External Abdominal Oblique Muscles
The external abdominal oblique is a pair of broad, thin, superficial muscles that lie on the lateral sides of the abdomen. Contraction of these muscles can lead to different actions, but they are most well recognized for their lateral flexion and twisting of the trunk known as a lateral bending. The external oblique muscles derive their name from their position in the abdomen, more outward than the internal abdominal oblique, and from the direction of their fibres, which run obliquely (diagonally) on the sides of the abdomen.
The external abdominal obliques originate along the lateral ribs 5 to 12 and insert into the linea alba (midline tissue) of the abdomen, pubis and the hip bone. Their shape is approximately rectangular with the long axis running front (anterior) to posterior (back) along the linea alba. Muscle fibres in the external oblique run medially (close to the midline) and inferiorly (underneath) from the origin to the point of insertion across the sides of the abdomen and the end just lateral to the rectus abdominis muscle.
The location and structure of the external oblique muscles provide them with different possible actions. Contraction of both (left and right) external oblique abdominal muscles together results in the compression of the abdomen (such as in sucking in the intestine), or the bending of the trunk (such as when performing a crunch or sit-up). Contraction of one of the abdominal oblique results in the side bending and twisting of the trunk on the reverse side. In other words, the left external oblique twists and bends the body to the right. The external abdominal oblique muscle helps in the flexion of the trunk using bilateral action, rotation of the body to the opposite side by unilateral action and compression of the abdominal wall. (Helps with urination, defecation, vomiting, and childbirth)
Internal Abdominal Oblique Muscles
The internal abdominal oblique muscle is located at the sides and the front of the abdomen, and it is the intermediate muscle of three flat muscles in this region, beneath the external oblique and on top of the transverse abdominal muscle. It is wide, thin and irregularly four-sided and occupies the side walls of the abdomen, extending across to the front. Both sides, acting together, bend the spine by drawing the pubis to the xiphoid process (the shortest of the three parts of the breast bone). The internal oblique divides around the rectus abdominis, and joins the transverse abdominis at the back, and the external obliques in the front.
One side also flexes (bends) the spine laterally (sideways) and rotates it, pulling the shoulder of that side forward. The external abdominal oblique muscle is also unevenly four-sided in shape and is superficial (outermost) to the internal oblique muscle. Both sides, acting together, flex the spine, drawing cartilage down toward the pubic bone. One side works alone by bending the spine to one side, rotates it to bring the shoulder on the opposite side to the front. Both of the abdominal oblique muscles together contract to compress abdominal contents, help digestion and assist during forced expiration. The internal abdominal oblique muscle assists in flexion of the trunk through bilateral action, rotation of the trunk on the same side, lateral flexion of the trunk to the same side (unilateral action) and compression of the abdominal wall. Helps with urination, defecation, vomiting, and childbirth.