What connects the sacrum to the pelvic girdle?

The sacrum connects to the pelvic girdle primarily through the vertebrae, specifically at the lumbosacral junction. This connection is crucial for transferring weight from the upper body to the lower limbs and plays a vital role in stability and mobility. The sacrum itself is formed from several fused vertebrae and integrates into the iliac bones of the pelvis. This anatomical arrangement allows the pelvic girdle to support the weight of the upper body, serves as an attachment point for various muscles, and facilitates movements such as walking, running, and jumping.

The other options pertain to different structures or do not provide the same level of connection to the pelvic girdle. Muscle fibers, while important for movement and stability, do not form a direct structural connection between the sacrum and the pelvic girdle. Smooth muscle tissue is primarily found in organs and is not involved in structural connections within the skeletal system. The coccyx, or tailbone, is located at the very end of the vertebral column and plays a role in pelvic support but does not serve as the structural connection between the sacrum and the pelvic girdle itself.