The serratus posterior muscles are thin quadrilateral muscles of the intermediate back layer beneath the rhomboids and trapezius, lying between the deep back muscles and the superficial muscles. The serratus posterior superior is an inspiratory muscle; the inferior is an expiratory/stabilizing muscle. Both are too thin to contribute significantly to trunk movement but serve respiratory and proprioceptive functions.
| Origin | Superior: ligamentum nuchae and spinous processes of C7-T3; Inferior: thoracolumbar fascia and spinous processes of T11-L2 |
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| Insertion | Superior: upper borders of ribs 2-5 (lateral to their angles); Inferior: lower borders of ribs 9-12 (lateral to their angles) |
| Nerve Supply | Superior: intercostal nerves 2-5; Inferior: intercostal nerves 9-12 |
| Blood Supply | Posterior intercostal arteries |
| Actions | Superior: elevates ribs 2-5 during inspiration (respiratory muscle); Inferior: depresses ribs 9-12 and counteracts the pull of the diaphragm on the lower ribs during forced expiration |
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The serratus posterior inferior is a trigger point site producing lateral chest and flank pain that can mimic kidney pain. The muscles are relevant in thoracic and lumbar approach anatomy as they are divided during posterolateral approaches to the thoracic cage. MRI identification of the serratus posterior layer helps plan reconstructive procedures in complex back defects. The thoracolumbar fascia attachment of the inferior serratus posterior is significant in core stability models.
Myofascial trigger points in the serratus posterior inferior refer pain to the lateral lower chest and flank in a pattern indistinguishable from renal colic; the absence of urological symptoms, normal urine analysis, and pain reproduction by local pressure at the thoracolumbar junction distinguish the muscular source.
