The iliocostalis is the most lateral column of the erector spinae group, spanning the full length of the back from the iliac crest to the cervical spine. Along with the longissimus and spinalis, it forms the erector spinae mass responsible for maintaining upright posture and extending the spine against gravity. Its lateral position makes it the primary muscle for ipsilateral lateral flexion within the erector spinae group.
| Origin | Iliac crest and thoracolumbar fascia; Lower six ribs medial to the angles; Upper six ribs at the angles |
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| Insertion | Lower six rib angles; Upper six rib angles and C7 transverse process; Posterior tubercles of C4 through C6 transverse processes |
| Nerve Supply | Posterior rami of spinal nerves from C4 to L3 |
| Blood Supply | Posterior intercostal and lumbar arteries |
| Actions | Extension of the vertebral column when acting bilaterally; Ipsilateral lateral flexion of the vertebral column; Depression of the ribs during forced expiration; Maintains erect posture against gravity |
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Acting bilaterally it resists forward flexion of the spine under load, providing the anti-flexion force required during lifting tasks, and eccentrically controls the forward bend in activities such as picking up objects from the floor.
The iliocostalis is a common site of muscle strain in acute low back pain, particularly in the lumbar portion during sudden unguarded lifting or rotational movements. Chronic low back pain frequently involves iliocostalis trigger points that refer pain across the low back and into the buttock region. The erector spinae group, including iliocostalis, is a primary target of spinal manipulation and massage therapy for low back pain.
The iliocostalis is palpable as the most lateral band of the erector spinae mass, running parallel to the spinous processes but 4 to 6 centimetres lateral to the midline, becoming firm during resisted spinal extension or ipsilateral lateral flexion.
Acute muscle fibre tears from sudden loading or unguarded movement, producing sharp unilateral low back pain that limits spinal movement and worsens with extension and ipsilateral lateral flexion against resistance.
