Rectus abdominis muscles:

‘The rectus abdominis muscles, commonly referred to as the “abs,” are a pair of long, flat muscles that extend vertically along the entire length of the abdomen adjacent to the umbilicus. Each muscle consists of a string of four fleshy muscular bodies connected by narrow bands of tendon, which give it a lumpy appearance when well defined and tensed. This lumpy appearance results in the rectus abdominis muscles being referred to as the “six-pack.”’

Image: Google images