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The muscle primarily responsible for extending the back is the erector spinae. This group of muscles runs along the spine from the sacrum to the skull and is crucial for maintaining an upright posture as well as facilitating backward bending of the spine. The erector spinae consists of three columns: the iliocostalis, longissimus, and spinalis, which work together to extend the vertebral column. When these muscles contract, they pull the spine into a more upright position or arch it backward, aiding in movements such as lifting or reaching.

The latissimus dorsi, while involved in various movements of the shoulder and upper body, does not play a primary role in spinal extension. The multifidus also supports stabilization and small movements of the spine, but it is not the main muscle for extension. The trapezius primarily affects the shoulders and neck, helping in movements like shrugging and turning the head rather than extending the back. Hence, the erector spinae is the key muscle for this action.