Infraspinatus arises from beneath a dense fascia from the medial three-fourths of the infraspinous fossa. Fibrous intramuscular septa give added attachment to the multipennate fibers, which converge to slide freely over the bare area of the scapula at its glenoid angle. A bursa lies between the two and it sometimes communicates with the shoulder joint. Here tendon replaces the muscle fibers, and is blended with the capsule of the shoulder joint, greatly increasing its strength. The tendon is inserted into the smooth area on the central facet of the greater tuberosity of the humerus.
Nerve supply of infraspinatus:
From the suprascapular branch (C5 and 6) of the upper trunk of the brachial plexus. Passing under the transverse scapular ligament and giving branches to supraspinatus, the nerve passes around the lateral border of the scapular spine and enters the infraspinous fossa to give two branches to the muscle.
Action of infraspinatus:
Apart from acting to brace the head of the humerus against the glenoid fossa, giving stability to the joint in movements of the peripheral parts of the limb, the muscle is also a powerful lateral rotator of the humerus.