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Serratus Posterior Inferior

Muscles

The serratus posterior inferior is a thin, quadrilateral muscle of the lower posterior trunk that depresses the lower ribs during forced expiration, lying deep to the latissimus dorsi. It is a minor accessory expiratory muscle whose clinical relevance centers on its contribution to lower thoracic and upper lumbar back pain and its role as a stabilizer of the lower ribs for diaphragmatic function.

Origin, Insertion, Action, Innervation

  • Origin: Spinous processes of T11–L2 (via a broad aponeurosis continuous with the thoracolumbar fascia)
  • Insertion: Inferior borders of ribs 9–12 (lateral to their angles)
  • Action:
  • Primary: Depression of ribs 9–12 during forced expiration
  • Stabilization of the lower ribs against the pull of the diaphragm during inspiration (prevents the diaphragm from elevating the lower ribs as it descends)
  • Innervation: Ventral rami of T9–T12 (intercostal nerves)

Palpation Guide

  • Client position: Prone with arms at the sides.
  • Landmark sequence:
  1. Serratus posterior inferior lies deep to the latissimus dorsi and superficial to the erector spinae in the lower thoracic/upper lumbar region.
  2. The muscle occupies the area between the spinous processes of T11–L2 and the angles of ribs 9–12 — this corresponds to the lower thoracic-upper lumbar transition zone.
  3. It is not individually palpable as a separate layer. Palpation of this region accesses latissimus dorsi (superficial), serratus posterior inferior, and erector spinae (deep).
  4. Deep tenderness at the thoracolumbar junction that is lateral to the erector mass (toward the rib angles) may involve serratus posterior inferior.
  • Tissue feel: Indistinguishable from surrounding layers. The lower thoracic-upper lumbar region feels like a multilayered muscle complex.
  • Confirmation test: No reliable confirmation test isolates this muscle.
  • Common errors:
  • Attributing all lower thoracic back pain to the erector spinae — serratus posterior inferior lies between the latissimus and the erectors and can contribute to the pain picture at the thoracolumbar junction.

Trigger Point Referral

  • Common TrP locations: TrPs are found in the lower thoracic region, deep to the latissimus, at approximately the T11–T12 level lateral to the erector mass.
  • Referral pattern: Refers to the lower thoracic back with an aching quality that may extend anteriorly along the lower ribs.
  • Clinical significance: The referral pattern overlaps with thoracolumbar junction pain from longissimus and kidney pain location. If lower thoracic pain wraps anteriorly along a rib and is not explained by facet, disc, or visceral pathology, consider serratus posterior inferior TrPs in addition to the erectors.

Trigger point referral diagram — coming soon

Image coming soon. For visual reference, see [Serratus Posterior Inferior at TriggerPoints.net](http://www.triggerpoints.net/muscle/serratus-posterior-inferior).

Clinical Notes

Common conditions:
  • Contributes to thoracolumbar junction pain alongside longissimus thoracis — the T11–L2 region is a biomechanical transition zone where multiple muscles converge. Serratus posterior inferior is one of several structures that can be involved.
  • Lower rib stabilization dysfunction — when serratus posterior inferior is weak or inhibited, the diaphragm may pull the lower ribs superiorly during inspiration instead of solely descending into the abdomen. This alters rib cage mechanics and can contribute to breathing dysfunction. This is more commonly discussed in respiratory rehabilitation than in MT practice.
What you'll typically find:
  • Serratus posterior inferior is rarely the primary pain generator. It is part of the multi-layer muscle complex at the thoracolumbar junction and is typically treated as part of the overall lower thoracic-upper lumbar region.
  • In clients with chronic lower back pain, this region is tender but the pain is generally attributed to the erector spinae and quadratus lumborum, which are more prominent structures.
Treatment effects:
  • Treated as part of the lower thoracic back region — deep sustained pressure through the latissimus layer targets serratus posterior inferior and the underlying erectors simultaneously.
  • The thoracolumbar junction benefits from specific attention regardless of which muscle layer is primarily involved — focused work at this transition zone resolves many cases of vague lower thoracic aching.
Cautions:
  • The kidneys lie deep to this region at the costovertebral angle. Apply the same cautions as for quadratus lumborum and the erector spinae regarding kidney palpation.
  • Do not confuse serratus posterior inferior tenderness with kidney tenderness — costovertebral angle tenderness that is worse with percussion (Murphy's kidney punch test) suggests renal pathology.
Clinical pearl:
  • Serratus posterior inferior is a muscle that students are required to know for anatomy exams but rarely encounter as a primary clinical target. Its practical value is in understanding the thoracolumbar junction as a multilayered structure — when you treat this region, you are working through latissimus, serratus posterior inferior, and erector spinae. Knowing what lies in each layer helps you calibrate your depth and direction.

Assessment

Manual muscle testing:
  • Not individually testable. Functional contribution is too minor for isolated assessment.
Stretch test:
  • No standard clinical stretch test. Assessment is through palpation of the thoracolumbar junction region.
Related special orthopedic tests:
  • Murphy's kidney punch test — to differentiate renal from musculoskeletal pain at the costovertebral angle

Muscle Groups

Lower thoracic back layer (anatomical): Accessory expiratory muscles (functional): Lower rib stabilizers (functional):

Related Muscles

Synergist for lower rib depression: Antagonist:

Key Takeaways

  • Serratus posterior inferior is a minor expiratory muscle whose primary clinical value is understanding the layered anatomy of the thoracolumbar junction.
  • The thoracolumbar junction (T11–L2) is a transition zone where multiple muscles converge — treat it as a layered complex, not as individual muscles.
  • Always differentiate musculoskeletal tenderness at the costovertebral angle from kidney pathology before applying deep treatment.

Related

Sources

  • Travell, J. G., & Simons, D. G. (1999). Myofascial pain and dysfunction: The trigger point manual (Vol. 2, 2nd ed.). Williams & Wilkins.
  • Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2023). Clinically oriented anatomy (9th ed.). Wolters Kluwer.
  • Biel, A. (2014). Trail guide to the body (5th ed.). Books of Discovery.
  • Rattray, F., & Ludwig, L. (2000). Clinical massage therapy: Understanding, assessing and treating over 70 conditions. Talus Incorporated.
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