Which muscle is primarily responsible for the extension of the spine?

The muscle primarily responsible for the extension of the spine is the erector spinae. This group of muscles runs vertically along the spine and is composed of three main parts: the iliocostalis, longissimus, and spinalis. When they contract, they work collectively to extend the vertebral column, allowing for movements such as standing up straight from a bent position or arching the back.

In contrast, the rectus abdominis is chiefly involved in flexing the spine, contributing to movements like bending forward or crunching. The latissimus dorsi, while important in shoulder movements and stabilization, does not play a direct role in extending the spine. The obliques aid in trunk rotation and lateral flexion rather than extension. Therefore, the erector spinae is the clear choice for the muscle responsible for spinal extension.