Scalenus Medius: Origin, Insertion, Nerve, Action & The "Side Bender"

The Scalenus Medius (Middle Scalene) is the largest and longest of the three scalene muscles. Located deep in the lateral neck, it acts as a primary lateral flexor of the cervical spine and forms the posterior border of the anatomically critical Interscalene Triangle.

[Image of Musculocutaneous nerve piercing coracobrachialis]

Quick Anatomy Snapshot

Group	Lateral Vertebral Muscles (The Scalenes).
Origin (Proximal)	Posterior tubercles of the Transverse Processes of C2 through C7.
Insertion (Distal)	Superior surface of the 1st Rib (posterior to the groove for the subclavian artery).
Nerve Supply	Anterior Rami of Cervical Spinal Nerves (C3-C8).
Primary Actions	Unilateral: Strong lateral flexion of the neck to the same side. Bilateral: Weak flexion of the neck. Inspiration: Elevates the 1st Rib (Accessory muscle of breathing).

Deep Dive: The Nerve Entrapper

The Scalenus Medius is distinct from its neighbors due to its size and its relationship with the nerves of the neck.

1. The Posterior Border

Along with the Anterior Scalene, the Middle Scalene forms the Scalene Interval (Interscalene Triangle).
• Anterior Border: Scalenus Anterior.
• Posterior Border: Scalenus Medius.
• Base: 1st Rib.
The Brachial Plexus and Subclavian Artery pass through this gap. Tightness in the Middle Scalene pushes these nerves forward against the Anterior Scalene, causing compression.

2. The Piercing Nerves

The Dorsal Scapular Nerve (supplying the Rhomboids) and the upper roots of the Long Thoracic Nerve (supplying Serratus Anterior) typically pierce directly through the belly of the Middle Scalene. Spasm in this muscle can therefore cause referred pain or weakness in the shoulder blade region ("winged scapula").

Physio Corner: Clinical Relevance

💪 Functional Fact: The Scalenus Medius is a "stabilizer" muscle. When you carry heavy groceries in one hand, the Scalenus Medius on the opposite side contracts isometrically to prevent your head from flopping toward the weight.

Palpation

1. Locate the clavicular head of the SCM. Move laterally into the posterior triangle of the neck.
2. Palpate deep, just above the clavicle. You will feel a bundle of nerves (Brachial Plexus).
3. Slide your fingers just posterior to these nerves to find the fleshy belly of the Middle Scalene.
4. Ask the patient to sniff or side-bend their head to confirm contraction.

⚠️ Clinical Pathology: Thoracic Outlet Syndrome (TOS)
While the Anterior Scalene gets most of the blame, the Middle Scalene is often the primary culprit in "scalenus anticus syndrome." Hypertrophy here narrows the exit space for the brachial plexus, leading to tingling in the arm and hand (C8-T1 distribution).

Manual Muscle Testing (MMT)

We test the scalenes as a group via lateral flexion.

Testing Tip: To bias the Middle Scalene, perform pure Lateral Flexion without rotation. (Anterior Scalene requires rotation to the same side; SCM requires rotation to the opposite side).

Step-by-Step Procedure (Side Bending)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Side-lying (head supported) or Sitting. Action: Patient tilts ear toward shoulder (Lateral Flexion). Ensure no rotation occurs. Resistance: Applied to the temporal region of the head, pushing down toward the opposite shoulder. Grade 3: Full range against gravity (side-lying). Grade 4/5: Holds against resistance.
Grade 0, 1 (Palpation)	Action: Palpate the lateral neck, posterior to the brachial plexus/SCM. Cue: "Try to touch your ear to your shoulder." Grade 1: Muscle tension felt. Grade 0: No activity.

Frequently Asked Questions

Does the Subclavian Vein pass through the triangle?

No. The vein passes anterior to the Anterior Scalene. Only the Artery and Plexus pass between the Anterior and Middle Scalenes.

What is the difference between Medius and Posterior Scalene?

Medius: Inserts on the 1st Rib. Largest belly.
Posterior: Inserts on the 2nd Rib. Smaller, deeper, and more posterior.

Can this muscle cause shoulder blade pain?

Yes. Because the Dorsal Scapular Nerve (which supplies the Rhomboids and Levator Scapulae) often pierces the Middle Scalene, tightness here can entrap the nerve, causing medial scapular pain.

Test Your Knowledge: Scalenus Medius Quiz

1. Where does the Scalenus Medius insert?

2. Which nerve frequently pierces the belly of the Middle Scalene?

3. The Scalene Triangle is formed by Anterior Scalene, 1st Rib, and:

4. What is the primary origin of Scalenus Medius?

5. What structures pass BETWEEN the Anterior and Middle Scalene?

6. What is the primary action of Scalenus Medius?

7. Which artery passes ANTERIOR to the Scalenus Medius insertion?

8. The Scalenus Medius helps with breathing during:

9. Scalenus Medius is located in which layer of neck muscles?

10. True or False: Scalenus Medius is the largest scalene muscle.

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.