Which muscle is NOT part of the erector spinae group?

The rectus abdominis is not part of the erector spinae group because it is primarily located in the anterior (front) part of the body, while the erector spinae muscles are situated along the posterior (back) side of the spine. The erector spinae group is composed of several specific muscles that play a critical role in maintaining an upright posture and facilitating movements of the vertebral column, including the iliocostalis, longissimus, and semispinalis. These muscles work together to extend, rotate, and laterally flex the spine.

The rectus abdominis, in contrast, is a major muscle of the abdominal wall responsible for flexing the lumbar spine, such as when one performs a sit-up. It also plays a role in stabilizing the pelvis and posture but operates in a different region and serves different functional purposes compared to the erector spinae muscles.