Keeping it simple...
Known as "the abs", the left and right rectus abdominis muscles lie side by side up the front of the abdomen.
They are like strong ribbons of muscle, extending from the front of the pelvis to the rib cage.
If you look at how the body is put together (see image below) it becomes obvious that the rectus abdominis should provide the support between the pelvis and upper body - and not the back/lumbar muscles (which can be the source of a lot of pain when the rectus abdominis muscles are not adequately used).
The rectus abdominis are powerful muscles that, when functioning at optimal, allow the body to bend and flex in all possible directions.
Think of your rectus abdominis muscles as the body's 'core pillar of strength'.
Upper attachment to the chest:
Lower attachment to the pelvis:
Each rectus abdominis consists of several panels of muscle separated by strips of tough connective tissue.
These panels are what create the "6 pack look" but the number of sections of muscle depends on the individual - 4, 6, 8, 10 packs can occur.
Between left and right rectus abdominis muscles there is midline strip of tough connective tissue called the linea alba.
The linea alba is our primary guide for body alignment. We can feel, and alter, the positioning of the linea alba by working on activating and extending the rectus abdominis muscles to increase our state of alignment.
The rectus abdominis muscles are part your Base-Line, the key muscles to focus on to improve your posture and build the connection between body and mind.