Final answer:
The four large flat abdominal muscles are rectus abdominis, external obliques, internal obliques, and transversus abdominis. They are essential for trunk movement, core stabilization, and protection of internal organs.
Step-by-step explanation:
The four large, flat abdominal muscles are known as the rectus abdominis, external obliques, internal obliques, and transversus abdominis. These muscles cover the anterior and lateral regions of the abdomen and converge at the anterior midline. The rectus abdominis is the muscle that is often associated with 'six-pack abs' and is enclosed in the rectus sheath, a sheet of connective tissue. The external obliques form the most superficial layer, extending inferiorly and medially. The internal obliques are situated just under the external obliques, running in a perpendicular direction to them. Finally, the transversus abdominis is the deepest muscle, arranged horizontally around the abdomen. Together, these muscles support various movements and rotations of the trunk, stabilize the core, and protect internal organs.