<P> The supraspinatus muscle spreads out in a horizontal band to insert on the superior and middle facets of the greater tubercle . The greater tubercle projects as the most lateral structure of the humeral head . Medial to this, in turn, is the lesser tuberosity of the humeral head . The subscapularis muscle origin is divided from the remainder of the rotator cuff origins as it is deep to the scapula . </P> <P> The four tendons of these muscles converge to form the rotator cuff tendon . These tendinous insertions along with the articular capsule, the coracohumeral ligament, and the glenohumeral ligament complex, blend into a confluent sheet before insertion into the humeral tuberosities . The insertion site of the rotator cuff tendon at the greater tuberosity is often referred to as the footprint . The infraspinatus and teres minor fuse near their musculotendinous junctions, while the supraspinatus and subscapularis tendons join as a sheath that surrounds the biceps tendon at the entrance of the bicipital groove . The supraspinatus is most commonly involved in a rotator cuff tear . </P> <P> The rotator cuff muscles are important in shoulder movements and in maintaining glenohumeral joint (shoulder joint) stability . These muscles arise from the scapula and connect to the head of the humerus, forming a cuff at the shoulder joint . They hold the head of the humerus in the small and shallow glenoid fossa of the scapula . The glenohumeral joint has been analogously described as a golf ball (head of the humerus) sitting on a golf tee (glenoid fossa). </P> <P> During abduction of the arm, moving it outward and away from the trunk, the rotator cuff compresses the glenohumeral joint, an action known as concavity compression, in order to allow the large deltoid muscle to further elevate the arm . In other words, without the rotator cuff, the humeral head would ride up partially out of the glenoid fossa, lessening the efficiency of the deltoid muscle . The anterior and posterior directions of the glenoid fossa are more susceptible to shear force perturbations as the glenoid fossa is not as deep relative to the superior and inferior directions . The rotator cuff's contributions to concavity compression and stability vary according to their stiffness and the direction of the force they apply upon the joint . </P>

Which muscle is not a part of the rotator cuff