Longissimus thoracis is the largest muscle in the body by length, spanning from the pelvis to the upper thoracic vertebrae. It is the dominant component of the erector spinae group and the primary trunk extensor.
| Origin | Posterior sacrum, iliac crest, and lumbar spinous and transverse processes |
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| Insertion | Transverse processes of all thoracic vertebrae and ribs between the angles and tubercles |
| Nerve Supply | Dorsal rami of spinal nerves throughout the thoracic and lumbar levels |
| Blood Supply | Posterior intercostal and segmental lumbar arteries |
| Actions | Extends the thoracic and lumbar spine; Ipsilateral lateral flexion; The largest and most powerful trunk extensor |
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Longissimus thoracis volume and quality are the most important imaging predictors of low back pain recovery and surgical outcomes. Fatty infiltration of longissimus correlates strongly with failed back surgery syndrome. In scoliosis surgery, longissimus acts asymmetrically on the two sides of the curve.
The dominant paraspinal muscle mass — palpated from the lumbar region to the mid-thoracic spine during resisted trunk extension.
High longissimus fat fraction on pre-operative MRI predicting poorer outcomes after lumbar fusion, used to counsel patients and select appropriate operative approaches.
