The serratus posterior inferior counteracts the tendency of the diaphragm to pull the lower ribs inward and upward during forceful inspiration, maintaining their position as the diaphragm descends. It also depresses the lower ribs during forced expiration. Its origin from the thoracolumbar fascia links it mechanically to the thoracolumbar musculature and explains why lower thoracic and lumbar fascial restrictions can impair lower rib mobility and diaphragm function.
| Origin | Spinous processes of T11 through L2 via the thoracolumbar fascia |
|---|---|
| Insertion | Lower borders of ribs 9 through 12 |
| Nerve Supply | Anterior rami of T9 through T12 intercostal nerves |
| Blood Supply | Posterior intercostal arteries |
| Actions | Depresses the lower ribs during forced expiration; Fixes the lower ribs against the pull of the diaphragm during inspiration |
|---|
By anchoring the lower ribs during diaphragm contraction, it prevents the paradoxical inward rib movement that would reduce the efficiency of diaphragmatic breathing, ensuring that all of the diaphragm's descent is converted into thoracic volume increase rather than lower rib distortion.
Serratus posterior inferior trigger points produce lateral rib cage pain that is reproduced by deep palpation in the thoracolumbar junction region and sometimes confused with kidney or pleural pathology from its posterior flank location. The muscle is accessible to dry needling through the lower thoracic paraspinal region.
Palpable with deep pressure over the lower thoracic and upper lumbar paraspinal region immediately lateral to the lower erector spinae, at the level of the floating ribs.
Lateral flank aching from the serratus posterior inferior referred to the floating rib region, reproduced by deep thoracolumbar palpation and managed with trigger point therapy.
