Origin

• Pubic symphysis
• Pubic crest

Insertion

• Xiphoid process
• Costal cartilages of the 5th-7th ribs

The rectus abdominis is a long muscle that forms together with the pyramidalis muscle the anterior abdominal wall.

Inferiorly the rectus abdominis muscle is attached by two tendons; the larger one is attached to the pubic crest, from the pubic tubercle to the pectineal line, while the small, medial tendon is attached to the pubic symphysis.

The rectus abdominis muscle is a paired muscle that runs vertically, either side of the linea alba, and insert into the xiphoid process of the sternum and costal cartilages of the 5th to the 7th ribs.

In total there are three tendinous intersections in the rectus abdominis muscle. One of these horizontal intersections is present at the level of umbilicus, another at the level of xiphoid process and the third midway between them. These fibrous bands divide the muscle into segments, resulting in a  ‘six pack’ shape in those with low body fat.

Although the rectus abdominis is a single muscle, it is innervated by the anterior primary rami of the T7 to T12 spinal nerves. This segmental nerve supply allows each portion of the muscle to contract independently. For instance, by focusing on the part of the rectus abdominis below the belly button and extending to the pubic bone, individuals can specifically target and strengthen the lower segments of the muscle during exercises.

The rectus abdominis muscle, a key component of the anterior abdominal wall, is connected to the linea alba, a fibrous structure that runs down the midline of the abdomen, providing structural support and serving as the attachment point for the abdominal muscles.

On either side of the rectus abdominis, the pyramidalis muscle lies, a small triangular muscle that helps to tense the linea alba. Together, the rectus abdominis, linea alba, and pyramidalis work to stabilize the core and assist in movements such as flexion and rotation of the trunk.

PYRAMIDALIS MUSCLE

Origin
Pubic symphysis, pubic crest

Insertion
Linea alba

Function
Tenses the linea alba.

LINEA ALBA

Origin
Tip of the xiphoid process

Insertion
Anterior fibres: pubic symphysis,
Posterior fibres: pubic crests on both sides.

The aponeuroses of the internal and  external oblique abdominal muscles and the transversus abdominis muscle extend vertically across the anterior aspect of the abdomen. While crossing over to the opposite side, their aponeurotic fibers interweave along the midline of the abdomen, forming the thickening of the linea alba.

The linea alba generally lacks blood supply and innervation. Due to this, it is a common incision during abdominal surgery.

The umbilicus is located slightly inferior to the midpoint of the Linea alba, the linea alba is widest around the umbilicus. Its width and thickness also vary depending on one’s weight, age and sex.

ClLINICAL CORRELATION

The rectus abdominis diastasis, is defined as a gap of 2.7 cm or greater between the two sides of the rectus abdominis muscle. The distance between the right and left rectus abdominis muscle is created by the stretching of the linea alba. Diastasis of the rectus abdominis muscle most frequently occurs in newborns and pregnant women. In the newborn, the rectus abdominis muscle is not fully developed and may not be sealed together at midline (this is especially true for premature newborns).

The increasing weight during pregnancy combined with hormonal changes and altered posture can increase the load on the abdominal wall and result in the stretching of the linea alba. This linea alba stretching can also be attributed to excessive abdominal exercises after the first trimester of pregnancy.

In this case, a program of specific corrective Core exercises may help to reduce the size of the gap in pregnant or postpartum women. In extreme cases, diastasis is corrected with a cosmetic surgery  procedure known as abdominoplasty.

Trunk Flexion

• Primary muscle responsible for flexing the trunk, such as during bending forward or crunching movements.

Trunk Lateral Flexion (Ipsilateral)

• Aids in side-bending of the trunk toward the same side.

Trunk Rotation (Ipsilateral)

• Contributes to rotation of the trunk to the same side, working with the oblique muscles.

Postural Support

• Helps maintain upright posture, stabilizing the spine and preventing excessive arching in the lower back.

Intra-Abdominal Pressure Regulation

• Assists in compressing the abdominal cavity, playing a role in forced expiration, coughing, sneezing, and defecation.

Pelvic Stabilization

• Supports the pelvis during movements, particularly in dynamic activities like walking or running.

Assistance in Breathing

• Aids with forced exhalation, helping to expel air from the lungs by contracting and pushing the diaphragm upward.

Support During Childbirth

• Plays a role in expulsive efforts during labor, particularly during the final stages of delivery.

Act as Antagonist to the Diaphragm

• Contributes to reducing the volume of the chest cavity during exhalation by acting as an antagonist to the diaphragm.

Relations of the Rectus Abdominis

• The rectus abdominis muscle is contained within the rectus sheath, which is formed by the merging of the aponeurosis of the transversus abdominis muscle with the external and internal oblique abdominal muscles.