The scapula is a large flat triangular-shaped bone that lies in the upper back. Along with the clavicle and sternum it forms the shoulder girdle, which transfers force between the upper arm to the axial skeleton.
The scapula helps to form two joints in the body. The glenoid cavity of the scapula is a shallow concave surface that articulates with the head of the humerus forming a ball-and-socket joint known as the glenohumeral joint. The scapula also has an atypical articulation called the scapulothoracic joint where the scapula slides over the ribcage. The glenohumeral joint and the scapulothoracic joint are together called the shoulder complex. When lifting overhead, the scapulothoracic joint contributes to about one-third of the motion while the glenohumeral joint contributes the remaining two-thirds. The scapula functions as a mobile base for the multiple muscles controlling the upper arm. Movements of the shoulder complex require highly coordinated function of these muscles.
