The iliocostalis (Latin: musculus iliocostalis) is a deep back muscle that lies lateral to the longissimus. It is one of the three muscles forming the erector spinae - the most powerful extensor of the back, the other two being the longissimus and spinalis muscles. The main function of the iliocostalis is to extend the spine when contracting on both sides and to flex the spine laterally when contracting on one side.
The muscle has three parts according to their origin: iliocostalis cervicis, iliocostalis thoracis, and iliocostalis lumborum.
The iliocostalis cervicis (also known as cervicalis ascendens, Latin: musculus iliocostalis cervicis) is the portion of the iliocostalis that is located deep in the back of the neck.
The muscle fibers of the iliocostalis cervicis originate from the third to sixth rib angles.
The iliocostalis cervicis inserts into the posterior tubercles of the fourth to sixth cervical vertebrae (C4 - C6).
Upon bilateral activation (on both sides), the iliocostalis cervicis extends the cervical spine. Upon unilateral activation (on one side), it laterally flexes the spine to the same side (ipsilaterally). However, the iliocostalis cervicis is a muscle with very little force capacity.
The iliocostalis cervicis is innervated by the lateral branches of the posterior (dorsal) rami of the lower cervical spinal nerves (C6 - C8).
The iliocostalis thoracis (also known as iliocostalis dorsalis or musculus accessorius, Latin: musculus iliocostalis thoracis) is the thoracic portion of the iliocostalis muscle.
The iliocostalis thoracis muscle fibers originate from the seventh to twelfth rib angles.
The fibers of the iliocostalis thoracis insert into the angles of the first to sixth ribs. However, some of its fibers attach to the transverse process of the seventh cervical vertebra (C7).
Upon bilateral activation, the iliocostalis thoracis extends the thoracic part of the vertebral spine, while upon unilateral activation, it laterally flexes the thoracic spine to the same side.
The innervation of the iliocostalis thoracis is provided by the lateral branches of the posterior (dorsal) rami of the spinal nerves.
The iliocostalis lumborum (or sacrolumbalis muscle Latin: musculus iliocostalis lumborum) is the third part of the iliocostalis located in the lower back.
The iliocostalis lumborum arises from the lateral crest of the sacrum, the medial end of the iliac crest, and the thoracolumbar fascia.
The iliocostalis lumborum inserts via flat tendons into the fifth to twelfth rib angles, the transverse processes of the first to fourth lumbar vertebrae (L1 - L4), as well as the adjacent thoracolumbar fascia.
The iliocostalis lumborum extends the lumbar spine by contracting bilaterally, while upon contracting unilaterally, it flexes the spine laterally to the same side.
The iliocostalis lumborum is innervated by the lateral branches of the posterior (dorsal) rami of the spinal nerves.