Infraspinatus Muscle

Total flexion of the shoulder joint can also be divided into three phases. In the first phase (0 to 60 degrees), the muscles used are the anterior fibers of the deltoid, the coracobrachialis, and the clavicular fibers of the pectoralis major. Motion is limited by the tension of the coracohu-meral ligament and by the resistance offered by the teres minor, teres major, and infraspinatus muscles. 

Yang WT, Chui PT, Metreweli C (1998) Anatomy of the normal brachial plexus revealed by sonography and the role of sonographic guidance in anesthesia of the brachial plexus. AJR Am J Roentgenol 171 1631-1636 Yao L, Metha U (2003) Infraspinatus muscle atrophy implications Radiology 226 161-164... 

The capsule of the glenohumeral joint is loose and pleated. The ligaments, which are merely thickenings in the capsule, provide little support. The major support of the humerus into the fossa is provided by the rotator cuff muscles, which hold the head into the fossa. These muscles include the supraspinatus, infraspinatus, teres minor, and subscapularis. Abduction-adduction and axial and horizontal rotations are coupled in that angular motions are accompanied by translatory slides. The caudal slide of the humeral head with abduction confers increased freedom of motion on the supraspinatus tendon beneath the coracoacromial ligament. 

From the anatomic point of view, the muscles of the shoulder may be subdivided into two main groups intrinsic musdes (subscapularis, supraspinatus, infraspinatus, teres minor, teres major and deltoid), which originate and insert on the skeleton of the upper limb, and extrinsic muscles, which join the upper limb with either the spine (trapezius, latis-simus dorsi, levator scapulae and rhomboid) or the thoracic wall (serratus anterior, pectoralis minor... 

Fig. 6.6). The posterior infraspinatus and teres minor muscles act as external rotators of the arm. 

Fig. 6.40a,b. Normal infraspinatus and teres minor muscles, a Sagittal 12-5 MHz US image over the infraspinous fossa demonstrates the upper infraspinatus (InfraS) and the lower teres minor (Tm), each of which is characterized by an individual hyperechoic aponeurosis (arrows), b Same scanning plane in another patient. A single posterior muscle (arrowheads) formed by the union of the two bellies of the infraspinatus and teres minor is found. Observe the wide central aponeurosis of this muscle (arrows). Asterisk, spine of the scapula. The insert at the upper left side of the figure indicates transducer positioning... 

The main function of this muscle is to abduct the arm. In conjunction with the Infraspinatus and teres minor, they stabilize the gleno humeral joint during abduction of the arm. 

---