Iliocostalis muscle

Origin and insertion

Iliocostalis muscle spans the entire back, from the neck to the pelvis. Depending on which region of the back the parts of iliocostalis attach, the muscle is divided into cervical, thoracic and lumbar portions.

• Iliocostalis cervicis arises from the angles of ribs 3-6 and inserts to the transverse processes of vertebrae C4-C6. 
• Iliocostalis thoracis originates from the angles of ribs 7-12 and inserts to the angles of the upper six ribs and transverse process of vertebra C7.
• Iliocostalis lumborum is divided into lumbar and thoracic parts. 

The lumbar part of iliocostalis lumborum originates from the lateral crest of sacrum, medial end of iliac crest and thoracolumbar fascia. It courses superiorly and inserts to the transverse processes of vertebrae L1-L4 and to the adjacent part of the middle layer of thoracolumbar fascia. The muscle fibers are laminated so that those attaching to L1 are the most superficial and medial, while those inserting to L1 are the deepest and most lateral. 

The thoracic part of iliocostalis lumborum originates from the same site as the lumbar, in a form of an aponeurotic sheath covering the lumbar part. Fibers course superiorly over the back and insert to the angle of ribs 5-12, laterally to iliocostalis thoracis. The muscle fibers of thoracic part are serially aligned, so that those attaching to the fifth rib are the most medial and inferior, while those inserting to the twelfth rib are the most lateral and superior.

Relations

Iliocostalis muscle lies over the posterior surface of the thoracic cage, in a gap between the longissimus muscle medially, and an imaginary line extending through the medial edge of scapula laterally. 

The dorsal surface of its cervical part is covered by the splenius capitis and rhomboid muscles, while the thoracic and lumbar segments lie beneath the thoracolumbar fascia. Moreover, the superior and inferior portions of the thoracic part lie deep to the serratus posterior superior and inferior muscles, respectively.

Innervation

Iliocostalis muscle is innervated by the lateral branches of posterior rami of cervical, thoracic and lumbar spinal nerves.

Blood supply

Arterial supply to iliocostalis has regional systematization;

• Iliocostalis cervicis is supplied by the occipital, deep cervical and vertebral arteries
• Iliocostalis thoracis is vascularized by the dorsal branches of posterior intercostal and subcostal arteries
• Iliocostalis lumborum receives blood from the dorsal branches of lumbar and lateral sacral arteries

Function

Iliocostalis muscle acts in synergy with the two other erector spinae components (longissimus and spinalis) to produce movements of the vertebral column. When these muscles contract unilaterally, they produce ipsilateral lateral flexion of the spine, while when acting bilaterally, they extend the spine.

Lateral flexion and extension are the movements produced during concentric contraction of erector spinae. During forward flexion of the spine, erector spinae muscles eccentrically contract to smoothen the bending of the vertebral column.