Rectus Abdominis Muscle Anatomy

Location & Overview

The rectus abdominis is one of the five abdominal muscles. The other four are the external oblique, internal oblique, transversus abdominis, and pyramidalis.^[1] The rectus abdominis muscle is a long and somewhat narrow set of paired vertical muscles located on either side of the midline of the abdomen. It is located in the rectus sheath, running vertically and parallel to the linea alba. The linea alba is a long white fibrous line running down the midline of the rectus sheath; linea alba translates from Latin to English as white line.^[2]

The rectus sheath is a fibrous, resilient, and durable compartment. It extends upward from the pubic crest to the inferior costal margin and the costal cartilages of ribs 5 to 7. Contained within the rectus sheath are the rectus abdominis and pyramidalis muscles. The rectus sheath is composed of the aponeuroses of the transversus abdominis, internal oblique, and external oblique muscles.^[3]

The belly of the rectus abdominis muscle has divisions that are created by three tendinous intersections. This creates four muscle segments, sometimes referred to as a ‘six-pack’ (or an ‘eight-pack’ for those rare individuals who are genetically blessed enough to have clearly visible surface separations for all eight segments!). These tendinous intersections are connected to the anterior rectus sheath, which covers them.^[2]

Origin & Insertion

The rectus abdominis originates from the pubic symphysis and pubic crest located on the pelvis. It then inserts superiorly at the fifth to seventh costal cartilages of the ribs and the xiphoid process of the sternum.^[2]

Action & Function

The primary action of the rectus abdominis muscle is to cause flexion of the trunk. It can also tense the abdominal wall, which provides stability to the torso and helps prevent herniation. Tension of the abdominal wall assists in increasing intra-abdominal pressure, which in turn creates a more rigid and stable torso under load. Intra-abdominal pressure created via abdominal muscle contraction (or the Valsalva maneuver) can also help expel feces more rapidly.^[1]

Innervation

The rectus abdominis muscle is innervated segmentally by the thoraco-abdominal nerves. In many anatomical descriptions, this is given as the T7-T11 intercostal nerves and the T12 subcostal nerve. However, cadaveric studies of the anterior abdominal wall have reported broader and overlapping contributions, including T6 and L1. For this reason, a cautious way to describe the rectus abdominis nerve supply is that it is supplied mainly by T7-T12, with variable or overlapping contributions from T6 and L1 also reported.^[4,5]

These nerves pass through the abdominal wall toward the rectus sheath and enter the rectus abdominis from its deep or lateral aspect. The nerve supply is segmental, but not completely isolated by level, because adjacent nerves can branch and communicate within the abdominal wall and rectus sheath. This helps explain why individual nerve injury may not always correspond neatly to a single spinal segment, although clinically important denervation can still occur if larger motor branches are damaged.^[6,7,8]

Blood Supply

Blood is supplied to the rectus abdominis primarily by the superior and inferior epigastric arteries. These arteries also supply the rectus sheath.^[3,9] The superior epigastric artery arises from the internal thoracic artery and descends within the rectus sheath, supplying the upper portion of the rectus abdominis. The inferior epigastric artery arises from the external iliac artery and ascends toward the rectus sheath, supplying the lower portion of the muscle. These vessels form an anastomosis with each other within the anterior abdominal wall, helping provide collateral blood flow between the upper and lower parts of the rectus abdominis and rectus sheath.^[3]