The internal oblique muscle, a key component of the abdominal wall, plays an important role in various bodily functions. It is one of three flat muscles forming the lateral part of the abdomen, working with others to support the trunk. Understanding its actions provides insight into how the body moves and maintains stability.
Locating the Internal Oblique
The internal oblique muscle is situated in the middle layer of the abdominal wall. It lies deep to the external oblique muscle, which is the outermost layer, and superficial to the transversus abdominis, the deepest abdominal muscle. This broad, thin sheet of muscle extends across the side of the abdomen.
Its fibers run obliquely, typically upward and forward, perpendicular to the external oblique’s fibers. They originate from the iliac crest, inguinal ligament, and thoracolumbar fascia, inserting into the lower ribs, linea alba, and pubic bone.
Primary Roles in Movement
The internal oblique muscle moves the trunk in various ways. When both internal oblique muscles contract simultaneously, they contribute to trunk flexion, bending the torso forward, as seen in a crunch. This bilateral contraction also helps compress abdominal contents.
When only one internal oblique muscle contracts, it facilitates lateral flexion, or side-bending, of the trunk to the same side. For instance, the right internal oblique would cause the trunk to bend to the right. Additionally, unilateral contraction of the internal oblique is responsible for ipsilateral trunk rotation, meaning it rotates the torso to the same side. This muscle works with the external oblique on the opposite side to achieve complex twisting movements, such as rotating the left shoulder towards the right hip.
Its Role in Breathing and Core Stability
Beyond visible movements, the internal oblique muscle is also important for breathing and core stability. It acts as an accessory muscle of respiration, particularly during forced exhalation. By compressing abdominal contents and pushing the diaphragm upward, it helps reduce chest cavity volume, expelling air from the lungs. This action is evident during coughing, sneezing, or strenuous exercise.
The internal oblique contributes to core stability by increasing intra-abdominal pressure (IAP). This pressure helps brace the spine and support physical activities like lifting or pushing. A stable core, maintained by the internal oblique and other abdominal muscles, aids overall body mechanics and injury prevention.