Often mistakenly called the diaphragm by singers and by some teachers, the abdominal muscles are linked with the breath when they are used to "support the breath" or when they stretch as the belly distends due to that action of the diaphragm. The abdominal muscles are a number of muscles including the
- external obliques,
- internal obliques,
- transversus abdominis and
- rectus abdominis
The centre line of the abdominals in front, going from the xiphoid process (see sternum) down to the pubic bone, passes through your navel. In individuals with athletic builds, this line - the linea alba can be seen and felt clearly. It is the centre of the abdominal aponeurosis which is a sheet of tendon that the muscles of the front of the abdomen are attached to one another and the skeleton. From the linea alba, the aponeurosis splits into two layers, one deep, the other superficial, making what I like to imagine is a "muscle sandwich" of the rectus abdominis. This is called its "sheath". Beyond the rectus abdominis, there is another band of aponeurosis, the linea semilunaris, which separates the rectus abdominis from the external and internal obliques. As there are two layers to these muscles, the aponeurosis sheaths these as well by dividing into three here, and attaches at the ribs. At the bottom of the abdomen, the aponeurosis forms the inguinal ligament which joins the hip crest to the pubic bone.
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Starting from the linea alba and working out, we first come to the rectus abdominis. [Shown in the image above with black vertical arrows. This is the left side of the body.] Starting from the pubic bone, these muscles connect with the fifth, sixth and seventh ribs and xiphoid process. They are divided into compartments by strips of tendon that creates what body builders inaccurately call a "six pack", as there are 8 or possible 10 segments, or 4 to 5 pairs. These pairs can, if trained to do so, contract individually.
The deepest muscles of the abdominal muscles, the transversus abdominis go horizontally from front to back. [In the image above, these muscles are shown with black horizontal arrows, representing the right side of the body. The rectus abdominis has been removed, as have the external obliques.] Some of the muscle fibers interdigitate (fit together like clasped fingers) with the diaphragm and the transversus thoracis (see internal intercostals) Unlike the other abdominal muscles, the transversus abdominis are not postural muscles. They are very important in respiration, and are probably instrumental in forced exhalation.
Sandwiched in the middle between the external obliques and the transversus abdominis, the internal obliques begin from the side of the hip crest and the inguinal ligament, and connect with the linea semilunaris.[Shown with black arrows here, the transversus muscles have been removed.] Their direction is down and out, or the opposite of the external obliques. Can be used to compress the abdomen for exhalation.
the opposite of the internal obliques, they course downward and inward, largest and strongest abdominal muscles. They are on the surface and begin at ribs 5 to 12 and connect to the linea semilunaris. [In this image you can see them represented by black arrows. Also note the aponeurosis covering the rectus abdominalis.]
The muscles work posturally by contracting and may flex the spine (rectus abdominis, obliques working both sides at once) or twist the spine (individual obliques or opposite internal/external obliques working together). By compressing the abdomen, these muscles create higher pressures in the abdomen and thorax, essential for "voiding" various things from the body. In other words, one can feel these muscles in defecation, urination, vomiting, childbirth, and most interestingly here, forced expiration. Also, by relaxing these muscles, one can allow the distention of the belly to be more free - making the action of the diaphragm more easily felt.
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More on abdominals...
There may be the Abdomizer© in your closet, but nothing beats Pilates for targeting your abs so that you are truly "locked on" for effective, appropriate use of these core muscles for everything you do, and every breath you take.