Latissimus Dorsi: Origin, Insertion, Nerve, Action & The "Swimmer's Muscle"

The Latissimus Dorsi (Latin for "broadest of the back") is a large, flat, triangular muscle covering the lower back. It connects the upper limb directly to the axial skeleton, bridging the gap between the arm and the pelvis. It is often called the "Climbing Muscle" or "Swimmer's Muscle" due to its powerful downward pull.

[Image of Latissimus Dorsi muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Spinous processes of T7-L5. Thoracolumbar Fascia. Posterior Iliac Crest. Lower 3-4 Ribs. Inferior Angle of the Scapula.
Insertion (Distal)	Floor of the Intertubercular (Bicipital) Groove of the Humerus.
Nerve Supply	Thoracodorsal Nerve (C6, C7, C8).
Blood Supply	Thoracodorsal Artery.
Primary Actions	Shoulder Extension: Pulls the arm backward. Adduction: Pulls the arm toward the body. Internal Rotation: Turns the arm inward. Depression: Pulls the shoulder girdle down.

Deep Dive: The Lady Between Two Majors

To remember the insertion of the Latissimus Dorsi, anatomists use a famous mnemonic regarding the Bicipital Groove.

1. The Mnemonic

"The Lady between Two Majors."
• Pectoralis Major: Inserts on the Lateral Lip.
• Teres Major: Inserts on the Medial Lip.
• Latissimus Dorsi ("The Lady"): Inserts on the Floor between them.

2. The Spiral

As the muscle fibers travel from the lower back to the arm, they twist 180 degrees. The lowest fibers from the hip actually insert highest on the humerus, while the upper thoracic fibers insert lower. This spiral increases the muscle's leverage and power for pulling motions.

Physio Corner: Clinical Relevance

💪 Functional Fact: This is the primary muscle for Closed Chain Depression. When your arms are fixed (like on parallel bars, crutches, or doing a pull-up), the Lats pull your body up toward your hands. It is essential for paraplegic transfers (lifting the pelvis to move).

Palpation

Have the patient place the back of their hand against their hip and push inward (adduct). Palpate the posterior axillary fold (the back of the armpit). The large muscular web you feel is the Latissimus Dorsi wrapping around the Teres Major.

⚠️ Clinical Pathology: Tight Lats
Because the Latissimus Dorsi is an internal rotator and extender, tightness here restricts Shoulder Flexion and External Rotation. Patients with tight lats often arch their lower back (lumbar extension) when trying to raise their arms overhead, leading to back pain.

Manual Muscle Testing (MMT)

To test the Lats, we simulate the "handcuff" position.

Testing Tip: Ensure the patient keeps the elbow straight. If they bend the elbow, they might try to substitute with the posterior deltoid.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Gravity)	Position: Prone (face down). Arm adducted at side, shoulder internally rotated (palm facing up/out). Action: Patient lifts the arm straight up towards the ceiling (Extension). Resistance: Applied at the distal forearm, pushing down and out (into flexion and abduction). Grade 3: Full range against gravity. Grade 4/5: Holds against strong resistance.
Grade 2 (Gravity Eliminated)	Position: Side-lying on the non-test side. Therapist supports the arm. Action: Patient moves the arm backward into extension. Result: Full range of motion.
Grade 0, 1 (Palpation)	Action: Palpate the lateral side of the thorax below the armpit. Cue: "Try to cough" or "Push your arm down." Grade 1: Muscle contraction felt (Coughing forces Lats to contract). Grade 0: No activity.

Frequently Asked Questions

What is its relationship with Teres Major?

They are "Twin Muscles." They share the same actions (Extension, Adduction, IR) and similar insertions. Teres Major is often called "Lat's Little Helper."

Does it act on the pelvis?

Yes. Because it attaches to the iliac crest and lumbar fascia, tight lats can pull the pelvis into an anterior tilt (lordosis), especially when the arms are raised.

Why is it called the "Cough Muscle"?

It assists in forceful expiration (coughing/sneezing) by compressing the posterior rib cage. You can feel your lats contract violently when you cough.

Test Your Knowledge: Latissimus Dorsi Quiz

1. Where does the Latissimus Dorsi insert?

2. Which nerve supplies the Latissimus Dorsi?

3. Which of the following is NOT an action of the Lats?

4. Which muscle is the "Little Helper" of the Latissimus Dorsi?

5. The Latissimus Dorsi is crucial for which activity?

6. The Thoracodorsal Nerve is a branch of the:

7. Tight Lats can limit which shoulder motion?

8. The origin of the Lats includes the:

9. "The Lady between Two Majors" refers to Lats lying between:

10. True or False: The Latissimus Dorsi attaches to the inferior angle of 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.