Pectoralis Major provides movement and support in the front of the shoulder. The muscle has two heads; the clavicular head originates from the more midline half of the clavicle, and the sternocostal head originates from the manubrium and sternum (chest bone). This muscle inserts into the lateral lip of the intertubercular sulcus on the humerus. When the two heads of the pectoralis major act together, they flex, adduct and medially rotate the arm at the glenohumeral joint.
The trapezius muscle has an extensive origin, which includes back of the skull, and most of the vertebrae of the spine. It inserts on to the clavicle, acromion and crest of the spine of scapula. The trapezius muscle is a powerful elevator of the shoulder and also rotates the scapula to extend the reach upwards.
Latissimus dorsi muscle originates from the spinous process of the lower six thoracic vertebrae, lumbar vertebrae, sacral vertebrae, the iliac crest of the hip bone and the lower three or four ribs. It finally inserts on to the bottom of the intertubercular groove. Latissimus dorsi extends, adducts and medially rotates the arm. It also draws the shoulder downward and backward and keeps the inferior angle of the scapula against the chest wall.
Side (lateral) muscles of the shoulder:
The large deltoid muscle provides the characteristic contour of the shoulder and is the largest, strongest muscle of the shoulder. It originates in three portions, the anterior, middle and posterior portions. The anterior portion arises from the anterior border and superior surface of the clavicle. The middle portion from the acromion process and the posterior portion from the lower border of the crest of the spine of the scapula. These three portions come together and insert into the deltoid tuberosity on the shaft of the humerus. Each portion has a different action on the body. The anterior portion flexes and medially rotates the arm, while the middle portion abducts the arm. Finally the posterior portion extends and laterally rotates the arm. The deltoid muscle takes over lifting the arm once the arm is away from the side.
Diagnose your shoulder
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