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The rectus abdominis is a paired set of thin muscles running vertically along the anterior, or front, of the human body, from the pelvis to the sternum. The two long muscles are connected by a band of tissue, and work to protect the internal organs of the abdomen while enabling flexion, or bending, of the torso. Together, the muscles of the rectus abdominis make up what is referred to as "six-pack" or "washboard" abs.
Horizontally, the muscles of the rectus abdominis are segmented into eight parts, separated by the same kind of dense tissue known as fascia, which connects them vertically. At its upper end, the rectus abdominis connects to the sternum between the fifth, sixth, and seventh ribs, near the bottom of the ribcage. At its lower end, the muscles connect to the pubis area at the front of the pelvis.
Functionally, it is largely the flexing of these muscles that causes a person to sit up from a prostrate position, as the spine curves. They are also crucial, along with the oblique muscles on the sides of the abdomen, in the stabilization of the upper body when it is supporting a load. The abdominal muscles are enclosed in what is called the rectus sheath, which helps them maintain their shape and compress the contents of the abdomen.
Due to popular emphasis on toned abdominal muscles, a number of exercise devices have been invented that attempt to isolate the rectus abdominis in an effort to develop the "six-pack." For the most part, however, these products have questionable effect. The "six-pack" effect is largely considered by nutritionists and fitness experts to be more a product of diet than abdomen-specific strength training, as only a thin layer of fat can obscure even the most developed abdominal muscles. Researchers estimate it takes 250,000 crunches to burn a pound of fat, making it a highly inefficient exercise for improving, or creating, "washboard" abs.
As with any muscles, the rectus abdominis are susceptible to injury. Damage from trauma or over-exertion can range from mild strains that heal naturally, to outright tears of the muscle that require surgery to repair. Rarely, the bottom of the rectus abdominis can detach entirely from the pelvic bone, which also requires surgery for repair. Such injuries can be troublesome and take a long time to heal, as it is difficult to isolate and rest the abdominal muscles.