Which muscle is NOT part of the muscles of the back?

The rectus abdominis is a muscle that is not part of the muscles of the back; instead, it is one of the primary muscles located in the abdominal region. This muscle plays a key role in flexing the spine and stabilizing the pelvis, but it is situated anteriorly (towards the front) in the body.

In contrast, the trapezius, rhomboid, and quadratus lumborum are all classified as back muscles. The trapezius is involved in moving, rotating, and stabilizing the shoulder blade as well as extending the neck. The rhomboid muscles, located between the shoulder blades, facilitate the retraction and stabilization of the scapula. The quadratus lumborum, located in the lower back, aids in lateral flexion of the vertebral column and stabilizes the pelvis. Thus, identifying the rectus abdominis as not part of the back muscles clarifies its anatomical location and function in relation to other muscle groups.