The rectus abdominis muscle is one of four abdominal muscles spanning the front of the body from the pubic bone to the ribcage. All of the abdominal muscles are paired and the rectus abdominis runs vertically on either side of the abdomen with the linea alba, a long strip of connective tissue, between them and connecting them.
It is a strange and troubling muscle. For some it is too short because thousands of sit-ups and crunches have made it hard as a rock. For others, and this category is in the majority, it is too long because of poor postural patterns that make the front of the body too long and the back of the body too short.
The span of the rectus abdominis muscle is so long on the front of the trunk that it can’t be effective on its own. No muscle can cover so much territory without the support of bones to help with the effort, so there are three tendinous bands inserted between the muscles essentially making them eight different muscles.
These tendinous insertions are what create the “six pack” appearance of an overly developed rectus abdominis muscle. Tendons don’t grow through exercise so they always remain the same size but enough crunches or sit-ups will get the individual muscles of the rectus abdominis to grow and bulge away from the tendons inserted in between.
Functionally, the rectus abdominis flexes the lumbar spine. If you are lying on your back and do a sit up, the rectus abdominis muscle draws the rib cage towards the pelvis.
If you are on your back and bring both legs up in the air and reach the feet up towards the ceiling lifting the hips up as well, the rectus abdominis will move the pelvis towards the ribcage.
If neither the pelvis nor the ribcage is fixed the rectus abdominis will pull the pelvis and ribcage towards each other equally.
While all of the abdominal muscles are distinct with unique functions they are also intimately connected through fascia and a sheet of connective tissue called an aponeuroses. In general, a balanced body is a healthy body but balance within the four abdominal muscles is fairly elusive. The rectus abdominis is the only set of abdominals to run vertically and the tendons of the other abdominal muscle encircle it at different points along the muscle.
Being separate yet connected in this way means that the tone of the rectus abdominis, or its lack thereof, will affect all of the other abdominal muscles as well.
We need to build balanced tone to all of the abdominals paying special attention to the transverse abdominis, the deepest of the abdominals which provides essential support to the lumbar spine.