Final answer:
The rectus abdominis and quadratus lumborum muscles provide trunk stability in the frontal plane.
Step-by-step explanation:
The muscles that provide trunk stability in the frontal plane are the rectus abdominis and the quadratus lumborum.
The rectus abdominis is a long, linear muscle that extends along the middle of the trunk and helps to flex and stabilize the vertebral column.
The quadratus lumborum is located in the posterior abdominal wall and plays a role in posture and stabilization of the body.