The upper extremity is connected to the axial skeleton and thoracic cage by the shoulder girdle. The unique arrangement of the skeletal and soft tissue structures of the shoulder allows for the greatest range of motion of any joint in the human body. For these same reasons, the shoulder joint is the least stable of all joints making it prone to dislocation and instability.

The glenohumearal joint has a greater range of motion than any other joint in the body. The small size of the glenoid fossa and the relative laxity of the joint capsule renders the joint relatively unstable and prone to subluxation and dislocation.

Range of motion:

Flexion, extension, abduction, adduction, circumduction, medial rotation, and lateral rotation.
* Flexion: Pectoralis major, deltoid, coracobrachialis, & biceps muscles
* Extension: Deltoid & teres major muscles.
– If against resistance, also latissimus dorsi & pectoralis major.
* Abduction: Deltoid & supraspinatus muscles.
– Subscapularis, infraspinatus, & teres minor exert downward traction
– Supraspinatus contribution controversial 
* Medial rotation: Pectoralis major, deltoid, latissimus dorsi, & teres major muscles.
– Subscapularis when arm at side
* Lateral rotation: Infraspinatus, deltoid, & teres minor muscles.

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2. The osseous structures of the shoulder girdle are the clavicle, scapula, and humerus. Medially, the clavicle articulates with the manubrium of the
sternum at the sternoclavicular (SC) joint. This joint serves as the only true articulation between the shoulder girdle and the axial skeleton. Laterally, the clavicle articulates with the acromion process of the scapula at the acromioclavicular (AC) joint

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