Which of the following is NOT part of the Deep Longitudinal System?

• A. Erector Spinae
• B. Quadrates lumborum
• C. Thoracolumbar fascia
• D. Latissimus Dorsi

Answer: (D)

The Deep Longitudinal System forms a vertical, spine-to-leg chain that stabilizes and transmits forces along the posterior axis. Its key components are the erector spinae muscles running along the spine, and the thoracolumbar fascia which acts as the central connective tissue link. The quadratus lumborum also participates in this system by helping stabilize the lumbar region and pelvis, aligning with the spine’s longitudinal support.

Latissimus dorsi, while important for shoulder movement and connected to the thoracolumbar fascia, is not considered a core part of the Deep Longitudinal System. It mainly operates within the posterior oblique sling, linking the arm to the pelvis through the fascia and contributing to shoulder–hip coordination rather than forming the spine-to-leg longitudinal support itself.

So, the muscle not part of the Deep Longitudinal System is the latissimus dorsi.