Semispinalis Thoracis: Origin, Insertion, Nerve, Action & Posture

The Semispinalis Thoracis is a deep muscle of the back, part of the Transversospinalis group. It is a long, slender muscle that consists of fascicles extending from the lower thoracic vertebrae to the upper thoracic and lower cervical vertebrae. It is a key stabilizer and extensor of the mid-back.

[Image of Iliocostalis Thoracis anatomy]

Quick Anatomy Snapshot

Group	Transversospinalis (Deep Back Muscles).
Origin (Proximal)	Transverse processes of the lower 6 Thoracic Vertebrae (T6-T12).
Insertion (Distal)	Spinous processes of the upper 4 Thoracic (T1-T4) and lower 2 Cervical (C6-C7) vertebrae.
Nerve Supply	Medial branches of the Posterior (Dorsal) Rami of thoracic spinal nerves.
Primary Actions	Bilateral: Extension of the thoracic spine. Unilateral: Rotation of the thoracic spine to the Opposite (Contralateral) side.

Deep Dive: The "Span" Rule

To distinguish the Semispinalis from its deeper neighbors (Multifidus and Rotatores), you must look at the length of the fibers.

1. The Longest of the Deep

The name Transversospinalis tells you the direction: fibers run from the Transverse Process (below) to the Spinous Process (above).
• Rotatores: Span 1-2 segments.
• Multifidus: Spans 2-4 segments.
• Semispinalis: Spans 4-6 segments. It has the longest fascicles of the deep group.

2. The Tendinous Difference

The Semispinalis Thoracis ends in long, thin tendons at its superior attachment (Spinous Processes), distinguishing it from the fleshier insertions of the Multifidus deeper to it.

Physio Corner: Clinical Relevance

💪 Functional Fact: This muscle acts as a primary "Anti-Kyphosis" muscle. It works constantly to maintain thoracic extension. Weakness here leads to the common "slouched" posture or "Round Back" seen in office workers.

Palpation

Palpation is difficult because it is covered by the Trapezius, Rhomboids, and part of the Longissimus. It lies in the paravertebral gutter between the transverse and spinous processes. It is felt as a deep resistance during thoracic extension.

⚠️ Clinical Pathology: Thoracic Kyphosis
In hyper-kyphosis (Dowager's Hump), the Semispinalis Thoracis becomes "locked long" (stretch weakness). Strengthening this muscle is crucial for restoring upright posture, but it is often inhibited by tight chest muscles (Pecs).

Manual Muscle Testing (MMT)

Testing thoracic extension targets the Semispinalis Thoracis along with the Erector Spinae.

Testing Tip: Ensure the patient is extending from the mid-back, not just lifting the head (cervical extension) or arching the lower back (lumbar extension).

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Gravity)	Position: Prone (face down), with the upper trunk hanging off the table (supported by therapist initially). Action: Patient lifts the chest to horizontal (extension). Resistance: Applied to the upper thoracic spine (between scapulae), pushing down. Grade 3: Lifts chest to horizontal. Grade 4/5: Holds against resistance.
Grade 0, 1 (Palpation)	Action: Palpate the paravertebral muscles in the mid-thoracic region. Cue: "Try to lift your chest slightly." Grade 1: Contraction felt. Grade 0: No activity.

Frequently Asked Questions

Does it rotate to the same side?

No. All Transversospinalis muscles (Semispinalis, Multifidus, Rotatores) run Up and In. When they contract unilaterally, they pull the spinous process toward the transverse process, causing rotation to the Opposite (Contralateral) side.

Does it attach to the head?

No. The Semispinalis Thoracis stops at the lower cervical vertebrae. The Semispinalis Capitis is the part that attaches to the skull.

Is it superficial or deep to Spinalis?

It is deep to the Spinalis (part of Erector Spinae). The Spinalis is the most medial column of the Erector Spinae and lies directly over the spinous processes and the Semispinalis Thoracis.

Test Your Knowledge: Semispinalis Thoracis Quiz

1. What is the fiber direction of Semispinalis Thoracis?

2. How many vertebral segments does a Semispinalis fascicle span?

3. Where does Semispinalis Thoracis originate?

4. What is the unilateral action of this muscle?

5. Which group does Semispinalis belong to?

6. Which nerve supplies this muscle?

7. Weakness in Semispinalis Thoracis leads to:

8. Where does it insert?

9. Is Semispinalis Thoracis superficial or deep to Longissimus?

10. True or False: Semispinalis Thoracis helps with forced 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.