Which muscle originates from the sacrum and iliac crest via the thoracolumbar fascia?

• A. Spinalis cervicis
• B. Latissimus dorsi
• C. Longissimus thoracis
• D. Quadratus lumborum

Answer: (C)

The muscle described uses the thoracolumbar fascia as a conduit from the sacrum and iliac crest to its origin, which is a hallmark of the latissimus dorsi. This broad back muscle pulls the arm back and toward the body, so it needs a wide, pelvis-to-spine origin that can anchor a powerful humeral movement. The latissimus dorsi arises from the thoracolumbar fascia along the lower spine and pelvis (including the sacrum and iliac crest), then fans up to insert on the intertubercular groove of the humerus. Its actions include extending, adducting, and medially rotating the arm, and it’s innervated by the thoracodorsal nerve (C6–C8).

Longissimus thoracis, by contrast, primarily attaches along the transverse processes of the lumbar and thoracic vertebrae and to the thoracolumbar fascia, but its main role is spine extension and lateral flexion, not direct humeral movement. It does not originate from the sacrum and iliac crest in the same direct way as the latissimus dorsi.