Transversus Abdominis: Origin, Insertion, Action & The "Corset" Muscle

The Transversus Abdominis (TvA) is the deepest of the flat abdominal muscles. Unlike the obliques which rotate the trunk, or the rectus which flexes it, the Transversus fibers run horizontally. Its primary job is to act as a natural corset, compressing the abdomen and stabilizing the lumbar spine.

[Image of Internal Oblique muscle anatomy]

Quick Anatomy Snapshot

Layer	Deepest layer of the abdominal wall (Deep to Internal Oblique).
Origin (Proximal)	Thoracolumbar Fascia (posteriorly). Internal lip of the Iliac Crest. Lateral 1/3 of the Inguinal Ligament. Costal cartilages of the lower 6 ribs (7-12).
Insertion (Distal)	Linea Alba (via aponeurosis) and the Pubic Crest (via the Conjoint Tendon).
Nerve Supply	Thoracoabdominal Nerves (T7-T11), Subcostal (T12), and Iliohypogastric/Ilioinguinal (L1).
Primary Actions	Compression: Compresses abdominal contents (increasing intra-abdominal pressure). Stabilization: Stabilizes the lumbar spine via the thoracolumbar fascia tension.

Deep Dive: The Biological Corset

The Transversus Abdominis is the only abdominal muscle whose fibers run Horizontally (Transversely), wrapping around the torso like a belt.

1. The Spinal Connection

The TvA is unique because it originates largely from the Thoracolumbar Fascia in the back. When the TvA contracts, it pulls this fascia tight. This tension stabilizes the lumbar vertebrae, creating a "rigid cylinder" effect that protects the lower back during movement.

2. The "Feed-Forward" Mechanism

In healthy individuals, the TvA contracts milliseconds before you move your arm or leg. This Feed-Forward mechanism prepares the spine for the shift in weight. In people with chronic lower back pain, this automatic firing is often delayed or absent.

Physio Corner: Clinical Relevance

💪 Functional Fact: The TvA is the primary target of "Core Stability" exercises. However, it is not strengthened by crunches. It is activated by the "Drawing-In Maneuver" (pulling the belly button toward the spine) or forced exhalation (like blowing up a balloon).

Palpation

Because it is deep to the Internal Oblique, it is hard to palpate directly.
Technique: Locate the ASIS (front hip bone). Move your fingers 1 inch medial and 1 inch inferior. Ask the patient to gently "draw their belly button in" without moving their pelvis. You should feel a slow, deep tensioning under your fingers (not a bulge).

⚠️ Clinical Pathology: Low Back Pain (LBP)
Inhibition of the Transversus Abdominis is strongly correlated with chronic LBP. The muscle does not spontaneously recover after an episode of back pain; it must be specifically retrained using motor control exercises.

Functional Assessment

We cannot perform a standard break test (MMT) for the TvA. We test its ability to stabilize and fire.

Testing Tip: Use a Pressure Biofeedback Unit (blood pressure cuff) under the lumbar spine to objectively measure TvA activation.

Step-by-Step Procedure (Drawing-In Maneuver)

Test	Patient Action & Observation
Prone Instability Test	Position: Prone, torso on table, legs hanging off edge. Action: Patient activates TvA ("hollow the stomach") while therapist applies PA pressure to lumbar spine. Result: If pain decreases when TvA is active, it suggests lumbar instability.
Supine Drawing-In	Position: Supine, knees bent (Hook lying). Action: Patient breathes out and gently pulls navel to spine. Success: The abdominal wall should flatten. If the belly pooches out (doming), the Rectus Abdominis is overpowering the TvA (Compensation).

Frequently Asked Questions

Does it flex the spine?

No. Its fibers run horizontally. Contraction compresses the gut (reducing waist diameter) but provides negligible torque for spinal flexion (bending forward).

What is the Conjoint Tendon?

The lowest aponeurotic fibers of the Transversus Abdominis join with the Internal Oblique to form the Conjoint Tendon, which attaches to the pubic crest. This structure reinforces the inguinal canal to prevent hernias.

Is it active during coughing?

Yes. It is a primary muscle of forced expiration. It rapidly compresses the abdomen to push the diaphragm up, expelling air for coughing, sneezing, or shouting.

Test Your Knowledge: Transversus Abdominis Quiz

1. What is the fiber direction of the Transversus Abdominis?

2. Where does the TvA originate posteriorly?

3. What is the primary action of the TvA?

4. Which nerve supplies the lowest fibers of the TvA?

5. The TvA forms the Conjoint Tendon with which other muscle?

6. In chronic low back pain, the firing of the TvA is often:

7. Which exercise best targets the TvA?

8. The TvA interdigitates with which muscle at the costal margin?

9. Where is the TvA located relative to the Internal Oblique?

10. True or False: The TvA assists in childbirth and defecation.

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.
• Richardson, C. A., et al. (1999). Therapeutic Exercise for Spinal Segmental Stabilization. Churchill Livingstone.