Intermediate Back Muscles

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There are three major groups of back muscles:

  • Superficial: attached to the shoulder girdle
  • Intermediate: attached to the posterior thorax
  • Deep: attached to the vertebral column

The first two groups (superficial and intermediate) are referred to as the extrinsic back muscles. The deep group is the intrinsic muscle group. This article will focus on the intermediate group.

The intermediate back muscles are covered superficially by the superficial back muscles. The intermediate group includes:

  • Serratus posterior superior
  • Serratus posterior inferior
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Serratus posterior superior (SPS)

The serratus posterior superior (SPS) attaches the vertebral column to the ribs, providing stabilisation to the ribs and promoting forceful expiration. When looking at the SPS on both sides of the spine, it has the appearance of a triangle.

Origin and insertion

The SPS originates from the inferior portion of the nuchal ligament, and the spinous processes of C7-T3. It inserts onto the posterior aspect of ribs 2-5.

Fibre orientation

The SPS consists of β€˜fingers’ of muscle fibres that each resemble a short strap.


The function of the SPS is to stabilise the ribs during passive respiration andΒ elevate the ribs (2-5) during active or forceful inspiration.


The intercostal nerves of ribs 2-5 supply sensorimotor innervation to the SPS.

Arterial supply

The intercostal arteries, as part of the intercostal neurovascular bundle, supply the SPS.

Serratus posterior superior (SPS)
Figure 1. Serratus posterior superior (SPS).1

Serratus posterior inferior (SPI)

The serratus posterior inferior (SPI) has a similar shape to the SPS but appears lower on the vertebral column and has an inverted appearance. It looks like an inverted triangle.

Origin and insertion

The SPI originates from the spinous processes of T11-L3 andΒ inserts onto the posterior surface of ribs 9-12.

Fibre orientation

Like the SPS, the fibres of SPI are finger-like projections of a strap muscle.


The function of SPI is to stabilise the thorax during passive respiration andΒ depress ribs 9-12 during active inspiration.


Sensorimotor innervation is supplied to the SPI through the intercostal nerves, and occasionally the subcostal nerve (T12).

Blood supply

The intercostal arteries, as part of the intercostal neurovascular bundle, supply the SPI.

Serratus posterior inferior
Figure 2. Serratus posterior inferior.2


Put your knowledge of the intermediate muscles of the back to the test with our quiz.


Reference images

  1. Serratus Posterior Superior: Anatomography. License: [CC BY-SA 2.1]
  2. Serratus Posterior Inferior: User: Mikael HΓ€ggstrΓΆm. License: [Public domain]

Reference texts

  • Sinnatamby, C. S. (2011). Last’s Anatomy, International Edition: Regional and Applied. Elsevier Health Sciences.
  • Moore, K. L., Dalley, A. F., & Agur, A. M. (2013). Clinically oriented anatomy. Lippincott Williams & Wilkins.

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