Locating the Rectus Abdominis
Description
There are two rectus abdominis muscles, one on each side of the midline of the belly. Among the abdominal muscles, these are the only ones that are found at the front. They form the only muscle layer at the front of the belly.
The rectus abdominis muscles have an easily recognizable form: the contractile fibers (reddish) are broken by noncontractile zones (whitish). This structure gives the muscles their “six-pack” form.
The rectus abdominis muscles extend vertically along the entire length of the belly.
Insertion
At the top, each rectus abdominis muscle attaches on either side of the sternum at the cartilage of the fifth, sixth, and seventh ribs. They narrow as they descend almost directly down the front of the belly and attach to the front of the pelvis at the pubis.
How the Rectus Abdominis Acts on the Skeleton
1 • Retroversion
The rectus abdominis can pull on the pelvis, bringing the pubis toward the sternum and eliminating the arch of the lower back (the lower back rounds and the buttocks tuck under). This tucking of the pelvis is called retroversion. The rectus abdominis also prevent the pelvis from moving in the opposite direction.
2 • Dropping the Ribs
The rectus abdominis can pull the sternum and the front of the rib cage toward the pelvis, which causes the ribs to drop. It can fix the ribs in this dropped position and also prevent them from lifting. (Note that when we drop the ribs in this fashion, we tend to exhale.)
3 • Flexion of the Spine
Indirectly, by pulling on the pelvis or the ribs, the rectus abdominis can cause flexion (rounding) of the spine. It can also inhibit the spine’s movement in the opposite direction (for example, it prevents arching the back).
Important Note
We need to note three things here:
• The rectus abdominis does not act directly on the spine because it isn’t attached to any vertebrae.
• By acting on the lower thoracic rib cage and the pelvis, the rectus abdominis affects the lower thoracic vertebrae, the lumbar vertebrae, and the lumbothoracic and lumbosacral “hinges.”
• Of these affected regions, the most mobile is the lumbothoracic hinge—where the twelfth thoracic vertebra meets the first lumbar vertebra. As a result, the rectus abdominis has greatest effect in this area; it tends to round the midback (in the region of the lower ribs) more than it reduces the curve of the lower back (in the lumbar region). If we want it to flatten the lumbar curve, we first need to inhibit flexion in the midback.
How the Rectus Abdominis Acts on the Viscera
When contracted, the rectus abdominis pushes the viscera backward, moving them closer to the spine.
Important Note
When pushed, the viscera don’t decrease in volume. This is impossible, as they are like a volume of incompressible liquid. Instead the viscera change form, compacting out to the sides of the abdomen, up toward the thorax, and down toward the pelvis.
The rectus abdominis rarely contracts along its entire length; instead, it contracts region by region. For example, it can contract uniquely below the ribs, at the level of the navel, or above the pelvis.
Fully contracted rectus abdominis
Contraction below the ribs
Contraction at the level of the navel
Contraction above the pelvis
The rectus abdominis’s ability to move the belly by successively contracting at different levels means that it can move the viscera toward either the thorax or the pelvis.
The rectus abdominis is the director-in-chief of the belly, which it can move upward or downward. Its action is often coupled with the action of other abdominal muscles, but it’s the action of the rectus that most profoundly influences the movement of the belly.