Infraspinatus: Origin, Insertion, Nerve, Action & Rotator Cuff Role

The Infraspinatus is a thick triangular muscle occupying the chief part of the infraspinous fossa of the scapula. It is the "I" in the SITS acronym for the Rotator Cuff. It is the primary external rotator of the shoulder and is crucial for posterior stability of the joint.

[Image of Infraspinatus muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Medial two-thirds of the Infraspinous Fossa of the Scapula.
Insertion (Distal)	Middle facet of the Greater Tubercle of the Humerus.
Nerve Supply	Suprascapular Nerve (C5, C6).
Blood Supply	Suprascapular and Circumflex Scapular Arteries.
Primary Actions	External Rotation: Primary rotator of the humerus. Dynamic Stability: Holds the humeral head in the glenoid cavity.

Deep Dive: The Middle Facet

As part of the Rotator Cuff, the precise insertion of the Infraspinatus is important to understand relative to its neighbors.

1. The "SITS" Relationship

On the Greater Tubercle of the Humerus, the rotator cuff muscles insert in a specific order (Superior to Inferior):
• Supraspinatus: Superior facet.
• Infraspinatus: Middle facet.
• Teres Minor: Inferior facet.

2. The Nerve Path

The Suprascapular Nerve enters the supraspinous fossa through the suprascapular notch, supplies the Supraspinatus, and then winds around the Spinoglenoid Notch to supply the Infraspinatus. Cysts or ligaments at this notch can compress the nerve, causing isolated atrophy of the Infraspinatus.

Physio Corner: Clinical Relevance

💪 Functional Fact: The Infraspinatus is the primary "braking mechanism" (decelerator) for the arm during the follow-through phase of a baseball pitch. This eccentric load makes it highly susceptible to strain in throwing athletes.

Palpation

Locate the Spine of the Scapula. Move your fingers inferiorly into the large fossa below the spine. Ask the patient to externally rotate their arm against resistance (push wrist out). You will feel the muscle belly contract beneath your fingers.

⚠️ Clinical Pathology: Referred Pain
Surprisingly, trigger points in the Infraspinatus typically do not cause pain in the back of the shoulder. Instead, they refer pain deep into the front of the shoulder and down the lateral arm. This is often misdiagnosed as Bicipital Tendinitis.

Manual Muscle Testing (MMT)

We test external rotation with the arm at the side to bias the Infraspinatus over the Teres Minor/Deltoid.

Testing Tip: Place a towel roll between the elbow and the torso. This prevents the patient from "cheating" by abducting the arm to mimic rotation.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Prone (face down) with arm abducted 90° and elbow hanging off edge (gravity resisted) OR Sitting with elbow at side flexed to 90°. Action: Patient rotates the forearm outward (External Rotation). Resistance: Applied at the distal forearm (near wrist), pushing inward (Internal Rotation). Grade 3: Full range against gravity (prone). Grade 4/5: Holds against resistance.
Grade 2 (Gravity Eliminated)	Position: Sitting with arm supported on a table, elbow flexed 90°. Action: Patient slides forearm outward across the table. Result: Full range of motion.
Grade 0, 1 (Palpation)	Action: Palpate inferior to the scapular spine. Cue: "Try to turn your arm out." Grade 1: Contraction felt. Grade 0: No activity.

Frequently Asked Questions

How do I distinguish it from Teres Minor?

Functionally, they are synergists (both externally rotate). Anatomically, Teres Minor is lower and supplied by the Axillary Nerve, while Infraspinatus is higher and supplied by the Suprascapular Nerve.

What is the "Hornblower's Sign"?

This is a test for massive tears of the posterior rotator cuff (Infraspinatus/Teres Minor). The patient cannot externally rotate the arm when it is abducted to 90°, so the hand drops in front of the mouth (like blowing a horn).

Does Infraspinatus abduct the arm?

It assists slightly in abduction by depressing the humeral head, allowing the deltoid to work, but it is not a primary abductor. Its upper fibers may contribute slightly.

Test Your Knowledge: Infraspinatus Quiz

1. Which nerve supplies the Infraspinatus?

2. Where does the Infraspinatus insert?

3. What is the primary action of the Infraspinatus?

4. Infraspinatus trigger points commonly refer pain to the:

5. Which muscle acts as a synergist to the Infraspinatus?

6. The Suprascapular nerve passes through which notch to reach the Infraspinatus?

7. To test MMT Grade 5, resistance is applied to the:

8. The Infraspinatus is part of which muscle group?

9. Where does the Infraspinatus originate?

10. True or False: The Infraspinatus is an Internal Rotator.

References

• Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2018). Clinically Oriented Anatomy. 8th ed. Philadelphia: Wolters Kluwer.
• Standring, S. (2016). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 41st ed. Elsevier.
• Travell, J. G., & Simons, D. G. (1999). Myofascial Pain and Dysfunction: The Trigger Point Manual. 2nd ed. Williams & Wilkins.