Final answer:
The rectus abdominis, known as the "sit-up" muscle, is described as a long, vertical muscle encased in the rectus sheath, and it's highlighted in the context of the question.
Step-by-step explanation:
The rectus abdominis is the muscle highlighted in this context. The rectus abdominis is a prominent muscle of the anterior abdominal wall, commonly known as the "sit-up" muscle. It extends vertically along the length of the abdomen, enclosed by the rectus sheath. It is responsible for flexing the lumbar spine, as when performing a sit-up, and contributes to the appearance of "six-pack abs."
The reference to Figure 11.16 and Table 11.6 indicates the rectus abdominis as the medial muscle of the abdomen that is encased by a connective tissue called the rectus sheath, differentiating it from the internal oblique, external oblique, and transversus abdominis muscles which are layered laterally and have different orientations to support various movements and rotations of the trunk as well as aiding in the protection of the internal organs.