Musculus infraspinatus
The infraspinatus originates from the infraspinous fossa of the scapula and inserts on the greater tubercle of the humerus and the joint capsule. Its actions are abduction of the shoulder, lateral rotation of the humerus, and lateral stabilization of the shoulder joint. It works closely with the supraspinatus. Innervation is the suprascapular nerve (primarily C6–C7). A bursa lies deep to its strong tendon and can become inflamed. The equine shoulder joint lacks strong collateral ligaments, so the surrounding muscles (including the infraspinatus) provide essential dynamic stability.
Origin: Infraspinous fossa of the scapula
Insertion: Greater tubercle of the humerus (caudal part) and joint capsule
Action: Abducts the shoulder; laterally rotates the humerus; lateral shoulder stabilization
Innervation: Suprascapular nerve (C6-C7)
Damage to the suprascapular nerve (commonly from trauma to the cranial border of the scapula) causes atrophy of the infraspinatus and supraspinatus. This produces the classic Sweeney appearance: a prominent scapular spine and lateral instability (“popping”) of the shoulder during weight-bearing. Nerve regeneration is possible but often takes months. The infraspinatus bursa can also become inflamed.
## Overview Infraspinatus contracture is a rare condition in which the infraspinatus muscle becomes fibrotic and shortened, locking the shoulder in external rotation. The horse swings the forelimb ou
Injury to the suprascapular nerve as it crosses the cranial border of the scapula, typically from direct trauma (collision with doorframes, trees). Causes denervation atrophy of the supraspinatus and
Sudden hollowing over the shoulder blade or outward popping of the shoulder after trauma (running into a post or another horse) needs veterinary evaluation. Most cases improve with rest; severe nerve damage may leave permanent atrophy.
## Overview Infraspinatus contracture is a rare condition in which the infraspinatus muscle becomes fibrotic and shortened, locking the shoulder in external rotation. The horse swings the forelimb outward (circumducts) during protraction. The gait abnormality is mechanical and usually non-painful. ## Causes Trauma or compartment-syndrome-like injury leading to fibrosis, similar to the condition in dogs. ## Diagnosis Characteristic gait and palpation of a tight infraspinatus. Imaging rules out other shoulder problems. ## Treatment Surgical release of the fibrotic muscle in severe cases. Many horses adapt to the gait change. ## Recovery and Prognosis The mechanical lameness is often permanent without surgery. Function is usually adequate for lower-level use. ## Prevention None specific. Signs include: forelimb circumduction (swings outward); inability to advance the limb straight; mechanical non-painful lameness
Injury to the suprascapular nerve as it crosses the cranial border of the scapula, typically from direct trauma (collision with doorframes, trees). Causes denervation atrophy of the supraspinatus and infraspinatus muscles. Signs include: Characteristic lateral shoulder instability ('shoulder slip') during weight bearing; rapid atrophy of supraspinatus and infraspinatus creating a visible hollow above the scapular spine; lateral subluxation of the shoulder during weight bearing; Rapid dramatic wasting of the muscles on either side of the scapular spine; the scapular spine becomes extremely prominent; shoulder instability; the shoulder may pop laterally during weight bearing; often from running into a solid object.; Obvious muscle wasting below the scapular spine; shoulder visibly slips laterally during walking; angular shoulder appearance; often recovers spontaneously over months if nerve not severed.
Damage to the suprascapular nerve (commonly from trauma to the cranial border of the scapula) causes atrophy of the infraspinatus and supraspinatus. This produces the classic Sweeney appearance: a prominent scapular spine and lateral instability (“popping”) of the shoulder during weight-bearing. Nerve regeneration is possible but often takes months. The infraspinatus bursa can also become inflamed.
Sudden hollowing over the shoulder blade or outward popping of the shoulder after trauma (running into a post or another horse) needs veterinary evaluation. Most cases improve with rest; severe nerve damage may leave permanent atrophy.
See the Infraspinatus on our interactive 3D horse model. Open 3D Model