Scalenus Posterior: Origin, Insertion, Nerve, Action & The "2nd Rib" Lifter

The Scalenus Posterior (Posterior Scalene) is the smallest and deepest of the three scalene muscles. While the Anterior and Middle Scalenes work on the 1st rib, the Posterior Scalene is unique because it bypasses the first rib to attach to the second rib, acting as a key accessory muscle for deep inspiration.

[Image of Musculocutaneous nerve piercing coracobrachialis]

Quick Anatomy Snapshot

Group	Lateral Vertebral Muscles (The Scalenes).
Origin (Proximal)	Posterior tubercles of the Transverse Processes of C5, C6, and C7.
Insertion (Distal)	Outer surface of the 2nd Rib (occasionally the 3rd).
Nerve Supply	Anterior Rami of Cervical Spinal Nerves (C6, C7, C8).
Primary Actions	Unilateral: Lateral flexion of the neck to the same side. Inspiration: Elevates the 2nd Rib.

Deep Dive: The Deepest Scalene

The Scalenus Posterior is often difficult to distinguish from the Scalenus Medius, as they run parallel to each other. In some people, the fibers may actually blend together.

1. The Second Rib Connection

This is the defining feature of this muscle.
• Anterior & Middle Scalenes: Insert on Rib 1.
• Posterior Scalene: Skips Rib 1 and inserts on Rib 2.
This attachment gives it a unique role in expanding the upper chest, particularly the diameter of the thorax, during heavy breathing.

2. The Levator Scapulae Neighbor

The Posterior Scalene is located immediately anterior to the Levator Scapulae. Clinically, tightness in the posterior neck is often a combination of Levator Scapulae and Posterior Scalene tension, as they share similar attachment sites on the transverse processes.

Physio Corner: Clinical Relevance

💪 Functional Fact: While less involved in Thoracic Outlet Syndrome (TOS) than its anterior counterpart, a tight Scalenus Posterior can elevate the 2nd rib. A "raised 2nd rib" can mechanically block the movement of the 1st rib, creating a chain reaction of stiffness in the upper thorax.

Palpation

1. Locate the Upper Trapezius and Levator Scapulae at the base of the neck.
2. Move your fingers just anterior to the Levator Scapulae, into the posterior triangle.
3. Palpate deep behind the larger Middle Scalene.
4. Ask the patient to take a sharp, deep breath or tilt their head to the side to feel the contraction.

⚠️ Clinical Pathology: Whiplash & COPD
In patients with chronic respiratory conditions (COPD, Asthma), the Scalenus Posterior becomes hypertrophied and short due to constant overuse in lifting the ribs. In Whiplash injuries, it acts as a lateral stabilizer and is frequently strained.

Manual Muscle Testing (MMT)

The Posterior Scalene is tested as part of the lateral flexion group.

Testing Tip: It is nearly impossible to isolate the Posterior Scalene from the Middle Scalene manually. We test lateral neck flexion strength and assess rib mobility to determine dysfunction.

Step-by-Step Procedure (Lateral Flexion)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Side-lying (to eliminate gravity) or Sitting. Action: Patient brings ear toward shoulder. Resistance: Applied to the side of the head above the ear. Differentiation: The Posterior Scalene is biased slightly if the head is turned away from the side of flexion, but this is a subtle distinction.
Grade 0, 1 (Palpation)	Action: Palpate the lateral neck base, deep to the Trapezius edge. Cue: "Tilt your head to the side." Grade 1: Deep tension felt. Grade 0: No activity.

Frequently Asked Questions

Does it pass through the Scalene Triangle?

No. The Scalene Triangle is formed by the Anterior and Middle Scalenes. The Posterior Scalene lies behind the Middle Scalene, outside of the neurovascular compression zone.

Why is it the smallest scalene?

It originates from fewer vertebrae (usually C5-C7) than the others and has a shorter lever arm for head movement compared to the Middle Scalene.

Does it rotate the neck?

Its contribution to rotation is debated and minor. If anything, it may assist slightly in rotation to the same side (ipsilateral), but its main job is lateral flexion.

Test Your Knowledge: Scalenus Posterior Quiz

1. Where does the Scalenus Posterior insert?

2. What is the primary action of the Scalenus Posterior?

3. Which muscle lies immediately anterior to the Scalenus Posterior?

4. What is the origin of Scalenus Posterior?

5. Which nerve supplies the Scalenus Posterior?

6. Does the Scalenus Posterior cause Thoracic Outlet Syndrome?

7. The Scalenus Posterior assists in which type of breathing?

8. Which muscle lies immediately posterior to the Scalenus Posterior?

9. What structure passes between the Middle and Posterior Scalenes?

10. True or False: Scalenus Posterior is the largest of the three scalenes.

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.