The external oblique muscle is the largest and most superficial of the three flat muscles that make up the lateral abdominal wall. This broad, sheet-like muscle stabilizes the trunk and facilitates movement. It protects the abdominal contents and contributes to the strength and function of the torso. Understanding its attachment points and actions is important for grasping its role in human anatomy.
Location and General Structure of the External Oblique
The external oblique muscle lies deep to the skin on the sides and front of the abdomen, forming the anterolateral abdominal wall. The muscle fibers run downward and medially, much like placing one’s hands into a jacket pocket.
As the fibers move toward the midline, they transition into a flat, sheet-like tendon called an aponeurosis. This aponeurosis extends across the anterior abdomen and interweaves with the aponeurosis from the opposite side. The external oblique is the outermost layer, situated superficial to the internal oblique muscle.
The Origin of the External Oblique Muscle
The external oblique muscle originates from the external surfaces of the lower eight ribs, ribs 5 through 12. It arises from the middle of the shaft of these ribs via eight fleshy projections, or digitations.
These digitations are arranged in an oblique line that runs inferiorly and anteriorly. The five upper slips interlock with corresponding slips of the serratus anterior muscle. The three lower slips interdigitate with the latissimus dorsi muscle.
The origin points are situated on the side of the rib cage. The fibers proceed from this superior and posterior attachment point, traveling across the side of the body toward the pelvis and the midline.
Insertion and Primary Actions
The muscle’s insertion is its distal, more movable attachment. The broad aponeurosis of the external oblique attaches along the entire length of the linea alba, a fibrous band that runs vertically down the midline of the abdomen.
Inferiorly, the muscle inserts onto the pubic crest and the pubic tubercle. The most posterior fibers insert onto the anterior half of the outer lip of the iliac crest, which is the curved upper border of the hip bone. The thickened inferior border of the aponeurosis, spanning between the anterior superior iliac spine and the pubic tubercle, forms the inguinal ligament.
The external oblique performs actions depending on whether one side or both sides contract simultaneously. When both muscles contract together (bilateral contraction), they cause trunk flexion, pulling the chest toward the hips, as seen during a sit-up. Bilateral contraction also compresses the abdominal contents, aiding in forced expiration, coughing, and defecation.
A unilateral contraction, where only one side shortens, results in two movements. It causes lateral flexion (side-bending) of the trunk toward the same side (ipsilateral). A unilateral contraction is also a primary driver of trunk rotation to the opposite side (contralateral rotation), meaning the right external oblique rotates the torso to the left.