Final answer:
The transversus abdominis is the abdominal muscle that only compresses the abdomen. It is the deepest muscle layer in the abdominal wall and acts like a corset to increase intra-abdominal pressure. Unlike other abdominal muscles, it does not aid in trunk movements such as flexion or rotation.
Step-by-step explanation:
The abdominal muscle that can only compress the abdomen is called the transversus abdominis. This muscle is the deepest layer of the antero-lateral wall of the abdomen. Its muscle fibers run horizontally, or transversely, which is where it gets its name.
The transversus abdominis muscle plays a crucial role in maintaining abdominal wall tension and compressing the abdominal organs, effectively increasing intra-abdominal pressure. It acts much like a corset, tightening around the viscera. This muscle, along with the internal and external obliques and the rectus abdominis, comprises the abdominal muscles that cover the anterior and lateral abdominal region. This group of muscles contributes to various movements and rotations of the trunk but only the transversus abdominis functions solely to compress the abdomen without contributing to movements such as flexion or rotation like the other abdominal muscles do.
The rectus abdominis is another important muscle in the abdominal group, often referred to as the "sit-up" muscle. This muscle extends through the entire length of the trunk and is characterized by the tendinous intersections that give rise to the 'six-pack' appearance in a well-trained abdomen.