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Anatomy of the Shoulder Joint and Shoulder Girdle

Posted by KinX Learning on June 30, 2017 . 0 Comments

The shoulder joint is the most freely movable of all the body's ball-and-socket joints. Because of this, the greatest variety and combination of movements at a joint can be executed by the arm from the shoulder joint. 

The bony arrangement of the shoulder joint consists of a shallow socket (glenoid fossa) into which the spherical head of the humerus fits. More specifically, the shoulder joint is formed by the articulation of the glenoid fossa of the scapula (shoulder blade) and the head of the humerus (upper arm bone). It should be noted that less than half of the humerus is in the socket at any time. Because of this, the bony arrangement is very weak and therefore the strength of the musculature around the shoulder is very important for stability. 

It is impossible to talk about the movements of the shoulder joint without also discussing the shoulder girdle, which consists of the scapula and clavicle (collar bone). The clavicle joins the sternum (breast bone) at the sternoclavicular joint, which allows for full-range movement of the outer (shoulder) end of the clavicle. The outer end of the clavicle joins the scapula at the acromion in what is known as the acromio-clavicular joint. 

Since the clavicle cannot move by itself, movements of the shoulder girdle are usually referred to as movements of the scapula that can be moved in all directions. Thus, scapula movements allow for a greatly increased range of motion in the shoulder joint by changing the position of the joint.  

Because the shoulder is designed for mobility, its stability is reduced. The muscular arrangements of the shoulder girdle and the shoulder joint are such that they provide the stability lacking as a result of the weak arrangements of the bones and ligaments. However, the muscles must be strong enough to provide the necessary stability. A lack of upper body strength accounts for many of the injuries in the shoulder region. 

Injurious stress on the ligaments and muscles of the shoulder girdle is possible if the stabilizing components of the muscles are not strong enough to hold the joint together. Also, since the shoulder girdle is fairly mobile relative to the trunk, in many instances it must become a stable base against which the muscles of the shoulder joint pull. During forceful overarm motions the strength of the agonist and antagonist muscles surrounding the shoulder girdle prevents overuse strains on the surrounding tissues. 

In most activities involving the upper extremity the shoulder girdle is responsible for the initiation of the movements. For example, elevation of the scapula initiates lifting the arm; depression precedes pulling the arm downward; protraction occurs before reaching, throwing or pushing forward; retraction initiates pulling backward; upward rotation takes place for increasing the range of overhead reaching; and downward rotation for forceful arm adduction at the shoulder joint.