Supinator: Origin, Insertion, Nerve, Action & The Arcade of Frohse

The Supinator is a deep muscle of the posterior forearm that wraps around the upper third of the radius. As its name implies, it turns the palm upward (supination). While the Biceps Brachii is the powerhouse supinator, the Supinator muscle is the "always-on" worker that handles low-load rotation.

[Image of Extensor Indicis muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Lateral Epicondyle of Humerus. Radial Collateral Ligament & Annular Ligament. Supinator Crest of the Ulna.
Insertion (Distal)	Lateral, posterior, and anterior surfaces of the proximal third of the Radius.
Nerve Supply	Posterior Interosseous Nerve (Deep branch of Radial Nerve) - C6, C7.
Blood Supply	Radial Recurrent Artery and Posterior Interosseous Artery.
Primary Actions	Supination: Rotates the radius to turn the palm anteriorly (up).

Deep Dive: The Nerve Tunnel

The Supinator consists of two layers of fibers (superficial and deep), and the relationship between these layers is clinically critical.

1. The Arcade of Frohse

The Deep Branch of the Radial Nerve enters the forearm and dives between the two layers of the Supinator muscle. The superior fibrous arch of the superficial layer is called the Arcade of Frohse. This arch is the most common site for compression of the Posterior Interosseous Nerve (PIN).

2. Biceps vs. Supinator

Why do we have two supinators?
• Supinator: Works in all elbow positions (flexed or extended) and for slow, low-resistance movements.
• Biceps Brachii: Only works effectively when the elbow is flexed and high power/speed is needed (like using a screwdriver).

Physio Corner: Clinical Relevance

💪 Functional Fact: If you supinte your hand with your elbow straight, you are using your Supinator. If you bend your elbow to 90 degrees and supinte forcefully, you recruit the massive Biceps Brachii.

Palpation

Palpation is difficult as it lies deep to the extensor muscles.
1. Locate the Lateral Epicondyle and the head of the Radius.
2. Move distally and posteriorly on the forearm.
3. Push the extensor muscle group aside gently.
4. Ask the patient to supinte slowly against resistance with the elbow extended. You may feel deep tension over the proximal radius.

⚠️ Clinical Pathology: Supinator Syndrome (PIN Entrapment)
Compression of the nerve at the Arcade of Frohse mimics Tennis Elbow (lateral elbow pain).
Key Difference: Tennis Elbow usually has pain at the epicondyle. Supinator Syndrome has pain about 4cm distal to the epicondyle and may cause weakness in finger extension (Finger Drop) without sensory loss (because the PIN is purely motor).

Manual Muscle Testing (MMT)

To isolate the Supinator, we must mechanically disadvantage the Biceps Brachii.

Testing Tip: Fully Extend the Elbow. This stretches the Biceps but puts it in a mechanical line of pull where it cannot effectively rotate the radius. Any supination force generated here is primarily the Supinator muscle.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Sitting or Standing. Shoulder neutral, Elbow Fully Extended. Forearm fully pronated. Action: Patient supinates the forearm (turns palm forward/out). Resistance: Applied at the distal forearm (wrist), attempting to twist the hand back into Pronation. Grade 3: Full range. Grade 4/5: Holds against resistance.
Grade 2 (Gravity Eliminated)	Position: Sitting with shoulder flexed to 90° and elbow flexed (arm supported on table, hand pointing up). Action: Patient rotates the palm toward their face. Result: Full range of motion.
Grade 0, 1 (Palpation)	Action: Palpate deep on the dorsal/lateral aspect of the upper forearm. Cue: "Try to turn your palm up." Grade 1: Deep contraction felt. Grade 0: No activity.

Frequently Asked Questions

Does the Supinator act on the elbow joint?

Minimally. While it crosses the joint, its fibers are transverse/oblique, designed for rotation (spin) rather than flexion or extension.

Which nerve pierces the Supinator?

The Deep Branch of the Radial Nerve. Once it exits the Supinator muscle distally, it is renamed the Posterior Interosseous Nerve (PIN).

How do I stretch the Supinator?

Extend the elbow and fully Pronate the forearm. Adding wrist flexion can increase the stretch across the extensor mass as well.

Test Your Knowledge: Supinator Quiz

1. Which nerve passes between the two heads of the Supinator?

2. What is the fibrous arch at the superior edge of the Supinator called?

3. Where does the Supinator insert?

4. To isolate the Supinator from the Biceps, the elbow should be:

5. Where does the Supinator originate on the Ulna?

6. PIN Entrapment (Supinator Syndrome) primarily causes:

7. Which ligament serves as an origin for the Supinator?

8. The Supinator is located in which compartment?

9. The primary difference between Biceps and Supinator action is:

10. True or False: The Deep Radial Nerve provides motor innervation to the Supinator *before* it becomes the PIN.

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.
• Magee, D. J. (2014). Orthopedic Physical Assessment. 6th ed. Elsevier.