The Rectus Abdominis Muscle is a long, flat, paired skeletal muscle located on the anterior surface of the abdominal wall. It is commonly known as the "abs" or "diamondback muscle." This bilateral muscle is separated from each other by the midline connective tissue line, the linea alba. Each muscle bundle is surrounded by a serrated connective tissue border called the linea semilunaris. The rectus abdominis originates inferiorly from the pubic symphysis, pubic crest, and pubic tubercle, and extends superiorly to the costal cartilages of the 5th to 7th ribs and the xiphoid process.
Anatomy
Morphology and Connection Points
The Rectus Abdominis Muscle is a long, vertical muscle that runs along the anterior abdominal wall. The right and left rectus abdominis muscles are separated by a fibrous structure called the linea alba. The muscle's characteristic appearance arises from its tendinous intersections, which divide the muscle into segments, creating smaller "muscle ventricles." This segmented structure, which gives it a diamond-shaped appearance, is particularly evident in individuals with low body fat. The muscle fibers are most often attached to the cartilages of the upper ribs and can sometimes extend as far as the anterior end of the fifth rib. The rectus abdominis is enclosed in a fibrous structure called the rectus sheath. This sheath is a multilayered membrane formed by the aponeuroses of the external oblique, internal oblique, and transversus abdominis muscles, surrounding the muscle on both the anterior and posterior surfaces. The muscle typically has an average thickness of around 10 mm; this can reach up to 20 mm in individuals who exercise regularly. In terms of volume, it is approximately 300 cm³ in sedentary individuals and 500 cm³ in active individuals.
Embryological Origin and Classification
The Rectus Abdominis Muscle is part of the anterolateral abdominal wall muscles, embryologically derived from the ventrolateral muscle mass. This muscle, particularly with the uprighting of human posture, has assumed an important protective and functional role in the anterior abdominal wall, continuing its development from top to bottom while maintaining its segmental structure.
Nerve Innervation and Vascular Supply
The Muscle is innervated by the thoracoabdominal nerves T7–T11. These nerves are continuations of the intercostal nerves and pass through the anterior surface of the rectus sheath to reach the muscle's motor fibers. Sensory innervation is provided by the thoracic nerves T7–T12. Vascular supply to the muscle comes from several sources: the superior epigastric artery, a continuation of the internal thoracic artery, supplies blood to the upper portion of the muscle. The inferior epigastric artery supplies the lower portion and, along with the vein of the same name, provides vascular return. The lower intercostal arteries also provide support via segmental branches. These vessels, located on both the anterior and posterior surfaces of the muscle, form an extensive blood supply network.
Function and Clinical Relationships
The primary function of the Rectus Abdominis Muscle is trunk flexion. If the pelvis is stable, the muscle pulls the rib cage downward; this effect is clearly observed in exercises like crunches. If the rib cage is stable, the muscle pulls the pelvis upward; this is effective, for example, in leg raises. When both ends work together, a forward bending of the trunk occurs. The muscle is also actively recruited in situations requiring high intra-abdominal pressure, such as during forced expiration (coughing, sneezing, childbirth, vomiting, or heavy lifting). It also plays a role in maintaining posture, supporting internal organs, and increasing intra-abdominal pressure, facilitating physiological functions such as digestion and defecation. Among the variations, the sternalis muscle, observed in some individuals, stands out. This structure can be considered a variant of the pectoralis major or rectus abdominis muscle. The attachment of muscle fibers to the xiphoid process or costoxiphoid ligaments is also a rare but anatomically significant variation. Clinically, the rectus abdominis muscle is an important structure in abdominal surgery—especially in median and paramedian incisions. It is also an area that should be evaluated in cases of trauma or muscle tears (e.g., rectus abdominis strain in athletes).
