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Iliocostalis lumborum muscle

The iliocostalis lumborum is the most inferior portion of the iliocostalis muscle group, which itself is part of the erector spinae complex. It forms an important paraspinal muscle mass in the lumbar region and contributes to extension, lateral flexion, and stabilization of the spine.

The muscle lies lateral to the longissimus thoracis and medial to the quadratus lumborum, occupying the groove between the spinous and transverse processes of the lumbar vertebrae. It plays a critical role in posture and is often involved in lower back pain syndromes, making it a frequent focus in musculoskeletal imaging.

Synonyms

  • Iliocostalis lumborum of erector spinae

  • Lower iliocostalis muscle

  • Lumbar iliocostalis

Origin, Course, and Insertion

  • Origin:

    • Arises from the sacrum, spinous processes of T11–L5, posterior part of the iliac crest, and thoracolumbar fascia

  • Course:

    • Fibers ascend obliquely and laterally from the sacral and iliac regions toward the ribs

    • Forms part of the erector spinae mass along with longissimus and spinalis muscles

  • Insertion:

    • Lower borders of the angles of ribs 6–12

    • Transverse processes of upper lumbar vertebrae

Relations

  • Anteriorly: Quadratus lumborum and psoas major

  • Posteriorly: Thoracolumbar fascia and superficial back muscles (latissimus dorsi)

  • Medially: Longissimus thoracis, multifidus

  • Laterally: Abdominal wall muscles (external and internal obliques)

  • Superiorly: Continuous with iliocostalis thoracis

  • Inferiorly: Attached to sacrum and iliac crest

Function

  • Extension: Extends the lumbar spine when acting bilaterally

  • Lateral flexion: Flexes the trunk to the same side when acting unilaterally

  • Stabilization: Helps maintain erect posture and stabilizes lumbar vertebrae during motion

  • Load distribution: Contributes to shock absorption and load transfer between spine and pelvis

Clinical Significance

  • Frequently involved in lower back pain, strain, or spasm

  • Can undergo atrophy or fatty infiltration in chronic low back pain or radiculopathy

  • Important landmark in lumbar spine imaging and surgery

  • May be injured in lifting, twisting, or sports-related activities

  • Evaluation is critical in MRI for paraspinal myopathy, infection, or tumor infiltration

MRI Appearance

T1-weighted images:

  • Muscle shows low-to-intermediate signal intensity

  • Fatty infiltration or degeneration shows as bright areas

T2-weighted images:

  • Normal muscle shows low-to-intermediate signal intensity

  • Strain, edema, or inflammation appears as bright signal intensity

STIR (Short Tau Inversion Recovery):

  • Normal muscle: low-to-intermediate signal

  • Pathology: bright hyperintensity indicating edema or myositis

T1 Fat-Sat Post-Contrast:

  • Normal muscle enhances mildly and uniformly

  • Abnormal enhancement in myositis, tumor infiltration, or abscess formation

3D T2 SPACE / CISS:

  • Muscle appears with intermediate signal intensity compared to surrounding fat

  • Provides high-contrast visualization of fascial planes and muscle borders

  • Useful for detecting small lesions or fascial extension of pathology

CT Appearance

Non-Contrast CT:

  • Muscle appears as soft tissue density lateral to lumbar spinous processes

  • Fatty infiltration shows as low attenuation regions

  • Calcification (rare) may be seen in chronic myopathy

Post-Contrast CT:

  • Normal muscle enhances mildly and homogeneously

  • Pathology (inflammation, tumor) appears as heterogeneous or intense enhancement

  • Abscess: rim enhancement with central low density

MRI images

Iliocostalis lumborum muscle MRI axial anatomy image-img-00000-00000

MRI images

Iliocostalis lumborum muscle mri coronal image

CT image

Iliocostalis lumborum muscle ct axial anatomy image-img-00000-00000