The Intrinsic Back Muscles

Intrinsic Muscles of the Back

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Last updated 30 March 2006

Intrinsic Muscles of the Back

Click here for the Extrinsic Muscles of the Back.

The intrinsic or deep muscles of the back are concerned with the maintenance of posture and movements of the vertebral column and head.
The muscles are named according to their relationship to the surface: (1) a superficial layer, (2) an intermediate layer, and (3) a deep layer.

Superficial Layer of Deep Back Muscles

The Splenius Muscles (pp. 351, 353)

These are bandage-like muscles (G. splenion, bandage) are applied to the sides and back of the neck, somewhat like spiral bandages.
They ascend from the medial plane of the neck and the transverse processes of the superior cervical vertebrae to the base of the skull. Each muscle is divided into a cranial portion, splenius capitis and a cervical portion, splenius cervicis.

Origin: inferior half of the ligamentum nuchae and the spinous processes of T1 to T6 vertebrae.
Insertion: splenius capitis inserts into the lateral aspect of the mastoid process and the lateral third of the superior nuchal line of the occipital bone (deep to the sternocleidomastoid muscle). Splenius cervicis insets into the posterior tubercles of the transverse processes of C1 to C4 vertebrae (posterior to the levator scapulae muscle).
Innervation: dorsal rami of inferior cervical nerves.

Acting alone, the splenius muscles laterally flex and rotate the head and neck to the same side. Acting together, they extend the head and neck.

Intermediate Layer of Deep Back Muscles

The Erector Spinae Muscle (p. 353)

Click here for a diagram of the Erector Spinae Muscle.

This massive muscle forms a prominent bulge on each side of the vertebral column. It lies within a fascial compartment between the posterior and anterior layers of the thoracolumbar fascia.
The erector spinae is arranged in three vertical columns: iliocostalis (lateral column); longissimus (intermediate column); and spinalis (medial column).
The common origin of the three columns is though a broad tendon that is attached inferiorly to the posterior part of the iliac crest, the posterior aspect of the sacrum, the sacroiliac ligaments, and the sacral and inferior lumbar spinous processes.

The Iliocostalis Muscle (p. 353)

This is the lateral column of the erector spinae and it arises from the common origin and inserts into the angles of the ribs.
It may be divided into three parts according to the region involved.
Iliocostalis lumborum is attached to the inferior six ribs; iliocostalis thoracis is attached to all the ribs; and iliocostalis cervicis is attached to the superior six ribs and the posterior tubercles of C4 to C6 vertebrae.

The Longissimus Muscle (p. 353)

This is the intermediate column of the erector spinae muscle and it arises from the common origin and is attached to the transverse processes of the thoracic and cervical vertebrae, and the mastoid process of the temporal bone of the skull.
This gives this muscle a herringbone appearance.
The longissimus can also be divided into three parts according to the regions it traverses.
Longissimus thoracis inserts into the tips of the transverse processes of all of the thoracic vertebrae, and into the tubercles of the inferior nine to ten ribs; longissimus cervicis extends from the superior thoracic transverse processes to the cervical transverse processes; the longissimus capitis arises in common with the cervical part and attaches to the mastoid process of the temporal bone.

The Spinalis Muscle (p. 353)

This is the narrow medial column of the erector spinae muscle and is relatively insignificant. It arises from the common origin and extends from the spinous processes in the superior lumbar and inferior thoracic regions to the spinous processes in the superior thoracic region.
It may also be divided into three parts (spinalis thoracis, spinalis cervicis, and spinalis capitis).

Actions of the Erector Spinae Muscles (p. 353)

Acting bilaterally, all three columns of the erector spinae extend the head and part or all of the vertebral column.
Acting unilaterally, the erector spinae laterally flexes the head or the vertebral column.
In addition, the longissimus capitis muscle rotates the head so that it is turned to the same side.
The erector spinae muscles are the chief flexors of the vertebral column. They straighten the flexed column and can bend it posteriorly. They also "pay out" (release) during its flexion so that the movement is slow and controlled.

Deep Layer of Intrinsic Back Muscles

When the massive erector spinae muscles are removed, several short muscles (semispinalis, multifidus, and rotatores) are visible in the groove between the transverse and spinous processes of the vertebrae.
Collectively, this group of muscles is known as the transversospinal muscle because their fibres run from the transverse processes to the spinous processes of the vertebrae.

The Semispinalis Muscle (p. 355)

This muscle arises from about half of the vertebral column.
It can be divided into three parts according to their superior attachments: semispinalis thoracis, semispinalis cervicis, and semispinalis capitis.
Semispinalis thoracis and cervicis pass superomedially from the transverse processes to the thoracic and cervical spinous processes superiorly.
Semispinalis capitis arises from the transverse processes of T1 to T6 vertebrae and inserts into the medial half of the area between the superior and inferior nuchal lines on the occipital bone.
It forms the largest muscle mass in the posterior aspect of the neck.

Bilaterally, the semispinalis thoracis and cervicis extend the cervical and thoracic regions of the vertebral column.
Unilaterally, they rotate these regions towards the opposite side.
Bilaterally, the semispinalis capitis muscle extends the head.

Innervation: these muscles are supplied by the dorsal rami of the cervical spinal nerves.

The Multifidus Muscle (p. 355)

The name multifidus (L. multus, many, + findo, to cleave) indicates that this muscle is divided into several bundles.
It covers the laminae of S4 to C2 vertebrae. Its fibres pass superomedially from the vertebral arches to the spinous processes, spanning one to three vertebrae.

Acting unilaterally, the multifidus muscle flexes the trunk laterally and rotates it to the opposite side.
Acting bilaterally, they extend the trunk and stabilise the vertebral column.

Innervation: these muscles are supplied by the dorsal rami of the cervical spinal nerves.

The Rotatores Muscles (p. 355)

These short muscles, the deepest ones in the groove between the spinous and transverse processes, run the entire length of the vertebral column.
They are easiest to observe in the thoracic region where rotation of the vertebral column is greatest.
These rotators arise from the base of the spinous process of the vertebra superior to it. They rotate the superior vertebra to the opposite side; they also stabilise it.
They are innervated by the dorsal rami of the spinal nerves.

The Interspinales and Intertransversarii Muscles (p. 355)

These are small muscles that unite the spinous and transverse processes of consecutive vertebrae. They are well developed in the cervical region.
The interspinales help to extend the vertebral column.
The intertransversarius can produce lateral flexion of the superior vertebra, and acting bilaterally, they help to extend the vertebral column.
The interspinales are innervated by the dorsal rami of the cervical spinal nerves; the intertransversarii are supplied mainly by ventral rami of the cervical spinal nerves, but some are supplied by the dorsal rami.