External Intercostals: Anatomy, Action, Breathing & "Hands in Pockets"

The External Intercostal muscles are the most superficial layer of the muscles found between the ribs. There are 11 pairs in total. They are crucial for respiratory mechanics, specifically for expanding the chest cavity during inhalation. They are easily distinguished by their unique fiber direction.

[Image of External Intercostal muscles]

Quick Anatomy Snapshot

Origin (Proximal)	Inferior border of the Rib above.
Insertion (Distal)	Superior border of the Rib below.
Nerve Supply	Intercostal Nerves (Ventral rami of T1-T11).
Blood Supply	Posterior Intercostal Arteries and Anterior Intercostal Arteries.
Primary Actions	Inspiration: Elevates the ribs, increasing the thoracic volume. Support: Stabilizes the intercostal space during breathing.

Deep Dive: The "Pocket" Muscles

The intercostal space contains three layers of muscle (External, Internal, Innermost). The External Intercostals are the outermost layer.

1. Fiber Direction: "Hands in Front Pockets"

The fibers of the External Intercostals run Inferomedially (down and forward). This is the same direction your fingers point when you put your hands in your front pants pockets.
Contrast: The Internal Intercostals run perpendicular to this (Hands in Back Pockets).

2. The Anterior Membrane

Interestingly, the muscle fibers do not extend all the way to the sternum (breastbone). Between the costal cartilages and the sternum, the muscle is replaced by the External Intercostal Membrane. This thin connective tissue layer continues the connection to the sternum.

Physio Corner: Clinical Relevance

💪 Functional Fact: While the diaphragm is the primary muscle of quiet breathing, the External Intercostals are recruited during Forced Inspiration (exercise or respiratory distress). They lift the ribs like the handle of a bucket ("Bucket Handle Movement"), expanding the chest diameter.

Palpation

Palpation is done in the intercostal spaces between the ribs. It is difficult to distinguish the external from the internal layer by touch alone, but they are most palpable laterally. Tenderness here often indicates an intercostal strain from coughing or twisting.

⚠️ Clinical Pathology: Intercostal Strain
Often called a "pulled rib muscle," this injury is common in sports involving torso rotation (baseball, golf) or severe coughing bouts. Pain is sharp, localized between the ribs, and worsens with deep breaths, coughing, or sneezing.

Functional Assessment (Chest Expansion)

We do not use standard MMT (Oxford Scale) for individual rib muscles. Instead, we measure Chest Expansion.

Testing Tip: Use a tape measure. Measure the circumference of the chest at the level of the 4th intercostal space (nipple line) or the xiphoid process.

Measurement Procedure

Step	Action
1. Exhale	Ask the patient to exhale fully. Take a measurement.
2. Inhale	Ask the patient to inhale maximally (deep breath). Take a measurement.
Result	Calculate the difference. Normal expansion is 2.5 cm to 7.5 cm (1-3 inches). Reduced expansion may indicate ankylosing spondylitis, COPD, or rib cage pathology.

Frequently Asked Questions

Do External Intercostals help with exhaling?

No. Exhalation is usually passive (elastic recoil). Forced exhalation involves the Internal Intercostals and abdominal muscles, which depress the ribs.

Where is the neurovascular bundle located?

The Intercostal Vein, Artery, and Nerve (VAN) run in the Costal Groove at the bottom of the rib, sandwiched between the Internal and Innermost Intercostal muscles. This protects them from external trauma.

How many External Intercostal muscles are there?

There are 11 pairs (22 total), one for each intercostal space on both the left and right sides.

Test Your Knowledge: External Intercostals Quiz

1. What is the primary action of the External Intercostals?

2. What is the direction of the External Intercostal fibers?

3. Which nerve supplies these muscles?

4. Anteriorly, near the sternum, the muscle is replaced by:

5. Where does the External Intercostal originate?

6. Which layer of the intercostal spaces is the External Intercostal found in?

7. During which phase of respiration are these muscles most active?

8. Reduced chest expansion (< 2.5cm) may indicate:

9. The External Intercostals are continuous with which abdominal muscle?

10. True or False: External Intercostal muscles run deep to the Internal Intercostal muscles.

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.
• West, J. B. (2012). Respiratory Physiology: The Essentials. 9th ed. Lippincott Williams & Wilkins.