Spinalis: Origin, Insertion, Nerve, Action & The Medial Column

The Spinalis is the most medial (closest to the spine) column of the Erector Spinae group. It is a bundle of muscles and tendons that runs vertically along the spinous processes. While it is often blended with the Longissimus, it is the primary "spine-to-spine" extensor of the back.

[Image of Longissimus Capitis anatomy]

Quick Anatomy Snapshot

Group	Erector Spinae (Medial Column).
Divisions	Spinalis Thoracis: The most prominent/consistent part. Spinalis Cervicis: Inconstant/variable. Spinalis Capitis: Often fused with Semispinalis.
Origin (Proximal)	Spinous Processes of upper Lumbar (L1-L2) and lower Thoracic (T11-T12) vertebrae.
Insertion (Distal)	Spinous Processes of upper Thoracic (T1-T8) and Cervical vertebrae (and Occiput for Capitis).
Nerve Supply	Medial branches of the Posterior (Dorsal) Rami of spinal nerves.
Primary Actions	Bilateral: Extension of the vertebral column. Unilateral: Lateral flexion (Weak) to the same side.

Deep Dive: The Medial Pillar

The Erector Spinae group mnemonic "I Like Standing" (Lateral to Medial) ends with Spinalis. This muscle hugs the spinous processes tightly.

1. The Thoracis Portion

The Spinalis Thoracis is the main component. It arises from the inner side of the broad Longissimus tendon in the lower back and inserts into the spinous processes of the upper back. Because it connects "spinous to spinous," it has a very direct line of pull for extension.

2. The "Missing" Muscles

The Spinalis Cervicis (neck) and Spinalis Capitis (head) are notoriously variable.
• Cervicis: Often absent or consists of only a few fibers.
• Capitis: Usually blended so thoroughly with the Semispinalis Capitis that it is impossible to separate during dissection.

Physio Corner: Clinical Relevance

💪 Functional Fact: The Spinalis is a major postural muscle. In upright standing, it works constantly to prevent the thoracic spine from collapsing into flexion (Kyphosis). Weakness here contributes to the "Dowager's Hump" in elderly patients.

Palpation

Palpate the spinous processes of the mid-thoracic spine. Move just slightly laterally (less than 1 inch). The thin strip of muscle lying directly against the side of the spinous processes is the Spinalis. It is medial to the thicker, ropey Longissimus.

⚠️ Clinical Pathology: Ankylosing Spondylitis
In inflammatory conditions like AS, the ligaments and tendons attaching to the spinous processes can calcify ("Bamboo Spine"). Since the Spinalis attaches directly to these bones, it can become rigid and fused, leading to a total loss of spinal extension.

Manual Muscle Testing (MMT)

It is impossible to isolate the Spinalis from the Longissimus and Iliocostalis. They are tested as the "Erector Spinae Group."

Testing Tip: The Biering-Sørensen Test (holding the chest off a table while prone) tests the endurance of this entire group.

Step-by-Step Procedure (Thoracic Extension)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Gravity)	Position: Prone, with upper chest off the table end (or lifting off table). Action: Patient extends the thoracic spine (lifts chest). Resistance: Applied to the upper back (T1-T4 level), pushing down into flexion. Grade 3: Lifts chest to horizontal/clears table. Grade 4/5: Holds against resistance.
Grade 0, 1 (Palpation)	Action: Palpate immediately lateral to the thoracic spinous processes. Cue: "Lift your chest." Grade 1: Contraction felt medial to Longissimus. Grade 0: No activity.

Frequently Asked Questions

How is it different from Semispinalis?

Spinalis (Erector Spinae): Connects Spinous Process to Spinous Process (Vertical fibers). Superficial.
Semispinalis (Transversospinalis): Connects Transverse Process to Spinous Process (Oblique fibers). Deep.

Does it rotate the spine?

Minimally. Because it is so close to the midline and runs vertically, it has very poor leverage for rotation. Its primary job is pure Extension.

Is Spinalis Lumborum a distinct muscle?

Usually, no. In the lumbar region, the Spinalis is blended into the common erector spinae aponeurosis and is often indistinguishable from the Longissimus Lumborum.

Test Your Knowledge: Spinalis Quiz

1. Which column of the Erector Spinae is the Spinalis?

2. Where does the Spinalis primarily insert?

3. Which part of the Spinalis is most consistent and distinct?

4. What is the primary action of the Spinalis?

5. Which muscle lies immediately lateral to the Spinalis?

6. Which nerve supplies the Spinalis?

7. The Spinalis Capitis is often blended with which muscle?

8. Why does Spinalis have poor leverage for rotation?

9. Spinalis fibers run in which direction?

10. True or False: The Spinalis is deep to the Multifidus.

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.