoulder girdle, upper back, and neck form a single movement system. They all reciprocally influence all of the other elements of the body.