Infraspinatus muscle

The infraspinatus muscle occupies the infraspinous fossa, and crosses the lateral part of shoulder joint on 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.

Antagonist: Shoulder medial rotators, primarily the subscapularis and pectoralis major.

Vascularization: Supplied mainly by the suprascapular artery and the subscapular artery; venous drainage parallels arterial supply.

Nerve: Suprascapularis.

The infraspinatus arises from the entire infraspinous fossa, including the scapular spine and adjacent neck, forming a thick, triangular, fan-shaped muscle. Its fibers converge laterally to insert on the lateral aspect of the greater tubercle of the humerus. Functionally, it acts as a lateral rotator and stabilizer of the shoulder joint during weight-bearing and locomotion. It is located caudal to the supraspinatus, superficial to the scapular fossa, and dorsal to the shoulder joint capsule. The arterial supply is mainly via the suprascapular and subscapular arteries, and the muscle is innervated by the suprascapular nerve. In comparison to humans, the canine infraspinatus is relatively more robust and plays a larger role in weight-bearing stabilization.

Variations:

• In ruminants, the infraspinatus is a dominant and powerful muscle, with an independent tendon insertion that primarily stabilizes the weight-bearing shoulder during locomotion.

• In horses, the muscle is relatively large, originates on the caudal scapula, and inserts on the humerus; its main function is to stabilize the shoulder while allowing limited abduction and rotation of the forelimb.

• In domestic carnivores, the infraspinatus is less dominant than the subscapularis, with an independent tendon insertion, and contributes to both lateral rotation and passive stabilization of the shoulder joint.

• In humans, the infraspinatus is adapted for lateral rotation of the non–weight-bearing upper limb, with smaller muscles relative to body size and fused tendons forming a true rotator cuff to enable high mobility for manipulation and grasping.