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Longissimus thoracis muscle

The longissimus thoracis is the largest and most powerful part of the erector spinae muscle group. It extends longitudinally along the thoracic and lumbar spine, contributing significantly to spinal posture and movement. Together with iliocostalis and spinalis, it forms the erector spinae system, but longissimus thoracis is the most prominent muscle in the lumbar region.

It arises from the lumbar spine, sacrum, and iliac crest, and inserts onto the thoracic vertebrae, ribs, and transverse processes, forming an important connection between the lower back and thoracic cage. This muscle is vital for extension, lateral flexion, and stabilization of the vertebral column.

Synonyms

  • Thoracic longissimus

  • Longissimus dorsi (historical term)

  • Part of the erector spinae muscle group

Origin, Course, and Insertion

  • Origin:

    • Arises from the common erector spinae tendon, attached to the sacrum, iliac crest, spinous processes of lumbar vertebrae, and supraspinous ligament

  • Course:

    • Fibers ascend vertically along the thoracic and lumbar spine, forming a thick muscular column

    • Positioned medial to the iliocostalis and lateral to the spinalis muscles

  • Insertion:

    • Inserts into the transverse processes of all thoracic vertebrae and the ribs (2nd to 12th) between their tubercles and angles

Relations

  • Anteriorly: Thoracolumbar fascia, vertebral transverse processes

  • Posteriorly: Overlying latissimus dorsi and trapezius in upper thorax

  • Laterally: Iliocostalis muscle

  • Medially: Spinalis thoracis muscle and vertebral column

Function

  • Extension: Extends the vertebral column when acting bilaterally

  • Lateral flexion: Flexes the trunk laterally when acting unilaterally

  • Postural support: Maintains erect posture and stabilizes spine during movement

  • Respiratory role: Assists in rib movement during deep inspiration

Clinical Significance

  • Common site for paraspinal muscle strain and chronic back pain

  • Hypertrophy or spasm may contribute to scoliosis and spinal misalignment

  • Fatty degeneration may occur in chronic back pain syndromes or after spinal surgery

  • Evaluated in MRI for paraspinal pathology (myopathy, infection, tumor, denervation changes)

  • Landmark in spinal surgery and imaging-guided interventions

MRI Appearance

T1-weighted images:

  • Muscle shows low-to-intermediate signal intensity

  • Fatty infiltration appears as bright areas within the muscle

T2-weighted images:

  • Muscle shows low-to-intermediate baseline signal

  • Edema, strain, or myositis appear as bright signal intensity

STIR (Short Tau Inversion Recovery):

  • Normal muscle: low signal intensity

  • Inflammation, edema, or injury: bright hyperintense signal

T1 Fat-Sat Post-Contrast:

  • Normal muscle shows mild, uniform enhancement

  • Pathological processes (myositis, tumor, abscess) demonstrate heterogeneous or rim enhancement

3D T2 SPACE / CISS:

  • Muscle shows intermediate signal compared to adjacent tissues

  • Provides clear delineation of muscle boundaries, useful in evaluating muscle-tendon interfaces

CT Appearance

Non-Contrast CT:

  • Muscle appears as a homogeneous soft tissue density mass lateral to vertebral column

  • Chronic atrophy shows as low density due to fatty replacement

  • Acute hematoma appears as a hyperdense region within muscle belly

Post-Contrast CT:

  • Normal muscle enhances mildly and homogeneously

  • Inflammatory or neoplastic involvement shows heterogeneous or intense enhancement

  • Abscess shows rim enhancement with central low attenuation

MRI images

Longissimus thoracis muscle mri axial anatomy image-img-00000-00000

MRI images

Longissimus thoracis muscle mri coronal image

CT image

Longissimus thoracis muscle ct axial anatomy image-img-00000-00000