The oblique muscles are important components of the core musculature, contributing to torso stability and movement. These muscles form part of the abdominal wall, wrapping around the sides of the body. They support the trunk and maintain internal organ position. Understanding their arrangement helps explain how the body moves and stabilizes its midsection.
External Oblique Fiber Direction
The external oblique muscle is the largest and most superficial of the flat abdominal muscles, located on the lateral and anterior sides of the abdomen. It originates from the external surfaces of the lower eight ribs (5 through 12). From these origins, the muscle fibers course in an inferomedial direction, running downward and forward, often described as similar to placing hands into pockets.
The external oblique inserts into the linea alba, a fibrous band down the center of the abdomen, and also attaches to the pubic tubercle, pubic crest, and the anterior half of the iliac crest. This fiber direction enables the external oblique to flex the trunk when both sides contract. When one external oblique contracts, it contributes to lateral flexion of the trunk on the same side and rotation of the trunk to the opposite side.
Internal Oblique Fiber Direction
Deep to the external oblique lies the internal oblique muscle, a broad, thin sheet of muscle in the lateral abdominal wall. It originates from the thoracolumbar fascia, the anterior two-thirds of the iliac crest, and the lateral two-thirds of the inguinal ligament. The fibers of the internal oblique run in an oblique direction perpendicular to those of the external oblique.
These fibers travel in a superomedial direction, running upward and towards the midline. This contrasting fiber orientation allows for coordinated movements with the external oblique. The internal oblique inserts onto the inferior borders of ribs 10-12, the linea alba, and the pubic crest. When both internal obliques contract, they flex the trunk and compress the abdominal contents. Unilateral contraction results in lateral flexion of the trunk to the same side and rotation of the trunk to the same side.
Importance of Fiber Orientation
The distinct and opposing fiber orientations of the external and internal oblique muscles are important for torso movement and stability. The “criss-cross” arrangement, with external oblique fibers running downward and forward and internal oblique fibers running upward and forward, creates a strong muscular corset around the abdomen. This layering allows for powerful and coordinated actions not possible if the fibers ran in the same direction.
This opposing arrangement is important for rotational movements of the trunk. For instance, twisting the torso to the left involves the coordinated contraction of the right external oblique and the left internal oblique. This synergistic action, where muscles on opposite sides work together, provides efficient and strong rotational force. This fiber arrangement also contributes to lateral flexion and overall trunk stability, enabling dynamic movements while protecting the spine and abdominal viscera.