Rectus Abdominis

Origin: Pubic symphysis and pubic crest.

Insertion: Xiphoid process of the sternum and costal cartilages of the 5th, 6th, and 7th ribs.

Action: The rectus abdominis muscle is responsible for flexing the lumbar spine (bending the torso forward), as well as compressing the abdominal contents.

External Oblique

Origin: External surfaces of the 5th to 12th ribs.

Insertion: Linea alba (a tendinous seam running down the center of the abdomen) and anterior half of the iliac crest (hip bone).

Action: The external oblique muscle aids in ipsilateral (same side) rotation of the trunk and flexion of the lumbar spine. When both sides contract together, they assist in trunk flexion and compress the abdomen.

Internal Oblique

Origin: Thoracolumbar fascia (connective tissue on the lower back), iliac crest, and lateral half of the inguinal ligament (a ligament that runs from the pelvis to the pubic bone).

Insertion: Linea alba, pubic crest, and costal cartilages of the 8th to 12th ribs.

Action: The internal oblique muscle is involved in contralateral (opposite side) rotation of the trunk and flexion of the lumbar spine. When both sides contract together, they help in trunk flexion and compress the abdomen.

Transversus Abdominis

Origin: Inner surfaces of the 7th to 12th ribs, thoracolumbar fascia, and iliac crest.

Insertion: Linea alba and pubic crest.

Action: The transversus abdominis muscle is responsible for compressing the abdominal contents, providing stability to the lumbar spine, and assisting in forced expiration.