Zone of Apposition (ZOA)

ZOA Position & Mechanical Function

The diaphragm’s mechanical action and respiratory advantage depends on its relationship and anatomic arrangement with the rib cage8,16. The cylindrical aspect of the diaphragm that apposes the inner aspect of the lower mediastinal (chest) wall constitutes the zone of apposition. Its region extends from the diaphragm’s caudal insertion near the costal margin, cephalid to the costophrenic angle, where the fibers break away from the rib cage to form the free diaphragmatic dome16,17. The area of apposition of diaphragm to rib cage has a cephalad extreme at the beginning of dullness by percussion and a caudal extreme just above the costal margin16. For many of us the cephalid extreme begins immediately below T8 or below the cephalid aspect of the diaphragm’s dome. The zone of apposition, for the most part, is not influenced by height of diaphragm dome but rather by the orientation of the rib cage. Individuals with elevated anterior, externally rotated ribs will have a decrease in their zone of apposition on one side or both sides of their thoraco-abdominal, depending on their pattern of diaphragm opposition, abdominal weakness and use.

The area of apposition of diaphragm to rib cage makes up a substantial but variable fraction of the total surface area of the rib cage. It accounts for more than one half of the total surface at residual volume and decreases to zero at total lung capacity16. During quiet breathing in the upright posture, it represents one fourth to one third of the total surface area of the rib cage16. The zone of apposition has anatomic importance because it is controlled by the abdomen and oblique muscles and directs diaphragmatic tension. Accessory respiratory muscle overuse, chest wall mobility and lung hyperinflation are all influenced by diaphragm and zone of apposition resting positions at the end of exhalation10.

The rib cage and abdominal pathway are therefore always mechanically coupled through the zone of apposition1. Abdominal muscle resting tension opposes the inspiratory action of the diaphragm by facilitating an increase in pressure in the abdominal compartment rather than outward protrusion of the abdomen during diaphragmatic contraction19. Therefore, the zone of apposition and dome shape of the diaphragm are maintained during inspiration by abdominal muscle resting tension supporting the abdominal viscera and stomach up against the diaphragm’s undersurface.

In summary, the dome of the diaphragm corresponds to the central tendon and the cylindrical portion corresponds to the portion directly apposed to the inner aspect of the lower rib cage called the zone of apposition. In relationship to its function, the diaphragm can be considered as an elliptical cylindroid capped by a dome (see figures). In standing humans at rest, this zone of apposition represents about 30 percent of the total surface of the rib cage. When the diaphragm contracts during inspiration its muscle fibers shorten. The axial length of the apposed diaphragm diminishes and the dome of the diaphragm descends relative to its costal insertions. The height of the zone of apposition in normal subjects actually decreases by about 1.5 cm during quiet inspiration, while the dome of the diaphragm remains relatively constant in size and shape. Thus, the most important change in diaphragmatic shape, the one responsible for most of the diaphragmatic volume displacement during breathing, is a piston-like axial displacement of the dome related to the shortening of the apposed muscle fibers5. The most important change in diaphragmatic change, i.e. shortening of the apposed diaphragm muscle, is also dependent therefore on opposition of the anterolateral abdominal muscle for diaphragmatic respiratory mechanical advantage, action and position11.

ZOA Restoration

Apposition of the diaphragm can be lost unilaterally, almost always on the left or bilaterally; resulting in a left Anterior Interior Chain pattern (L AIC) or Posterior Exterior Chain pattern (PEC), respectfully.

Abdominal muscle, internal obliques, and transverse abdominis are primarily responsible for ipsilateral diaphragm leaflet opposition and for ipsilateral lower leaflet opposition upon contraction during inspiration, resulting in contralateral upper rib cage and apical chest wall expansion, especially during trunk rotation or gait. Loss of ipsilateral or bilateral abdominal opposition and diaphragm apposition results in hyperinflation. Studies have demonstrated that changes in diaphragm dimensions produced by chronic hyperinflation occur exclusively in the zone of apposition. Contraction of the diaphragm has been demonstrated to reduce the proportion of surface area apposed to the rib cage³.

Reducing physical and physiological symptoms associated with hyperinflation, paradoxical breathing and accessory respiratory muscle overuse requires repositioning and re-training of the diaphragm for normal zone of apposition activity.

Using the Postural Restoration Institute® L AIC manual technique, one can guide the rib cage and diaphragm into a position where the left leaflet of the diaphragm regains proper mechanical advantage to efficiently contract via the central tendon and where the dome can rest at expiration since tangential force is no longer needed for postural stabilization.

Proper position of the diaphragm is reached when expansion of the abdominal wall is no longer required during maximal opposition (internal rotation of the ipsilateral rib cage) at inspiration. Although simultaneous “belly” expansion and chest wall expansion is desirable upon inhalation via the nose without using accessory muscles of the neck; contralateral apical flexibility and chest wall mobility is needed during ipsilateral diaphragm apposition contraction for diaphragmatic breathing to occur effortlessly with assistance from external barometric pressure, chest wall re-coil, pleural elastic properties and negative internal mediastinal pressure.

A good example of active established ZOA occurs when one can perform a successful standing reach test, fingers to toes in standing, and inhale with anterior mediastinal compression and posterior mediastinal expansion. Passive ZOA can be reached through PRI manual techniques, if active ZOA is unobtainable. Maximum ZOA is completed passively, in supine, when at the end of the exhalation phase the trans-diaphragmatic strength during active ZOA contraction is the strongest at thoraco-lumbar flexion and the weakest at thoraco-lumbar extension. At the end stage of a L AIC manual restoration technique the anterior lower leaflet of the sighing patient will easily depress caudally and “drop” posteriorly, through internal rotation of the rib cage.

ZOA References

To read peer reviewed journal articles about PRI, click HERE.