The internal oblique is a large, prime-mover muscle located on the anterior trunk and part of the abdominal group. Relative to the other abdominal muscles, the internal oblique lies deep to the external oblique, superficial to the transverse abdominis, and lateral to the rectus abdominis. It is a thick, strong muscle that wraps around the lower torso, connecting the abdomen to the low back. The fibers of the internal oblique run at a downward angle from the anteriorly oriented abdominal aponeurosis to the iliac crest, then converge posteriorly at the thoracolumbar aponeurosis.

When all the abdominal muscles work together, the trunk flexes, rounding the torso and pulling the sternum toward the pubis. The internal and external oblique muscles also work in various combinations to laterally flex and rotate the trunk. Lateral flexion occurs when the internal and external oblique on the same side fire simultaneously. This means the right internal and external obliques laterally flex the trunk to the right and those on the left laterally flex the trunk left. For rotation, the internal oblique teams up with the external oblique on the opposite side. The right internal oblique works with the left external oblique to turn the trunk to the right and the left internal oblique works with the right external oblique to pull the trunk to the left. 

The internal oblique, external oblique, and transverse abdominis all compress and protect the abdominal contents and assist with forced exhalation. Activation of these muscles increases pressure within the abdominal cavity, helping to forcefully expel air from the lungs. Increasing intra-abdominal pressure also helps stabilize the spine during motions—like bending, lifting, and twisting—that stress the low back.

Activities that require strong trunk rotation and flexion, such as throwing overhand and pushing with one hand, may produce excessive tension in the abdominal muscles. Low-back pain may also prompt muscle guarding to limit motion and protect the spine. The internal oblique is often activated to brace the lower spine, since it wraps around the torso. Improving soft-tissue mobility and lengthening the internal oblique helps slack tissue in the low back, improving circulation and decreasing pain in that region.