Upper and Lower Abdominals - The rectus abdominis is the most visible abdominal muscle group in the center of the abdomen. It is responsible for the six-pack. Oblique Abdominals - The oblique muscles are the largest abdominal muscles that run vertically along both sides of the torso. Transverse Abdominals - The transversus abdominis is the deepest abdominal muscle. It sits below the oblique muscles.