Supraspinatus (Rotator Cuff)
The supraspinatus is the uppermost muscle of the rotator cuff. Its origin is the body of the scapula above the acromion process. Its insertion is at the top of the outside edge of the humerus - known as the greater tubercle.
Its primary function is abduction of the humerus up to 30° of abduction (laterally raising the arm). After 30°, the deltoid takes over abduction.
Functionally, the supraspinatus opposes the comparatively weak gravitational force. Follow-throughs with large downward components create forces much greater than gravity. The stress it creates is magnified by powerful internal rotation because it moves the muscle's insertion further from its origin.
It plays a very small role in external rotation.
Finally, the supraspinatus is also important for joint stabilizatoin, holding the humerus firmly against the glenoid fossa.