The rectus sheath envelops the straight abdominal muscles. It serves as protection from external influences and plays the crucial role in shaping the trunk, as long as it is not prevented from this function.
What is the rectus sheath?
The term rectus sheath is composed of the middle part of the name of the rectus abdominis muscle (straight abdominal muscle) and the original meaning of the word sheath = sleeve. It is a connective tissue sheath that wraps around the two muscle bellies that run next to the midline of the lower trunk. It actually forms a structure that can be compared to a sword sheath in terms of its shape. However, the movement aspect is missing. The muscles cannot be pulled out of the sheath like a sword. They are fixed and have only a slight displacement. The rectus sheath is formed from the aponeuroses (planar tendons) of the other abdominal muscles. These include the oblique abdominal muscles (M. obliquus abdominis externus and internus) and the transverse abdominal muscle (M. transversus abdominis). The tendon extensions, coming from the left and right, form a structure in which the fiber course of the individual sheets corresponds to the original direction of pull of the associated muscles. This creates a fiber network that systematically spans around the rectus abdominis muscle.
Anatomy and structure
The two straight abdominal muscles form two muscle cords that run laterally to the midline of the abdomen. They originate at the cartilages of the 5th – 7th rib and the tip of the sternum. From there they run straight down and attach to the symphysis. From top to bottom, they become continuously narrower. Between the two muscle bellies is a gap, the linea alba (white line), which is wider above the navel than below. The rectus sheath, formed by the other abdominal muscles, encompasses the two muscle bellies according to an organized system, which is divided into a part above and a part below the linea arcuata (arcuate line). The lower portion is only weakly and thinly expressed. For the organization of the upper section, the aponeurosis of the obliquus abdominis internus muscle divides into two sheets at the outer edge of the rectus. One pulls to its posterior side, the other covers the anterior side. The superficial leaflet is supplemented by extensions of the tendons of the obliquus abdominis externus muscle, the deep one by those of the transversus abdominis muscle. In the area of the linea alba, the fibers of the different parts cross and interlace.
Function and Tasks
The rectus sheath keeps the rectus abdominis muscle in shape. Together with the 3 – 4 intermediate tendons (intersectiones tendinae) that divide the two muscles into individual compartments, they form the basis for the six-pack (sometimes an eight-pack) when in good training condition. In the crossover area, it prevents the linea alba from spreading too far apart. An important function of all the components involved in the system is to protect the abdominal viscera. For this purpose, the rectus sheath is fused abdominally with the fascia and peritoneum so that, together with the muscle bellies, a strong protective blanket is formed at the front of the abdomen; laterally, the transversus abdominis muscle is mainly responsible for this function. The special construction of the rectus sheath with the crossing fibrous tracts of the various muscles is the decisive factor for the waistline of the trunk in this area. The construction corresponds to a supporting corset. The diagonal fibers of the rectus abdominis externus muscle of one side merge into those of the obliquus abdominis internus muscle of the other side without loss of continuity. This diagonal network is finally completed by the transverse fibers of the transversus abdominis muscle. Conscious tension of the muscles involved leads to an accentuation of the shapes in the middle of the body at normal body weight. The involvement of the aponeurosis of the transversus abdominis muscle at the edge of the rectus prevents the abdomen from being pulled in too much by the tension of this muscle. Such an action would not be beneficial to the underlying organs. The diagonal interconnection of two internal and external oblique abdominal muscles unites and reinforces their respective movement and stability functions. This is particularly significant in movements involving flexion and rotation of the spine, such as those used in anti-gravity abdominal training.
Diseases
A frequently transient dysfunction is the softening of the linea alba, rectus diastasis. It is caused by expansion of the abdomen. It typically occurs during pregnancy, when the abdominal wall widens to such an extent that the tensile forces that occur can no longer be compensated for by the tendon fibers. However, severe obesity or a pronounced weakness of the connective tissue can also cause this event. In the first two cases, the process is reversible if the abdominal circumference is restored to normal. Regular training of the oblique abdominal muscles promotes the regeneration process. In the area of the navel, the connective tissue structure of the linea alba is broken. Where the umbilical cord was in the embryo, there is a ring of muscles after birth, which is too weakly developed in infants, but also in some adults. If too much pressure is exerted, parts of the peritoneum can be pushed out through this opening, resulting in what is known as an umbilical hernia. This is often only visually conspicuous and without symptoms. An inflammation of the peritoneum, called peritonitis, occurs as a result of inflammation in the area of the abdominal organs and is usually caused by bacteria. The posterior leaflet of the rectus sheath, with its close proximity to the peritoneum, may be affected by the inflammatory process. A typical injury in which the rectus sheath can be affected is the abdominal muscle strain. Primarily, this results in a tear in the muscle fibers. But the causative sudden increase in tension also transfers to the connective tissue parts of the system. There, tears or circumscribed defects can also develop, which are highly painful.
