What prime mover of back extension a deep composite muscle consisting three columns?

The prime mover of back extension is the erector spinae, a deep composite muscle consisting of three columns:

- Iliocostalis: This column is located on the lateral side of the erector spinae and runs from the sacrum to the cervical vertebrae.

- Longissimus: This column is located in the middle of the erector spinae and runs from the lumbar vertebrae to the cervical vertebrae.

- Spinalis: This column is located on the medial side of the erector spinae and runs from the sacrum to the thoracic vertebrae.

The erector spinae muscles work together to extend the back and maintain posture. They are also responsible for rotating the spine and stabilizing the pelvis.