Infraspinatus !
The infraspinatus is a rotator cuff muscle on the back of the shoulder blade; it is the main muscle that turns the arm outward and steadies the ball.
Opens in the Durgical app, no sign-up needed.
What it is
The rotator cuff muscle below the spine of the shoulder blade; it turns the arm outward.
What it does
Turns the arm outward and steadies the ball.
Key facts
- Origin
- Infraspinous fossa
- Insertion
- Middle facet of the greater tubercle
- Action
- Laterally rotates the arm; stabilises the head
- Nerve
- Suprascapular nerve (C5–C6)
- Blood supply
- Suprascapular artery, circumflex scapular artery
Relations
- Superficial
- Rear deltoid laterally and lower trapezius medially; between them a small area lies just under the skin
- Deep
- Infraspinous fossa, with the suprascapular nerve and artery entering from the spinoglenoid notch; laterally the posterior shoulder capsule
- Above
- Spine of the scapula and the supraspinatus, whose tendon merges with it
- Below
- Teres minor, with which it may be fused
How to find it
Below the spine of the scapula, between the lower trapezius and the rear deltoid, the muscle is felt tightening when the forearm is turned outward against resistance with the elbow at the side, bent 90°.
Examination
- External rotation lag sign: the elbow at the side bent 90°, the arm is placed passively in near-full external rotation and released; if it springs back inward, the infraspinatus (often with the supraspinatus) is torn
- Resisted external rotation at the side: weakness suggests an infraspinatus tear or suprascapular nerve lesion; pain with good strength suggests tendinopathy
On imaging
On MRI, oblique sagittal images compare the bulk and fat content of each cuff muscle. Oedema or atrophy of the infraspinatus alone, with a cyst at the spinoglenoid notch, points to nerve compression rather than a tear.
Common problems
- Rotator cuff tear, partial or full, from wear with age or from a fall; the most common cause of shoulder weakness.
- Tendinopathy and impingement: pain lifting the arm and at night.
- Calcific tendinitis: calcium deposits that cause sudden severe pain.
How it is diagnosed
Weakness and pain on cuff tests (for example the empty-can test). Ultrasound or MRI shows the tear size and muscle quality.
Treatment
- Physical therapy, pain medicines and sometimes injections, especially for partial and age-related tears.
- Arthroscopic rotator cuff repair with suture anchors for many full-thickness tears, especially recent ones in active people.
- Reverse shoulder replacement for large, irreparable tears with arthritis.
Clinical notes
- Posterosuperior cuff tears spread from the supraspinatus into the infraspinatus; loss of external rotation makes eating and grooming hard
- When it cannot be repaired, latissimus dorsi or lower trapezius transfers aim to restore external rotation
Anatomical variants
- May be fused with the teres minor
Development
Forms from C5–C6 segmental tissue in the limb bud. When it and teres minor fail in Erb birth palsy, internal rotation goes unopposed; a lasting contracture can push the humeral head backward and deform the glenoid in a growing child.
Related conditions
Test yourself
Think of the answer, then tap the question to see it.
What are the main action and nerve supply of the infraspinatus?
It is the main lateral rotator of the shoulder, with the teres minor; suprascapular nerve (C5–C6).
Isolated wasting of the infraspinatus with a normal supraspinatus points to what?
The suprascapular nerve at the spinoglenoid notch, often a ganglion cyst from a labral tear or traction in overhead sports.
What is the external rotation lag sign, and what does it indicate?
Elbow at the side bent 90°, the arm is placed in near-full external rotation and released; if it springs back inward, the infraspinatus is torn.
When the infraspinatus cannot be repaired, which transfers aim to restore external rotation?
Latissimus dorsi or lower trapezius transfers.
Educational information, not medical advice. For any symptom, see a health professional.
Its names
- Official Latin (TA2)
- musculus infraspinatus TA2 2458
- Official Spanish (DeCS)
- Músculo infraespinoso
- English (TA2)
- Infraspinatus
Related
Sources: Terminologia Anatomica (TA2); Anatomy, Shoulder and Upper Limb, Infraspinatus Muscle; Anatomy, Rotator Cuff.
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.
