The zone of apposition refers to a vertical area of the diaphragms that begins at their insertion point on the inside of the lower ribs and extends to the top of the diaphragms. If the zone of apposition is present, the diaphragms will be dome-shaped and proper diaphragmatic breathing will occur without recruiting accessory muscles of respiration.
The zone of apposition is dependent on the position of the ribcage.
Ribs can either rotate externally, which is up, or they can internally rotate which is down.
An elevated ribcage has ribs that are in a state of external rotation. External rotation of a ribcage occurs when the lumbar spine is in a state of extension.
A proper ZOA will only occur when the the ribcage is in a state of neutrality, meaning the ribcage (and lower spine) are not in a state of extension. In other words, a proper ZOA can only occur when we can achieve thoraco-lumbar flexion.
Flexion is achieved through internal rotation of the ribs.
In PRI, thoracic/lumbar flexion and its accompanying internal rotation of the ribs is achieved through elongated exhalations that will recruit the external obliques. The internal obliques will rotate the ribs internally (down, back, and in) and then with the transverse abdominals hold them down during the next inhalation.
This is very important to understand.
If the ribs remain internally oriented during the next inhalation, this will force the ribcage to expand.
This ribcage expansion, instead of ribcage elevation, is what we are trying to achieve so that a proper zone of apposition can be found and maintained during respiration.
If the lower back arches upon inhalation, this means that the ribs are externally rotating, the ribcage is elevating as the lower back is extending, and the ZOA will be lost.
One way of helping people to maintain rib internal rotation during inhalation is by putting your hand under their lower back and instructing them to not leave your hand when they inhale. Usually this is done in the 90/90 position with the individual’s knees and hips bent at 90 degrees and feet on the wall.
If their lower back stays in contact with your hand as they inhale, this means that their ribs are internally rotated they are thus relying on rib expansion to breathe.
Because a proper ZOA depends on both proper pelvic and ribcage position, achieving a ZOA, particularly on the left, is an essential objective of all Postural Restoration programs.
For an example of how the ZOA can effect shoulder pain, you can check out this post.