Muscles of the Abdomen

The abdominal wall divides into two main regions: anterolateral and posterior. The anterolateral region contains five paired muscles organized into lateral and anterior groups. The posterior region lies between the last thoracic and lumbar vertebrae and includes the quadratus lumborum, psoas major, psoas minor, and iliacus muscles. Together, these regions provide flexible protection for internal organs.

The lateral group consists of three flat muscles arranged in layers. The external oblique is most superficial, the internal oblique runs perpendicular beneath it, and the transversus abdominis runs transversely around the abdominal wall. Their aponeuroses interlace to form the linea alba, a vertical fibrous connective tissue band extending from the sternum to the pubic symphysis.

The linea alba is a vertical band of fibrous connective tissue formed by the interlacing aponeuroses of the lateral abdominal muscles. It runs from the xiphoid process of the sternum to the pubic symphysis, serving as a central attachment point. The anterior muscles, rectus abdominis and pyramidalis, are positioned bilaterally to the linea alba.

The anterior abdominal wall contains two paired muscles: the rectus abdominis and the pyramidalis. The rectus abdominis is a long, flat muscle extending from the sternum and ribs to the pubic bone. The pyramidalis is a small, variable muscle located beneath the lower rectus abdominis that not everyone possesses.

The quadratus lumborum is a key stabilizer muscle located deep in the lower back on either side of the lumbar spine. It extends from the iliac crest to the lower ribs and lumbar vertebrae, aiding in maintaining upright posture. This muscle is essential for spinal stability and support during movement and standing.

The psoas major runs from the lumbar vertebrae to the lesser trochanter of the femur, while the iliacus is a fan-shaped muscle that originates from the iliac crest. These muscles work closely together, merging to form the iliopsoas muscle. Together, they extend into the lower limbs and facilitate hip flexion and movement.

The psoas minor is present in only about half the population and runs parallel to the psoas major. It is much smaller and thinner than the psoas major. When present, the psoas minor assists in flexing the pelvis and spine, though it is not essential for these movements.