The Science

Everything begins with posture.  PRI is focused on training providers in the science of postural adaptations, asymmetrical patterns and the influence of polyarticular chains of muscles.

The Basics Concepts

What is Posture?

Posture is the reflection of patterned airflow.

Airflow is a reflection of the “position” of many systems that are regulated, determined and created through limited functional patterns. These patterns reflect our ability and inability to breathe, rotate and rest symmetrically.

“Limited functional patterns” refers to movement that is restricted in directions, planes or normal boundaries of functional range, as a result of improper joint, muscle and mediastinum rest position. Function is therefore limited because soft tissue and osseous restrictions prevent one from using muscles and joints in their normal range. Adaptation and compensation for these limitations require neuromotor encoding and hyperactivity of muscle that is placed in improper positions that exceed normal physiological length or in positions that make them a mover or counter-mover in planes and directions that are not observed when one is in a neutral or more symmetrical state of rest. This compensatory activity and hyperactivity usually becomes dysynchronous in the accessory muscles of respiration and at the appendicular flexors and axial extensors, thus limiting functional rotation at the trunk and through the lumbo-pelvic-femoral and cranial-mandibular-cervical complex.


The human body is not symmetrical. The neurological, respiratory, circulatory, muscular and vision systems are not the same on the left side of the body as they are on the right, and vice versa. They have different responsibilities, function, position and demands on them. This system asymmetry is a good thing and an amazing design. The human body is balanced through the integration of system imbalances. The torso, for example, is balanced with a liver on the right and a heart on the left. Extremity dominance is balanced through reciprocal function; i.e. left arm moves with right leg and vice versa.

Postural Restoration Institute® (PRI) credentialed professionals recognize these imbalances and typical patterns associated with system disuse or weakness that develops because of dominant overuse. This dominant overuse of one side of the body can develop from other system unilateral overuse. For example, if the left smaller diaphragm is not held accountable for respiration as the right is, the body can become twisted. The right diaphragm is always in a better position for respiration, because of the liver’s structural support of the right larger diaphragm leaflet. Therefore, the left abdominals are always important to use during reciprocal function, such as walking, to keep the torso balanced.

Keeping the right chest opened during breathing is also challenging since there is no heart muscle inside the right side of the chest. Standing mainly on the right lower extremity to offset the weight of the left upper torso, assists in moving the pelvis forward on the left and the shoulder complex down on the right. This asymmetry compliments the special functions of the two sides of the brain. Although the two sides (hemispheres) of the brain share responsibilities for some functions, each hemisphere has its own “specialties”. Each hemisphere controls the opposite side of the body. The left brain has more responsibilities for speech and language and thus the right upper extremity becomes a dominant extremity in communication, growth and development. PRI credentialed professionals recognize when this normal pattern is not balanced sufficiently with left extremity neurologic and muscular activity.

When these normal imbalances are not regulated by reciprocal function during walking, breathing or turning, a strong pattern emerges creating structural weaknesses, instabilities, and musculo-skeletal pain syndromes. Balancing muscle activity around the sacrum (pelvis), the sternum (thorax) and the sphenoid (middle of the head) through a PRI approach best positions multiple systems of the human body for appropriate integrated asymmetrical function. PRI credentialed professionals incorporate reciprocal function to reduce ‘leading’ with the left pelvis and right arm, and respiratory function to maximize airflow in and out of the right lung.

Vision, occlusion, hearing, foot pressure, occupational demands, in-uterine position, etc. can all influence asymmetrical tendencies and patterns. Humpback whales bottom-feed on their right side, lemurs tend to be lefties when it comes to grabbing their grub, toads use their right forepaw more than their left, chimpanzees hold a branch up with the left hand and pick the fruit with their right hand, and humans usually balance their center of gravity over their right leg for functional ease and postural security. PRI credentialed professionals recognize the more common integrated patterns of human stance, extremity use, respiratory function, vestibular imbalance, mandibular orientation and foot dynamics; and balance these patterns, as much as possible, through specific exercise programs that integrate correct respiration with left side or right side inhibitory or facilitory function.

Introduction to PRI Clinical Considerations

Recognizing Asymmetry

Polyarticular Chains

Temporomandibular Cervical Chain (TMCC)
Muscles: Temporalis (ant. fiber), Masseter, Medial pterygoid, Rectus capitis posterior major, Obliquus capitis, Rectus capitis anterior, Longus capitis, Longus colli

Brachial Chain (BC)
Muscles: Anterior-Lateral Intercostals, Deltoid-Pectoral, Sibson’s Fascia, Triangularis Sterni, Sternocleidomastoid, Scaleni, Diaphragm

There are two brachial polyarticular muscular chains lying over the anterior pleural and cervical area. These chains influence cervical rotation, shoulder dynamics and apical inspirational expansion. They are composed of muscle that attaches to the costal cartilages and bone of ribs four through seven and xiphoid to the posterior, inferior occipital bone, anterior, inferior mandible and coracoid process of scapula. These two tracks of muscles, one on each side of the sternum, are anterior to the medial/upper mediastinum and upper thoracic cavity. They are composed of the triangular sterni, sternocleidomastoid, scalene, pectoralis minor, intercostals and muscles of the pharynx and anterior neck. They provide the support and anchor for cervical-cranial orientation, rotation and rib position. The right brachial chain muscle is opposed by the right posterior back muscles (PEC), lower trap, serratus anterior, external rib rotators and left internal abdominal obliques. The brachial chain muscle on the left is opposed by the left posterior back muscles (PEC), lower trap, serratus anterior, external rib rotators and right internal abdominal obliques.
Anterior Interior Chain (AIC)
Muscles: Diaphragm, Iliacus, Psoas, TFL, Vastus Lateralis, Biceps Femoris

There are two anterior interior polyarticular muscular chains in the body that have a significant influence on respiration, rotation of the trunk, ribcage, spine and lower extremities. They are composed of muscles that attach to the costal cartilage and bone of rib seven through 12 to the lateral patella, head of the fibula and lateral condyle of the tibia. These two tracts of muscles, one on each side of the interior thoraco-abdominal-pelvic cavity, are composed of the diaphragm and the psoas muscle. With the iliacus, tensor fasciae latae, biceps femoris and vastus lateralis muscles this chain provides the support and anchor for abdominal counter force, trunk rotation and flexion movement.

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.