The infraspinatus muscle occupies the infraspinous fossa, and crosses the lateral part of shoulder joint ot the roughened area of the greater tubercle of humerus. It acts as a lateral collateral ligament of the shoulder joint.
Origin: Infraspinous fossa, scapular spine and the caudal border of the scapula.
Insertion: Distal to the greater tubercle of the humerus.
Action: Outward rotator and abductor of the humerus and a flexor or extensor of the shoulder joint, depending on the position of the joint when the muscle contracts.