I am still not quite 100% clear on the mechanism behind the diaphragm and rib cage functional relationships. As I am currently understanding, because the right diaphragm is larger, has broader attachments along the spine and rib cage, and has the liver to help keep it in a domed position (better ZOA) it is therefore more powerful than the left. I don’t quite get why this rotates T8 down to the pelvis to the right? As a response to this right sided increased power the right abdominals must work harder to counterbalance the inspiratory force of the diaphragm and then become stronger than the left abdominals. Is it this abdominal strength imbalance left to right that causes the rotation?
The dominant pattern of right abdominal muscle activity is a component of the rotational force that orients the lumbar spine to the right, but is not the primary influence. The abdominals lack a direct attachment to the lumbar vertebral bodies, but they are able to work together with the diaphragm in a state called zone of apposition to influence the position of the lumbar spine because the diaphragm does have direct attachment. The position and influence of the diaphragm's right crural attachments with the lumbar vertebral bodies is really the central element of this right rotational force. The right crura are larger, thicker, attach lower on the lumbar spine and of course, are attached to the more powerful right half of the diaphragm. The diaphragm's crural attachment sites to the lumbar spine connect with the spine along the anterior aspect of the vertebral bodies and pull ipsilaterally toward the central tendon on the same side. This means the right crura, if not adequately opposed by a counter pull on the left side into left rotation, will rotate the lumbar spine to the right in the transverse plane. This dominant right diaphragm design issue is further exacerbated by the shorter position of the right abdominal wall as the thoracic spine and rib cage respond to the right oriented lumbar spine position by counter-rotating back to the left . You'll remember from the Diane Lee referenced drawings of rib cage mechanics during thoracic spine rotation that the rib cage on the side opposite the direction of thoracic spine rotation will move into internal rotation. This internal rotation of ribs on the right side gives the right abdominal muscles better leverage to work with the stronger and more influential right diaphragm. This is where the dominant abdominal strength comes into play, but it's still a secondary issue to the dominant role of the right hemi-diaphragm. - James Anderson, MPT, PRC