Which of the following statements is false about the muscles of the abdominal wall?

The statement regarding the internal oblique fibers running horizontally is incorrect. The internal oblique muscle fibers actually run at a diagonal or oblique angle, specifically upward and inward, which is opposite to the external oblique's downward and outward orientation. This arrangement allows the internal oblique muscles to assist in trunk rotation and lateral flexion effectively.

In the abdominal wall, the external oblique has fibers that run down and out, creating a V-shape, which contributes to abdominal compression and trunk movements. The rectus abdominis muscle has fibers that run vertically, which enables it to flex the lumbar spine and supports posture. The transverse abdominis muscle, on the other hand, does indeed have fibers that run horizontally and plays a critical role in stabilizing the core and compressing the abdominal cavity. These fiber orientations contribute significantly to the functional dynamics of the abdominal wall muscles.