Deep back muscles
The deep back muscles, also called intrinsic or true back muscles, consist of four layers of muscles: superficial, intermediate, deep and deepest layers. These muscles lie on each side of the vertebral column, deep to the thoracolumbar fascia. They span the entire length of the vertebral column, extending from the cranium to the pelvis.
The deep back muscles act together to provide support and maintain the body’s posture, as well as to produce movements of the head, neck, and trunk. The main functions of these muscles are flexion, extension, lateral flexion and axial rotation of the vertebral column.
All of these muscles are innervated by the segmental branches of the posterior rami of spinal nerves, and are supplied by several arteries along the various regions of the vertebral column.
This article will introduce you to the anatomy and function of the deep muscles of the back.
|Definition and function||The deep muscles of the back are a group of muscles that act to maintain posture and produce movements of the vertebral column.|
Superficial layer: splenius (splenius capitis, splenius cervicis)
Intermediate layer: erector spinae (iliocostalis, longissimus, spinalis)
Deep layer: transversospinales (semispinalis, multifidus, rotatores)
Deepest layer: segmental muscles (levatores costarum, interspinales and intertransversarii)
|Innervation||Posterior rami of spinal nerves|
|Blood supply||Vertebral, deep cervical, occipital, transverse cervical, posterior intercostal, subcostal, lumbar, and lateral sacral arteries|
- Superficial layer
- Intermediate layer
- Deep layer
- Deepest layer
The superficial muscle layer is composed of the splenius muscles (spinotransversales muscles), which are the splenius capitis and splenius cervicis. These flat muscles are located on the posterolateral aspect of the neck and the posterior upper thorax, overlying the deep muscles of the neck. The splenius muscles both originate from the spinous processes of cervical and thoracic vertebrae:
- The splenius capitis arises from the spinous processes of vertebrae C7-T3 and the lower half of the nuchal ligament. It then passes superolaterally to insert on the mastoid process and the lateral third of the superior nuchal line of the occipital bone.
- The splenius cervicis arises from the spinous processes of vertebrae T3-T6 and inserts onto the transverse processes of vertebrae C1-C3 or C4.
The splenius muscles are innervated by the posterior rami of the middle and lower cervical spinal nerves. The blood supply for both muscles comes from the vertebral, occipital, superior intercostal, deep cervical and transverse cervical arteries.
When acting together, both muscles produce extension of the neck. However, when acting individually, each muscle causes lateral flexion of the neck and rotation of the head to the same side.
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The intermediate layer contains the large erector spinae muscles which are sometimes called the long muscles of the back. This muscle group is the largest of the deep back muscles and lies on either side of the vertebral column between the spinous processes of the vertebrae and the angles of the ribs.
The muscles are composed of three vertical columns of muscle that lie side by side. From lateral to medial, these are the iliocostalis, longissimus and the spinalis muscles. Each muscle column is subdivided into regions (lumborum, thoracic, cervicis, capitis) based on which region of the axial skeleton it attaches to superiorly.
The iliocostalis muscle forms the lateral column of the erector spinae muscle group.
The muscle is divided into three regions according to its attachments:
- Iliocostalis cervicis
- Iliocostalis thoracis
- Iliocostalis lumborum
The attachments of the iliocostalis muscle are shown in the table below:
Origin: Angle of ribs 3-6
Inserition: Transverse processes of vertebrae C4-C6
Origin: Angle of ribs 7-12
Insertion: Angles of ribs 1-6, Transverse process of vertebra C7
Origin: Lateral crest of sacrum, Medial end of iliac crest, Thoracolumbar fascia
Insertion: Angle of ribs 5-12, Transverse processes of vertebrae L1-L4 (+ Adjacent thoracolumbar fascia)
The iliocostalis is innervated by lateral branches of the posterior rami of cervical, thoracic and lumbar spinal nerves. Each region of the iliocostalis muscle has a specific blood supply. The iliocostalis cervicis is vascularized by the occipital, deep cervical and vertebral arteries. The iliocostalis thoracis is supplied by the dorsal branches of posterior intercostal and subcostal arteries, while the dorsal branches of the lumbar and lateral sacral arteries supply the iliocostalis lumborum.
The function of the iliocostalis muscles is to produce ipsilateral lateral flexion of the spine when acting unilaterally and to extend the spine during bilateral contraction.
The longissimus muscle forms the central column of the erector spinae muscle group and is the longest and thickest of this group. It is divided into three regions based on their attachments:
- Longissimus capitis
- Longissimus cervicis
- Longissimus thoracis, which is further subdivided into thoracic and lumbar parts.
The attachments of the longissimus muscle are shown in the table below:
Origin: Transverse processes of vertebrae C4-T5
Inserition: Mastoid process of temporal bone
Origin: Transverse processes of vertebrae T1-T5
Insertion: Transverse processes of vertebrae C2-C6
- Lumbar part - Lumbar intermuscular aponeurosis, medial part of sacropelvic surface of ilium, posterior sacroiliac ligament
- Thoracic part - Spinous and transverse processes of vertebrae L1-L5, median sacral crest, posterior surface of sacrum and posterior iliac crest
- Lumbar part - Accessory and transverse processes of vertebrae L1-L5
- Thoracic part - Transverse process of vertebrae T1-T12, Angles of ribs 7-12
The nerve supply to the various parts of the longissimus muscle is by branches of the posterior rami of the corresponding regional spinal nerves. The longissimus capitis and cervicis are vascularized by the vertebral artery, deep cervical artery, superficial and deep descending branches of occipital artery and deep branch of the transverse cervical artery. The longissimus thoracis on the other hand is supplied by the dorsal branches of superior intercostal, posterior intercostal, lateral sacral and median sacral arteries.
During bilateral contraction, the longissimus muscle functions as a powerful extensor of the lumbar, thoracic and cervical spine, as well as an extensor of the head and neck. Unilateral contraction of the muscle results in ipsilateral lateral flexion of the spine.
The spinalis muscle is the smallest and most medial of the erector spinae muscle group.
Like the longissimus, the spinalis muscle is divided into three parts:
- Spinalis capitis
- Spinalis cervicis
- Spinalis thoracis
The attachments of the spinalis muscle are shown in the table below:
Origin: Spinous processes of vertebrae C7-T1
Inserition: Occipital bone (midline)
Origin: Spinous processes of vertebrae C7-T1, nuchal ligament
Insertion: Spinous process of vertebrae C2-C4
Origin: Spinous process of vertebrae T11-L2
Insertion: Spinous process of vertebrae T2-T8
The innervation of the spinalis muscle comes from the lateral branches of the dorsal rami of adjacent spinal nerves (cervical, thoracic and lumbar).
The blood supply of the spinalis cervicis and capitis muscles is provided by muscular branches of the vertebral, deep cervical, and occipital arteries. The spinalis thoracis muscle is supplied by dorsal branches of the superior and posterior intercostal arteries, and branches of the lumbar arteries.
As other erector spinae muscles, the main function of the spinalis muscle is extension of the vertebral column during bilateral contraction, and lateral flexion of the spine to the same side when acting unilaterally.
The deep layer contains the transversospinalis muscle group which is made up of the semispinalis, multifidus, and rotatores muscles. These muscles lie between the spinous and transverse processes of the vertebral column, deep to the erector spinae muscles. They arise from the transverse processes of the vertebral column and run upwards and medially in an oblique fashion to insert on the spinous processes of superior vertebrae. Generally, the muscles of the transversospinalis group stabilize the vertebrae during localized movements of the intervertebral joints of the vertebral column.
The most superficial muscle in this group is the semispinalis muscle, spanning the thoracic and cervical regions of the vertebral column, with an attachment on the occipital bone of the skull.
The semispinalis muscle has three parts:
- Semispinalis capitis
- Semispinalis cervicis
- Semispinalis thoracis
The attachments of the semispinalis muscle are shown in the table below:
Origin: Articular processes of vertebrae C4-C7, transverse processes of vertebrae T1-T6
Inserition: Between superior and inferior nuchal lines of occipital bone
Origin: Transverse processes of vertebrae T1-T6
Insertion: Spinous processes of vertebrae C2-C5
Origin: Transverse processes of vertebrae T6-T10
Insertion: Spinous processes of vertebrae C6-T4
The semispinalis capitis is innervated by the greater occipital nerve (posterior ramus of C2 spinal nerve) and spinal nerve C3, while both the semispinalis cervicis and the semispinalis thoracis are innervated by medial branches of posterior rami of spinal nerves.
The various parts of the semispinalis muscle are vascularized by branches of the occipital, deep cervical, vertebral and dorsal branches of posterior intercostal arteries.
The semispinalis muscle has a unique function due to its attachment to the skull. Bilateral contraction of this muscle draws the head posteriorly, extending the neck and thoracic spine. Unilateral contraction, on the other hand, causes ipsilateral flexion of the neck and thoracic spine with contralateral rotation of the head.
The multifidus belongs to the intermediate layer of the transversospinalis muscle group. This muscle is composed of many short, triangular muscles that span the entire length of the vertebral column, but are thickest and most developed in the lumbar region. The multifidus is divided regionally into three:
- Multifidus cervicis, that arises from the superior articular processes of vertebrae C4-C7.
- Multifidus thoracis, that originates from the transverse process of thoracic vertebra.
- Multifidus lumborum, that arises from the mammillary processes of lumbar vertebrae, posterior aspect of sacrum, posterior superior iliac spine (PSIS) of ilium and posterior sacroiliac ligament.
All three parts of the multifidus muscle insert on the lateral aspect and tips of the spinous processes of vertebrae 2-5 levels above origin.
The nerve supply to the multifidus muscle is derived from the medial branches of posterior rami of spinal nerves in the corresponding cervical, thoracic and lumbar regions. Its blood supply comes from the vertebral, deep cervical, occipital, posterior intercostal, subcostal, lumbar and lateral sacral arteries based on the regions the muscle parts occupy.
The main function of the multifidus is to stabilize the vertebrae during movements of the spine. Bilateral contraction of the muscle results in extension of the vertebral column at all levels, while unilateral contraction produces ipsilateral lateral flexion and contralateral rotation of the vertebral column.
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Deep to the multifidus are the small rotatores (rotator muscles), which are the deepest of this muscle group. Like the multifidus, the rotatores are also present along the entire length of the vertebral column, but are more prominent and best developed in the thoracic region. They consist of short rotatores (rotatores breves) which attach to the spinous processes of adjacent superior vertebrae and long rotatores (rotatores longi) which attach to vertebrae two levels up.
The attachments of the rotatores muscles are shown in the table below:
Origin: Transverse processes of vertebrae T2-T12
Inserition: Laminae/Spinous process of vertebra (1 level above origin)
Origin: Transverse processes of thoracic vertebrae
Insertion: Laminae/Spinous process of vertebra (2 levels above origin)
The rotatores are innervated by the medial branches of posterior rami of spinal nerves and receive their blood supply via dorsal branches of posterior intercostal and lumbar arteries.
Similar to the multifidus muscle, the major function of the rotatores is to stabilize the spine. Bilateral contraction of these muscles extends the vertebral column, while unilateral contraction causes rotation of the trunk to the contralateral side.
The levatores costarum, interspinales and intertransversarii muscles form the deepest layer of the deep back muscles and are sometimes referred to as the segmental muscles or the minor deep back muscles.
The levatores costarum muscles are located in the thoracic region of the vertebral column. They originate from the transverse processes of C7-T11 vertebrae and travel inferolaterally to insert between the tubercle and the angle of the corresponding rib below.
The levatores costarum are innervated by the lateral branches of the dorsal rami of thoracic spinal nerves (T1-T12), and vascularized by the dorsal branch of the posterior intercostal artery.
As their name suggests, the main function of these muscles is to elevate the ribs and facilitate inspiration during breathing.
The interspinales muscles are short, paired muscles that connect adjacent spinous processes of the vertebral column. These muscles are divided regionally into three parts; interspinales cervicis, thoracis and lumborum. They are well developed in the cervical and lumbar regions of the spine, but may be entirely absent in the thoracic region.
The attachments of the interspinales muscles are shown in the table below:
Origin: Superior aspect of spinous processes of vertebrae C3-T1
Inserition: Inferior aspect of spinous processes of vertebrae C2-C7
Origin: Superior aspect of spinous process of vertebrae T2, T11 & T12 (variable)
Insertion: Inferior aspect of spinous processes of vertebrae T1, T10 & T11
Origin: Superior aspects of spinous processes of vertebrae L2-L5
Insertion: Inferior aspects of spinous processes of vertebrae L1-L4
The interspinales muscles are innervated by the posterior rami of the respective spinal nerves. They receive blood supply from dorsal branches of respective regional arteries, namely the vertebral, deep cervical, occipital, transverse cervical, superior and posterior intercostal, subcostal and lumbar arteries.
The major function of these muscles is to stabilize the adjoining vertebrae of the vertebral column. They also assist with extension of the cervical and lumbar spine.
The intertransversarii muscles are small muscles that pass between the transverse processes of adjacent vertebrae and are most developed in the cervical and lumbar regions of the spine.
- Intertransversarii colli are found In the cervical region and are composed of anterior and posterior sets.
- Intertransversarii lumborum are found in the lumbar region. They consist of four pairs of muscles that extend between adjacent transverse processes of lumbar vertebrae.
The intertransversarii colli are innervated by the anterior and posterior rami of cervical spinal nerves, while lumbar intertransversarii are innervated by the anterior and posterior rami of lumbar spinal nerves. The intertransversarii colli receive their blood supply from the occipital, deep cervical, ascending cervical and vertebral arteries, while lumbar intertransversarii are vascularized by the dorsal branches of lumbar arteries.
The function of the intertransversarii colli muscles is to assist in lateral flexion and stabilization of the cervical spine. Similarly, the function of the lumbar intertransversarii is to aid in ipsilateral lateral flexion of the lumbar spine when acting unilaterally, and to stabilize the lumbar spine when acting bilaterally.
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