ACSM Personal Trainer Certification Practice Exam 2025 - Free Personal Trainer Practice Questions and Study Guide

The muscle responsible for the primary function of extension or hyperextension of the spine is the erector spinae. This group of muscles runs alongside the vertebral column and is essential for maintaining an upright posture as well as allowing for the bending and extending of the back. It consists of three main columns: the iliocostalis, longissimus, and spinalis muscles. When these muscles contract, they pull the spine into an extended position, which is critical for movements that require bending backward or standing tall.

In contrast, the other muscles listed have different primary functions. The biceps brachii is mainly involved in flexing the elbow and supinating the forearm. The rhomboids primarily facilitate the retraction of the scapula and are not involved in spinal extension. The triceps brachii's main role is to extend the elbow joint, making it another muscle that does not contribute to spine extension or hyperextension. Thus, the erector spinae is distinctly positioned as the key muscle associated with extending the spine.