Semispinalis Thoracis Muscle Under the Skin, Posterior View

Description

Running longitudinally in the posterior thorax, the semispinalis thoracis is presented as paired, narrow bands immediately adjacent to the thoracic spinous processes, coursing superomedially from transverse processes in the lower thoracic region toward spinous processes several segments above. Superficial contours of trapezius and the posterior deltoid frame the upper back and shoulder, while the scapular spines and medial borders provide lateral landmarks against the rib cage. Deep to the more superficial erector spinae mass (longissimus thoracis and spinalis thoracis), the transversospinalis column sits medial and slightly anterior, with fibers aligned for segmental extension and contralateral rotation of the thoracic vertebral column. Skin is kept intact to emphasize how little of this deep muscle is appreciable on surface inspection. Clinically, semispinalis thoracis matters because it lies in the same deep paraspinal corridor traversed during posterior thoracic approaches and percutaneous procedures, where clear separation of longissimus, multifidus, and transversospinalis layers reduces iatrogenic muscle stripping and postoperative axial pain. Its proximity to the dorsal rami and medial branches helps explain why focal paraspinal tenderness can track along thoracic facet arthropathy patterns and why needle placement for thoracic medial branch blocks must respect the bony landmarks rather than surface muscle contours. Deep, segment-spanning fibers also make it a frequent contributor to persistent myofascial pain after whiplash-type mechanisms, even when superficial muscles look unremarkable. Small muscle. Big symptoms. Use this posterior, subcutaneous presentation in gross anatomy labs, kinesiology teaching on thoracic extension and rotation, and spine-focused atlases that need accurate layering from skin to deep paraspinals around the scapulae and thoracic spinous processes. It also supports surgical education materials for posterior thoracic exposure, facet interventions, and documentation of paraspinal pain generators. Anatomical accuracy verified by SciePro's Medical Advisory Board.