Intrinsic Back Muscles

The true (intrinsic) back muscles are developmentally derived from the paraxial mesoderm alongside the vertebral column itself. They act directly on the spine: extension, rotation, lateral flexion, and continuous postural control.

They are uniformly innervated by the posterior rami of the spinal nerves at the corresponding segmental levels. This is the single defining feature distinguishing this entire muscle group from the extrinsic muscles of Section 5.

• Organised into three layers (superficial to deep): splenius, erector spinae (intermediate), transversospinalis (deep), plus a group of minor, short segmental muscles at the deepest level.

Splenius capitis:

• Origin: Ligamentum nuchae, spinous processes of C7-T3/T4.
• Insertion: Mastoid process, lateral superior nuchal line.
• Innervation: Posterior rami of middle cervical spinal nerves.
• Action: Bilaterally, extends the head/neck; unilaterally, laterally flexes and rotates the head/neck toward the same side.

Splenius cervicis:

• Origin: Spinous processes of T3-T6.
• Insertion: Transverse processes of C1-C3 (upper cervical vertebrae).
• Innervation: Posterior rami of lower cervical spinal nerves.
• Action: Bilaterally, extends the neck; unilaterally, laterally flexes and rotates the neck toward the same side, functioning as splenius capitis's lower cervical counterpart.

The two splenius muscles together form a broad, strap-like sheet covering the deeper intrinsic muscles in the cervical/upper thoracic region, named from the Greek "bandage", reflecting their bandage-like appearance overlying the deeper muscle layers.

• The main bulk of the intrinsic back musculature, forming a broad, thick mass running the length of the vertebral column. It lies within a fibrous compartment formed by the thoracolumbar fascia.
• It arises from a common broad tendinous origin (the common erector spinae tendon, from the sacrum, iliac crest, and lower lumbar/sacral spinous processes). It then divides into three parallel columns, each further subdivided regionally.

Iliocostalis:

• The most lateral column, subdivided into iliocostalis lumborum, iliocostalis thoracis, and iliocostalis cervicis (progressively more superior/medial as it ascends), each segment overlapping and continuing the one below.
• It attaches to the angles of the ribs and (in the cervical part) the transverse processes of the lower cervical vertebrae.
• Innervation: Posterior rami at the corresponding segmental levels throughout.
• Action: Bilaterally, extends the vertebral column; unilaterally, laterally flexes the column toward the same side.

Longissimus:

• The intermediate, largest column, subdivided into longissimus thoracis, longissimus cervicis, and longissimus capitis: extends from the common tendinous origin up to the mastoid process (via longissimus capitis, the most superior extension), attaching along its course to the transverse processes and adjacent rib regions.
• Innervation: Posterior rami at the corresponding segmental levels throughout.
• Action: Bilaterally, extends the vertebral column and head; unilaterally, laterally flexes the column and head/neck toward the same side, and assists slight rotation of the head toward the same side (via longissimus capitis).

Spinalis:

• The most medial, smallest and least distinct column, subdivided into spinalis thoracis, spinalis cervicis, and spinalis capitis (the cervical and capitis parts often poorly developed or absent, frequently blending with adjacent semispinalis capitis fibres).
• It attaches directly between adjacent spinous processes.
• Innervation: Posterior rami at the corresponding segmental levels throughout.
• Action: Bilaterally, extends the vertebral column (a comparatively minor contribution given its small size relative to iliocostalis/longissimus); unilateral lateral flexion contribution is minimal given its near-midline position.

Mnemonic for the three columns, lateral to medial: "I Love Spine" (Iliocostalis, Longissimus, Spinalis).

• A group of short, obliquely oriented muscles running from transverse processes upward and medially to spinous processes, lying deep to erector spinae.
• They are subdivided by the number of vertebral segments each muscle fibre spans, forming a graded, layered system providing fine segmental control of spinal movement.
• They also give critical proprioceptive feedback for postural control, given the exceptionally high density of muscle spindles found in these small, deep segmental muscles, particularly multifidus.

Semispinalis:

• The most superficial and longest-spanning transversospinalis muscle, spanning approximately 4-6 vertebral segments, subdivided into semispinalis thoracis, semispinalis cervicis, and semispinalis capitis.
• Semispinalis capitis is notably substantial, forming much of the posterior neck's bulk. It is the most superior extension, attaching to the occipital bone (between the superior and inferior nuchal lines).
• Innervation: Posterior rami at the corresponding segmental levels.
• Action: Extends and contralaterally rotates the vertebral column/head (unilateral contraction rotates the head/trunk to the opposite side, given the muscle's oblique fibre direction running from a lower transverse process to a higher, more medial spinous process on the opposite side).

Multifidus:

• Spans approximately 2-4 vertebral segments, present throughout the vertebral column but particularly well-developed in the lumbar region, where it is the largest and most functionally significant of the transversospinalis muscles.
• It arises from the sacrum, mammillary processes (lumbar), transverse processes (thoracic/cervical), inserting onto the spinous process 2-4 levels above.
• Innervation: Posterior rami at the corresponding segmental levels.
• Action: Extends and contralaterally rotates the vertebral column (same oblique-fibre principle as semispinalis).
• In the lumbar spine, it functions critically as a key dynamic segmental stabiliser, providing fine intersegmental control essential for lumbar spine stability during movement.
• Multifidus atrophy/dysfunction is a well-recognised, frequently studied finding in chronic low back pain, reflecting this muscle's central role in segmental stability.

Rotatores:

• The shortest-spanning transversospinalis muscles, spanning only 1-2 vertebral segments (rotatores brevis, one segment; rotatores longus, two segments), most numerous and best developed in the thoracic region.
• They arise from the transverse process, inserting onto the lamina/spinous process of the vertebra immediately (or one level) above.
• Innervation: Posterior rami at the corresponding segmental levels.
• Action: Extend and contralaterally rotate the vertebral column at the individual segmental level; given their very short span and small size, believed to function primarily as fine proprioceptive sensors for segmental position/movement, with a comparatively minor direct mechanical contribution to gross movement.

Interspinales:

• Small paired muscles connecting adjacent spinous processes, best developed in the cervical and lumbar regions (where segmental mobility is greatest), poorly developed or absent in the thoracic region.
• Innervation: Posterior rami at the corresponding segmental levels.
• Action: Assist extension and provide proprioceptive feedback at the individual segmental level.

Intertransversarii:

• Small paired muscles connecting adjacent transverse processes, similarly best developed in the cervical and lumbar regions.
• Innervation: Posterior rami (and, notably, a variable contribution from the anterior rami at some levels, one of the few intrinsic back muscle exceptions to the strict posterior rami rule, though this is a relatively minor, inconsistent variation rather than a defining feature).
• Action: Assist lateral flexion and provide proprioceptive feedback at the individual segmental level.

Levatores costarum:

• Small muscles in the thoracic region, running from the transverse process of one vertebra obliquely down to the rib below (levatores costarum breves, to the immediately adjacent rib; levatores costarum longi, spanning to the rib one level further down).
• Innervation: Posterior rami of the corresponding thoracic spinal nerves.
• This is a notable exception among rib-related muscles, most of which: e.g., the intercostals, are innervated by anterior rami.
• Levatores costarum are intrinsic back muscles by both embryological origin and posterior rami innervation, despite their rib attachment.
• Action: Elevate the ribs, assisting inspiration (a minor accessory respiratory contribution) and assisting lateral flexion/rotation of the thoracic spine.

• Posterior rami innervation is the unifying anatomical thread across every muscle in this section.
• Each intrinsic back muscle receives its motor supply exclusively from the posterior ramus of the spinal nerve at (and, for longer-spanning muscles, spanning) its own segmental level. This directly reflects the muscles' shared paraxial mesodermal developmental origin alongside the vertebral column.
• Extension is the dominant action of the erector spinae (bulk, power) and, to a lesser degree, the transversospinalis group.
• Rotation is predominantly a transversospinalis function (contralateral, given the oblique fibre direction). It is most developed in the thoracic region (rotatores) but functionally most clinically significant in the lumbar region (multifidus, for segmental stability rather than gross rotational range, given the lumbar facet joints' own strong rotational restriction).
• Lateral flexion involves unilateral contraction of essentially the entire intrinsic muscle mass on one side, working together with the intertransversarii.
• Postural control depends heavily on the continuous, low-level tonic activity of the deep transversospinalis muscles (particularly multifidus) and the minor segmental muscles (interspinales, intertransversarii), given their high proprioceptive muscle spindle density.
• These small, deep muscles are now understood to function less as primary movement generators and more as fine-tuned segmental stabilisers and position sensors, continuously adjusting to maintain spinal alignment.
• They provide the sensory feedback that larger, more superficial muscles (erector spinae, and even the extrinsic muscles) use to generate coordinated, controlled gross movement.