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.” The name rectus abdominis comes from the Latin words for “straight” and “abdominal,” indicating that its fibers run in a straight vertical line through the abdominal region of the body. The rectus abdominis has its origins along the superior edge of the pubis bone and the pubic symphysis in the pelvis. Its insertions are at the inferior edges of the costal cartilages of the fifth through seventh ribs and at the xiphoid process of the sternum. A covering of connective tissue known as the rectus sheath surrounds the rectus abdominis muscles and provides attachment points for the internal and external oblique muscles that flank them on both sides. Between the rectus abdominis muscles is a thick mass of white fibrous connective tissue called the linea alba that unites the abdominal muscles of the left and right sides. The rectus abdominis muscle performs the important task of flexing the torso and spine in the abdominal region. It does this by pulling the ribcage closer to the pelvis. The rectus abdominis can also tense to contract the abdomen without moving the torso, as in sucking in one’s gut. Contraction of the abdomen results in increased pressure within the abdominopelvic cavity and is useful to push substances out of the body during exhalation, defecation, and urination. The external abdominal obliques are a pair of broad, thin, superficial muscles that lie on the lateral sides of the abdominal region of the body. Contraction of these muscles may result in several different actions, but they are best known for their lateral flexion and rotation of the trunk known as a side bend. The external obliques get their name from their position in the abdomen external to the internal abdominal obliques and from the direction of their fibers, which run obliquely (diagonally) across the sides of the abdomen.
The external abdominal obliques have their origins along the lateral ribs 5 through 12 and insert into the linea alba of the abdomen, the pubis, and the iliac crest of the hip bones. Their shape is roughly rectangular with the long axis running anterior to posterior along the linea alba. Muscle fibers in the external obliques run medially and inferiorly from the origins to the insertions across the lateral sides of the abdomen and end just lateral to the rectus abdominis muscles. The location and structure of the external abdominal obliques gives them many different possible actions. Contraction of both external obliques together results in the compression of the abdomen (as in sucking in the gut) or the flexion of the trunk (as in performing a crunch or sit-up). Contraction of one of the abdominal obliques results in the lateral flexion and rotation of the trunk on the opposite side; in other words, the left external oblique rotates and flexes the trunk to the right.