The rectus abdominis muscle, also known as the "six pack", is a paired muscle running vertically on each side of the anterior wall of the human abdomen (and in some other animals). There are two parallel muscles, separated by a midline band of connective tissue called the linea alba (white line). It extends from the pubic symphysis, pubic crest and pubic tubercle inferiorly to the xiphoid process and costal cartilages of ribs V to VII superiorly.
Superior to the arcuate line, It is contained in the rectus sheath.
The rectus is usually crossed by three fibrous bands linked by the tendinous intersections. While the "six pack" is by far the most common configuration of the muscle bellies of the rectus, there exist rare anatomic variations which result in the appearance of eight ("eight pack"), ten, or asymmetrically arranged segments.
The rectus abdominis is an important postural muscle. It is responsible for flexing the lumbar spine, as when doing a "crunch". The rib cage is brought up to where the pelvis
is when the pelvis is fixed, or the pelvis can be brought towards the
rib cage (posterior pelvic tilt) when the rib cage is fixed, such as in a
leg-hip raise. The two can also be brought together simultaneously when
neither is fixed in space.
The rectus abdominis assists with breathing and plays an important role in respiration in the event the patient is short of breath.
It also helps in keeping the internal organs intact and in creating
intra-abdominal pressure, such as when exercising or lifting heavy
weights, during forceful defecation or parturition (childbirth).
The rectus abdominis is a long flat muscle, which extends along the
whole length of the front of the abdomen, and is separated from its
fellow of the opposite side by the linea alba.
The muscle is inserted by three portions of unequal size into the cartilages of the fifth, sixth, and seventh ribs.
The upper portion, attached principally to the cartilage of the fifth
rib, usually has some fibers of insertion into the anterior extremity
of the rib itself.
Some fibers are occasionally connected with the costoxiphoid ligaments, and the side of the xiphoid process.
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