Posterior Inferior Serratus Muscle

Description

Posterior trunk anatomy is presented with emphasis on the serratus posterior inferior muscle on a male torso, lying deep to the external oblique and latissimus dorsi and superficial to the thoracolumbar fascia. Its muscular slips arise from the spinous processes of the lower thoracic and upper lumbar vertebrae (classically T11 to L2) and run superolaterally to attach to the inferior borders of ribs 9 to 12 near their angles. Inferiorly, the superior margin of the iliac crest and adjacent abdominal wall musculature frame the lower field, while the thoracic cage and thoracolumbar junction provide the medial bony landmarks. Functionally, serratus posterior inferior is often taught as an accessory breathing muscle because it can depress the lower ribs during forced expiration and help stabilize the posterior rib cage during respiration. Small but clinically relevant. Pain generators in this region are more often myofascial than primary muscle injury, and this view helps distinguish serratus posterior inferior from adjacent sources of posterior thoracoabdominal pain such as costovertebral joint irritation, quadratus lumborum trigger points, or lower rib dysfunction. The rib angles and the muscle’s oblique fiber direction also make clear why injections, dry needling, or surgical dissection in the posterolateral thorax must respect the intercostal neurovascular bundle coursing along the inferior rib margin. Suitable for gross anatomy and kinesiology teaching when covering the posterior thoracic wall, thoracolumbar fascia relationships, and the mechanics of forced expiration. It also fits well in physiotherapy and sports medicine materials addressing posterior trunk pain patterns, and in surgical education for posterolateral approaches where layered dissection over ribs 9 to 12 is discussed. Anatomical accuracy verified by SciePro's Medical Advisory Board.