Which muscle is known as the "six-pack" muscle?

The rectus abdominis is the muscle commonly known as the "six-pack." It is a long, flat muscle that runs vertically along the front of the abdomen, from the pubic bone to the sternum and lower ribs. Tendinous intersections (bands of connective tissue) cross the muscle horizontally, creating the segmented "six-pack" appearance when body fat is low enough for the muscle to be visible.

The rectus abdominis is responsible for flexing the lumbar spine (bending the trunk forward), compressing the abdominal contents, and assisting in forced expiration and stabilising the pelvis during movement.

The other muscles listed are distinct:

• The rectus femoris is one of the four quadriceps muscles in the front of the thigh, responsible for knee extension and hip flexion. Despite sharing the word "rectus" (meaning straight), it is a leg muscle, not an abdominal one.
• The transversus abdominis is the deepest abdominal muscle. It wraps around the torso like a corset and stabilises the core, but it lies beneath the other abdominal muscles and is not visible as the "six-pack."
• The obliques (internal and external) are located on the sides of the abdomen and are responsible for trunk rotation and lateral flexion.