Final answer:
The rectus abdominis muscle extends through the trunk and flexes the lumbar vertebrae, whereas the quadratus lumborum stabilizes the lower back and assists in lateral flexion.
Step-by-step explanation:
The muscle that flexes the lumbar vertebrae as part of its action is the rectus abdominis. This muscle extends through the entire length of the trunk and is a significant contributor to the movement of the vertebral column. The quadratus lumborum forms the posterior abdominal wall and it plays a key role in stabilizing these vertebrae and maintaining posture, but its primary function is not to flex the lumbar vertebrae. Instead, it helps with the lateral flexion of the spine and stabilizes the lower back.