Serratus Anterior: Origin, Insertion, Nerve, Action & The "Boxer's Muscle"

The Serratus Anterior is a broad, fan-shaped muscle located on the lateral wall of the chest. Its name comes from the Latin Serra (Saw), describing the jagged, saw-tooth appearance of its origins on the ribs. It is critical for shoulder stability and overhead movement.

[Image of External Oblique muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	External surfaces of Ribs 1-8 (Lateral aspect).
Insertion (Distal)	Anterior (Costal) surface of the entire Medial Border of the Scapula.
Nerve Supply	Long Thoracic Nerve (C5, C6, C7) - "C5, 6, 7 raises your arms to heaven."
Blood Supply	Lateral Thoracic Artery.
Primary Actions	Protraction: Pulls the scapula forward around the rib cage (e.g., punching). Upward Rotation: Rotates the glenoid up to allow overhead reaching (arm > 90°). Stabilization: Keeps the medial border of the scapula glued to the thoracic wall.

Deep Dive: The Saw-Tooth Muscle

The Serratus Anterior wraps around the rib cage from front to back, sliding underneath the scapula (shoulder blade).

1. The Finger Interdigitation

On the side of the ribs, the lower digitations of the Serratus Anterior fit together with the digitations of the External Oblique muscle like interlocking fingers. This "saw-tooth" pattern is often visible in lean athletes.

2. The Pivot Point

The muscle attaches to the entire medial border of the scapula, but the fibers converging at the Inferior Angle are the strongest. These lower fibers pull the bottom of the scapula outward, causing the glenoid (socket) to tilt upward. Without this rotation, you cannot raise your arm above your head.

Physio Corner: Clinical Relevance

💪 Functional Fact: It is nicknamed the "Boxer's Muscle" because it is responsible for protraction—the final phase of a punch where the shoulder blade slides forward around the ribs to extend the reach.

Palpation

Raise the patient's arm to 90 degrees. Palpate the lateral rib cage along the mid-axillary line (below the armpit). You can feel the ribs; the muscle overlying them is the Serratus Anterior. Ask the patient to punch forward or push against a wall to feel it contract.

⚠️ Clinical Pathology: Scapular Winging
Damage to the Long Thoracic Nerve (which runs superficially along the muscle) causes paralysis of the Serratus Anterior.
The Sign: When the patient pushes against a wall, the medial border of the scapula pops off the rib cage, looking like a bird's wing. This is known as Medial Winging.

Manual Muscle Testing (MMT)

We test the ability to protract the shoulder with force.

Testing Tip: Watch for the shoulder blade. If it pops out (wings) when they push, the muscle is weak, even if the patient is strong in other areas.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Gravity)	Position: Supine. Arm flexed to 90° (pointing at ceiling), elbow straight. Action: Patient punches up towards the ceiling (protracts shoulder), lifting the shoulder off the table. Resistance: Applied to the fist or elbow, pushing the shoulder down into the table. Grade 3: Completes punch against gravity. Grade 4/5: Holds against resistance.
Grade 2 (Gravity Eliminated)	Position: Sitting. Arm resting on a table at 90° flexion. Action: Patient slides the arm forward across the table (protraction). Result: Full range of motion.
Grade 0, 1 (Palpation)	Action: Palpate the lateral ribs just below the axilla. Cue: "Try to reach forward."

Frequently Asked Questions

Is it an accessory breathing muscle?

Yes. If the scapula is fixed in place (e.g., hands on knees after a run), the Serratus Anterior pulls the ribs outward, aiding in forced inspiration ("heaving chest").

What is the antagonist of the Serratus Anterior?

The Rhomboids (Major and Minor) and the Middle Trapezius. These muscles retract (adduct) the scapula, pulling it back toward the spine, opposing the protraction of the Serratus.

Where is the Long Thoracic Nerve located?

It runs vertically down the external surface of the Serratus Anterior muscle. Because it lies on top of the muscle (superficial), it is vulnerable to trauma from side impact or heavy backpacks.

Test Your Knowledge: Serratus Anterior Quiz

1. What is the nerve supply of the Serratus Anterior?

2. What is the primary action of the Serratus Anterior?

3. Where does the Serratus Anterior insert?

4. Damage to the Long Thoracic Nerve causes:

5. The Serratus Anterior helps lift the arm overhead by performing:

6. The "saw-tooth" origins interdigitate with which muscle?

7. Which nerve roots comprise the Long Thoracic Nerve?

8. Where does the Serratus Anterior originate?

9. Which muscle acts as an antagonist to Serratus Anterior during protraction?

10. True or False: The Serratus Anterior lies deep to the Scapula.

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.
• Kendall, F. P. (2005). Muscles: Testing and Function, with Posture and Pain. 5th ed. Lippincott Williams & Wilkins.