Why do people sport 4-, 6- or even 8-pack abs?

As you struggle through your third set of crunches at the gym, one motivator might be the promise of six-pack abs. But for some people, this may not be physically possible — not because they aren’t exercising hard enough, but because their toned core will instead reveal a four pack of abs or, in other cases, an eight pack. So what’s behind the variations in washboard abs?

Everyone’s abdominal wall is made up of five different muscles that run from the ribs to the pelvis. These muscles help to protect the spine, support balance and improve mobility, research shows. The top layer, known as the rectus abdominis, is the largest muscle of the abdominal wall and is comprised of three fibrous bands that run vertically up the abdomen. Several fibrous tissues known as tendinous intersections run horizontally across the center vertical band, cutting the abdominal wall into sections, which eventually can give way to the appearance of six-pack abs.