Rectus Abdominis: Origin, Insertion, Nerve, Action & The "Six-Pack"

The Rectus Abdominis is the most prominent superficial muscle of the anterior abdominal wall. Famous as the "Six-Pack" muscle (or "Washboard Abs"), it is a long, paired strap muscle that runs vertically down the entire length of the abdomen, separated by the Linea Alba.

[Image of External Oblique muscle anatomy]

Quick Anatomy Snapshot

Origin (Inferior)	Pubic Crest and Pubic Symphysis.
Insertion (Superior)	Xiphoid Process and Costal Cartilages of Ribs 5, 6, and 7.
Nerve Supply	Thoracoabdominal Nerves (Anterior rami of T7-T11) and Subcostal Nerve (T12).
Blood Supply	Superior Epigastric Artery and Inferior Epigastric Artery.
Primary Actions	Trunk Flexion: Bending the spine forward (crunch). Pelvic Tilt: Posteriorly tilts the pelvis (flattens lumbar spine). Compression: Increases intra-abdominal pressure (forced expiration).

Deep Dive: The Anatomy of the "Six-Pack"

The distinct segmented appearance of the Rectus Abdominis is due to specific anatomical structures.

1. Tendinous Intersections

The muscle belly is interrupted by 3 or 4 fibrous bands called Tendinous Intersections. These bands adhere the muscle to the anterior layer of the Rectus Sheath.
• Usually located: at the xiphoid, at the umbilicus, and halfway between them.
• When the muscle hypertrophies (grows) and body fat is low, these bands create the visual "pack" separation.

2. The Linea Alba

Running vertically down the midline is the Linea Alba ("White Line"). This is a tough, fibrous raphe where the aponeuroses of the oblique muscles and transversus abdominis fuse. It separates the left and right Rectus muscles.

Physio Corner: Clinical Relevance

💪 Functional Fact: The Rectus Abdominis is a powerful Posterior Pelvic Tilter. Tight hip flexors (Iliopsoas) pull the pelvis forward (anterior tilt/swayback). A strong Rectus Abdominis counteracts this pull, protecting the lower back from excessive arching (lordosis).

Palpation

Ask the patient to lie supine with knees bent (crook lying). Ask them to lift their head and shoulders off the table (partial crunch). You can easily palpate the medial and lateral borders of the muscle and feel the tendinous intersections as indentations.

⚠️ Clinical Pathology: Diastasis Recti
During pregnancy or with significant obesity, the Linea Alba can stretch and thin, causing the two Rectus muscles to separate laterally. This creates a gap or bulge along the midline when the patient engages their core. It is not a tear, but a stretching of the connective tissue.

Manual Muscle Testing (MMT)

Abdominal testing is unique because we use the weight of the upper body as resistance rather than manual pressure.

Testing Tip: Watch for the feet lifting off the table. If they lift, the patient is using their hip flexors (Psoas) to pull the torso up, rather than curling with the abs.

Step-by-Step Procedure (Trunk Flexion)

Grade	Patient Position & Action
Grade 5 (Normal)	Position: Supine, hands clasped behind the head. Action: Patient curls up until the inferior angles of the scapulae clear the table.
Grade 4 (Good)	Position: Supine, arms crossed over chest. Action: Patient curls up until scapulae clear the table.
Grade 3 (Fair)	Position: Supine, arms extended forward above plane of body. Action: Patient curls up until scapulae clear the table.
Grade 2 (Poor)	Position: Supine, knees bent. Action: Patient lifts head but cannot clear scapulae. Result: Only partial range of motion.

Frequently Asked Questions

Can you isolate "Upper" vs "Lower" abs?

Anatomically, it is one single muscle sheet. However, it is segmentally innervated (T7-T12). While you cannot completely isolate them, you can bias the upper section by fixing the pelvis and moving the ribcage (crunches), and bias the lower section by fixing the ribcage and moving the pelvis (leg raises).

What is the Pyramidalis?

It is a small, triangular muscle found in about 80% of people, located anterior to the Rectus Abdominis within the rectus sheath, just above the pubis. It tenses the Linea Alba.

What is the Arcuate Line?

The Arcuate Line is a horizontal line on the posterior aspect of the Rectus Sheath (about halfway between umbilicus and pubis). Below this line, the posterior rectus sheath disappears, and the Rectus Abdominis rests directly on the Transversalis Fascia.

Test Your Knowledge: Rectus Abdominis Quiz

1. What structures create the "Six-Pack" appearance?

2. Where does the Rectus Abdominis insert superiorly?

3. What is the primary action of the Rectus Abdominis on the pelvis?

4. Which nerve supplies the Rectus Abdominis?

5. Diastasis Recti involves the separation of the muscle along the:

6. To test Grade 5 strength, where should the patient's hands be?

7. Which small muscle lies anterior to the Rectus Abdominis distally?

8. Below the Arcuate Line, what forms the posterior rectus sheath?

9. Which artery runs within the Rectus Sheath?

10. True or False: The Rectus Abdominis is active during quiet inspiration.

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.