The Intrinsic Muscles of the Back – Anatomy of the Torso

Image: “Deep Muscles of the back (posterior view)” by Phil Schatz. License: CC BY 4.0

Intrinsic Back Muscles: Explanations

The autochthonous back muscles are also referred to as primary back muscles. In other sources, this group of muscles is also known as erector spinae muscles (spinal erectors) or erector trunci muscles (truncal erectors), since it specifically describes the primary function of this muscle group: the erection of the spine or the torso.

The primary back muscles are divided into medial and lateral tract with five (5) systems in total, in each system various muscles are grouped together. The names of the systems (muscle groups) contain the osseous insertion points of the respective muscles, which is a distinct advantage at learning.

In the event of specific innervation of a part of the system, lateral flexion and rotation can additionally be induced – these movements are, however, considered secondary movements. Also, the primary back muscles have a significant influence on respiratory functions as they can perform both inspiration and expiration due to their insertions at the spine and the ribs. The only function that is not performed by the autochthonous back muscles is flexion.

Without exceptions, all primary back muscles are innervated by the dorsal rami of the spinal nerves.

Sacrospinalis System of the Intrinsic Back Muscles

The sacrospinal system possesses the strongest muscle fibers. It belongs to the lateral tract and describes the intertransversal muscles. There are two groups: the iliocostalis and the longissimus group.

Sacrospinalis System: Iliocostalis Group

Image: “Deep Muscles of the back (posterior view)” by Phil Schatz. License: CC BY 4.0

The iliocostalis group consists of three muscles, with their name describing their location: lumbar iliocostalis muscle, thoracic iliocostalis muscle, and cervical iliocostalis muscle.

Lumbar iliocostalis muscle
The lumbar iliocostalis muscle originates from the external labium of the iliac crest, the dorsal aspect of the sacral bone and the thoracolumbar fascia. It is inserted at the costal angle of the lower six (7^th – 12^th) ribs and performs extension of the thoracic and lumbar spine. It is innervated by the dorsal rami (Th9-L1).

Thoracic iliocostalis muscle
The thoracic part of the iliocostalis group originates from the costal angle of lower six (7^th – 12^th) ribs and is inserted at the costal angle of the upper six (1^st – 6^th) ribs. It is innervated by the dorsal rami (Th2-Th9) and performs extension of the thoracic spine.

Special features: In some people, the thoracic iliocostalis muscle is also innervated by the segments Th1 and Th10 of the dorsal rami.

Cervical iliocostalis muscle
The cervical iliocostalis muscle originates from the costal angle of the 3^rd – 6^th rib and inserts at the posterior tubercle of the transverse processes of the 3^rd – 6^th cervical vertebra. It is innervated by the dorsal rami (Th1-Th2) and acts as an extensor of the cervical and the upper thoracic spine.

Special features: In some people, the cervical iliocostalis muscle is also innervated by the dorsal rami of the segments C8 and Th3.

Sacrospinalis System: Longissimus Group

Image: “Deep Muscles of the back (posterior view)” by Phil Schatz. License: CC BY 4.0

The longissimus group consists of three different muscles: thoracic longissimus muscle, cervical longissimus muscle, and longissimus capitis muscle.

Thoracic longissimus muscle
This muscle originates from the dorsal surface of the sacral bone, the spinous processes of the lumbar and the inferior thoracic spine. It inserts at the accessory processes of the lumbar spine, the transverse processes of the thoracic spine, and medially of the costal angle of the 2^nd – 12^th rib. It is innervated by the dorsal rami (Th3-Th5), and it extends the lumbar and the thoracic spine.

Special features: In some people, the thoracic longissimus muscle is also innervated by the dorsal rami of the segment Th2.

Cervical longissimus muscle
The cervical longissimus muscle originates from the transverse processes of the 1^st – 6^th thoracic vertebrae and inserts at the posterior tubercle of the transverse processes of the 2^nd – 5^th cervical vertebrae. It is innervated by the dorsal rami (C3-Th2) and acts as an extensor of the cervical and the superior thoracic spine.

Longissimus capitis muscle
The muscle originates from the transverse processes of the 3^rd thoracic to the 3^rd cervical vertebrae and inserts at the mastoid process of the temporal bone. Its neuronal supply is ensured by the dorsal rami (C1-C3) and acts as an extensor of the cervical spine and the head joints.

Special features: This muscle is inconsistent and does not occur in every person. If present, it can also be innervated by the dorsal rami of the segment C4.

Spinotransversalis System of the Intrinsic Back Muscles

The spinotransversalis system is only present in the superior thoracic and the cervical spine. It consists of only three muscles: the splenius cervicis, splenius capitis and the inferior oblique capitis muscle, the latter muscle is sometimes listed in the short neck muscles.

Image: “Deep Muscles of the back (posterior view)” by Phil Schatz. License: CC BY 4.0

Splenius Cervicis Muscle

The splenius cervicis muscle originates from the spinous processes of the 3^rd – 6^th cervical vertebrae and the supraspinal ligament. It is inserted at the posterior tubercle of the transverse processes of the 1st – 3^rd cervical vertebral bodies. It is innervated by the dorsal rami (C1-C6) and performs extension in the cervical spine.

Splenius Capitis Muscle

The splenius capitis muscle originates from the spinous processes of the 3^rd cervical vertebrae and 1st – 3^rd thoracic vertebrae, and the supraspinal ligament. It inserts at the lateral half of the superior nuchal line and at the mastoid process. In the event of active innervation via the dorsal rami (C1-C6), it acts as an extensor of the cervical spine and the head joints.

Inferior Oblique Capitis Muscle

This muscle originates from the spinous process of the axis (2nd cervical vertebra) and inserts at the transverse process of the atlas (1st cervical vertebra). It is innervated by the major occipital nerve (dorsal ramus C2) and serves as the primary stabilizer of the atlanto-axial joint.

Special features: In some people, the inferior oblique capitis muscle can additionally be innervated by the suboccipital nerve (dorsal ramus C1).

Intertransversalis System of the Intrinsic Back Muscles

The muscles of the intertransversalis system connect the transverse processes of the spine with each other. The system is divided into an intertransversal and an ‘others’ group.

Intertransversalis System: Intertransversal Group

Image: “Deep Muscles of the back (posterior view)” by Phil Schatz. License: CC BY 4.0

The intertransversarii are the small muscles that connect the transverse processes of the spine. This muscle group consists of three muscle lines: medial lumbar intertransversarii muscles, thoracic intertransversarii muscles, and posterior cervical intertransversarii muscles.

Medial lumbar intertransversarii muscles
These muscles originate from the mammillary processes and the accessory processes of all lumbar vertebrae. These are inserted on the respective adjoining osseous structures of the lumbar spine. They are innervated by the dorsal rami (L1-L5) and perform lateral flexion in the lumbar spine.

Thoracic intertransversarii muscles
These muscles originate from the transverse processes of the 10th – 12th thoracic vertebrae, and are inserted at the transverse processes of the 11th – 12th thoracic vertebrae and at the mammillary and accessory processes of the 1st lumbar vertebra. At active innervation via the dorsal rami (Th10-Th12), they act as lateral flexors of the thoracic spine.

Special features: These muscles are not inherent in every person.

Posterior cervical intertransversarii muscles
These small muscles originate from the posterior tubercle of the transverse processes of the 1st – 7th cervical spine and insert at the posterior tubercles of the respective adjoining cervical vertebra. They are innervated by the dorsal rami (C2-C7) and perform lateral flexion in the cervical spine.

Intertransveralis System: ‘Others’ Group

This group describes muscles which belong to the intertransversal system, but do not represent an own functional muscle group. They include the long and short levatores costarum muscles and the superior oblique capitis muscle, the latter is also often listed under the short neck muscles.

Long and short levatores costarum muscles

The long and short levatores costarum muscles are twelve (12) in number that originate from the transverse processes of the 7th cervical vertebra and 1st – 11th thoracic vertebrae. Each muscle is inserted at the rib below its origin. The short fibers insert at the ribs, which lay respectively one (1) segment below, the long fibers at the ribs, which lay respectively two (2) segments below. At active innervation via the dorsal rami (C8-Th11), they perform extension of the thoracic spine.

Special features: In some people, the long and short levatores costarum muscles are partially also innervated by the ventral rami (C8-Th11).

Superior oblique capitis muscle
The superior oblique capitis muscle originates from the transverse process of the atlas and inserts at the lateral third of the inferior nuchal line of the occipital bone. It is innervated by the suboccipital nerve (dorsal ramus C1) and leads to extension of the atlanto-occipital joint.

Transversospinalis System of the Intrinsic Back Muscles

The transversospinalis system represents the largest muscle group of the primary back muscles (medial tract). It consists of the semispinalis, the multifidi, and the rotators group. The muscles connect the transverse processes (homologues) with the spinous processes. Thus, they run obliquely from the caudo-lateral to the cranio-medial direction.

Transversospinalis System: Semispinalis Group

Image: “Deep Muscles of the back (posterior view)” by Phil Schatz. License: CC BY 4.0

The semispinalis group includes three muscles: thoracic semispinalis muscle, cervical semispinalis muscle, and semispinalis capitis muscle.

Thoracic semispinalis muscle
The thoracic semispinalis muscle originates from the transverse processes of the lower six thoracic vertebrae (7th-12th) and inserts at the spinous processes of the 6th cervical vertebrae to the 4th thoracic vertebrae. It is innervated by the dorsal rami (Th3-Th6), and act as extensors of the thoracic spine.

Cervical semispinalis muscle
The cervical semispinalis muscle originates from the transverse processes of the upper six thoracic vertebrae (1st – 6th) and inserts at the spinous processes of the 2nd – 7th cervical vertebrae. It is innervated by the dorsal rami (C3-C7) and extends the cervical spine and the superior thoracic spine.

Semispinalis capitis muscle
The semispinalis capitis muscle originates from the transverse processes of the 3rd cervical to 6th thoracic vertebrae. It runs to the occipital bone and inserts between the superior and inferior nuchal line. It is innervated by the dorsal rami (C1-C4), and extends the superior thoracic spine, the cervical spine, and the head joints.

Transversospinalis System: Multifidi Group

Image: “Deep Muscles of the back (posterior view)” by Phil Schatz. License: CC BY 4.0

The multifidi muscles have multiple points of origin: the dorsal surface of the sacral bone, the posterior sacroiliac ligaments, the iliac crest, the mammillary processes (lumbar vertebrae 1-5), transverse processes (thoracic vertebrae 1-12), and the articular processes (cervical vertebrae 4–7). They insert at the spinous processes of the 2nd cervical to the 5th lumbar vertebrae. They extend the complete spine as long as they are innervated by the dorsal rami (C3–S3).

Transversospinalis System: Rotator Group

The rotator group is formed by three muscles: lumbar rotator muscles, thoracic rotator muscles and cervical rotator muscles.

Lumbar rotator muscles
They originate from the mammillary processes of the lumbar spine and insert at the base of the spinous processes of the adjoining superior vertebra. They are innervated by the dorsal rami (L1–L5) and extend the lumbar spine.

Special features: These muscles are inconsistent and, thus, not present in every person.

Thoracic rotator muscles
The origins of these muscles are the transverse processes of the thoracic spine. They insert at the spinous processes of the adjoining superior vertebra. They are innervated via the dorsal rami (Th1–Th12), and extend the thoracic spine.

Cervical rotator muscles
These muscles originate from the transverse and articular processes of the cervical spine and insert at the spinous processes of the adjoining superior vertebra. They receive their neuronal supply through the dorsal rami (C2–C8) and are extensors of the cervical spine.

Special features: These muscles are inconsistent and, thus, not present in every person.

Spinalis and Interspinalis System of the Intrinsic Back Muscles

The muscles of the spinal and interspinal system form the deep layer of the medial tract and directly lie on the osseous structures. There are three different groups: interspinal, spinal and recti-capitis-group. They connect the spinous processes of the spine in its vertical course.

Image: “Deep Muscles of the back (posterior view)” by Phil Schatz. License: CC BY 4.0

Spinal and Interspinal System: Interspinal Group

The interspinal group contains three muscles: lumbar interspinales muscles, thoracic interspinales muscles, and cervical interspinales muscles.

Lumbar interspinales muscles
The origins of these muscles are the spinous processes of 2nd lumbar  to 1st sacral vertebrae. They are inserted at the spinous processes of the 1st – 5th lumbar vertebrae. They are innervated via the dorsal rami (L1–L5), and extend the lumbar spine.

Thoracic interspinales muscles
The thoracic interspinales muscles originate from the spinous processes of 2nd thoracic to 1st lumbar vertebrae, whereas they are often absent in the middle thoracic spine. They insert at the spinous processes of the thoracic vertebrae 1–12 and extend the thoracic spine as long as they are actively innervated by the dorsal rami (Th1–Th12).

Cervical interspinales muscles
These muscles originate from the spinous processes of 3rd cervical to 1st thoracic vertebrae. They are inserted at the spinous processes of the 2nd – 7th cervical vertebrae. They are innervated by the dorsal rami (C3–C8) and extend the whole cervical spine.

Spinalis and Interspinalis System: Spinal Group

The thoracic spinalis muscle, cervical spinalis muscle and spinalis capitis muscle are the three muscles of the spinal group.

Thoracic spinalis muscle
This muscle originates from the spinous processes of 10th thoracic to the 3rd lumbar vertebrae and inserts at the spinous processes of the thoracic vertebrae 2–8. It receives its neuronal supply via the dorsal rami (Th1–Th12) and is an extensor of the thoracic spine.

Cervical spinalis muscle
The cervical spinalis muscle originates from the spinous processes of the 6th cervical to the 2nd thoracic vertebrae and inserts at the spinous processes of the 2nd – 4th cervical vertebrae. It is innervated by the dorsal rami (C2–C4) and extends the cervical spine.

Spinalis capitis muscle
Originating from the spinous processes of the inferior cervical spine and the superior thoracic spine, the spinalis capitis muscle inserts between the superior and the inferior nuchal line at the occipital bone. It is innervated via the dorsal rami (C1–C2), and performs extension of the cervical spine and the head joints.

Special features: This muscle is inconsistent and is, thus, not present in every person.

Spinalis and Interspinalis System: Recti-Capitis-Group

The major posterior rectus capitis muscle and the minor posterior rectus capitis muscle form the rectus-capitis-group in the spinal and interspinal system. Both muscles are also included within the short neck muscles.

Major posterior rectus capitis muscle
This muscle originates from the spinous process of the axis and inserts at the middle third of the inferior nuchal line of the occipital bone. It is innervated by the suboccipital nerve (dorsal ramus C1) and is an extensor of the upper and lower head joint.

Minor posterior rectus capitis muscle

The ‘little brother’ of the major posterior rectus capitis muscle originates from the posterior arch and from the posterior tubercle of the atlas. It inserts at the medial third of the inferior nuchal line of the occipital bone and extends the atlanto-occipital joint at active innervation via the suboccipital nerve (dorsal ramus C1).

Intrinsic Back Muscles: Tabular Overview

You can learn everything about the extrinsic back muscles in a separate article.

Review Questions

The answers can be found below the references.

1. How many systems do the intrinsic back muscles consist of?

1. 1
2. 2
3. 3
4. 4
5. 5

2. Which of the listed functions is not performed by the intrinsic back muscles?

1. Flexion
2. Extension
3. Lateral flexion
4. Rotation
5. Stabilization in the atlanto-axial joint