**2.3. Asymmetries of respiration**

and better-supported right leaflet, and a smaller, less efficient, left leaflet. The diaphragm's respiratory mechanics exert a powerful asymmetrical influence on the torso. The crura of the right leaflet, which inserts onto three lumbar vertebrae L1–3, is also stronger and thicker than the left crura, which inserts on only two lumbar vertebrae L1, 2 [16] (see **Figure 2**). This distribution exerts a right rotational influence on the lumbar spine, orienting it to the right. Articulation of the lumbar spine with the sacrum orients the sacrum to the right. Strong ligaments bonding the sacrum to the pelvis effect right rotation of the pelvis as well. This right rotational orientation of the lower spine and pelvis is enhanced by the gravitational shift of the body over the

Asymmetry facilitates movement. In a balanced system, asymmetry is a positive, vitalizing force. In the human body, loss of balanced musculoskeletal function precipitates and reinforces overuse of dominant postures and patterns because of the underlying structural bias toward right stance, influenced by organ placement, weight distribution, and muscle attachment. Habit and repetition perpetuate and reinforce dysfunction. Innate physiological human asymmetry may well be a factor in the onset and development of scoliosis and other postural disorders.

**Figure 2.** Diaphragm with crura. Florida Center for Instructional Technology copyright 2004–2017.

Webster's New World Medical Dictionary defines "neutral posture" as the stance that is attained when the "joints are not bent and the spine is aligned and not twisted" [17]. Neutral posture gives rise to the concept of "ideal posture" in which the alignment of body segments involves a minimal amount of stress and strain and which is conducive to maximal efficiency in use of the body [18, 19]. Ideal posture is critical for proper respiratory action [20]. When the body is in its

Due to physiological asymmetry, a neutral posture does not imply strict symmetry; rather, it describes a position of relative structural balance and a readiness for movement in any direction. Loss of relative musculoskeletal balance reflects persistence of a structural bias

ideal or neutral alignment, diaphragmatic respiratory mechanics are optimized [16].

**2.2. Neutral posture reflects relative musculoskeletal balance**

right leg due to the weight of the liver on the right side of the body [1–4].

138 Innovations in Spinal Deformities and Postural Disorders

The influence of the respiratory system is significant and often underlies or is complicit with scoliosis and other postural disorders. Understanding the mechanisms of breathing and how the loss of diaphragmatic competency can precipitate biomechanical dysfunction is not sufficiently appreciated in most current rehabilitation practices. Since the ability to exchange air is crucial to life, the respiratory system is a core motivator for muscle activity to insure adequate oxygenation. Within the respiratory system, the diaphragm is considered the primary muscle of respiration; however, there are numerous accessory muscles of respiration to assist when supplemental ventilation is needed. For instance, running places higher oxygen demands on the body to support a higher level of physical exertion. The accessory muscles of respiration are designed to accommodate such needs. Loss of diaphragmatic effectiveness due to postural or biomechanical dysfunction will result in pathological, compensatory accessory muscle recruitment [30].

The respiratory diaphragm is centrally located in our asymmetrically organized trunk. It is highly asymmetrical in form, in muscle attachment, and in function. Most importantly, it is uniquely positioned to directly influence every aspect of the postural, skeletal, and muscular core, and it influences the position and function of all other body systems [31]. The respiratory diaphragm is comprised of two muscles: a right and left hemidiaphragm [32], each with its own central tendon and each innervated by a right and left phrenic nerve, respectively [16]. Together, these two muscles span the internal dimension of the body just below the lungs. They insert on the xiphoid process, on the inner surfaces of ribs 7–12, and on the anterior aspect of the spine. The right leaflet is larger in diameter, it has a thicker and larger central tendon, its dome is higher, and it is better supported than the left by the liver beneath it and by strong right eccentric abdominal activity [31]. The right crura anchors to L1–3 on the right, the left crura to L1, 2 on the left [16]. The diaphragm leaflets also insert into the fascia overlying quadratus lumborum and to the psoas muscles via the arcuate ligaments, creating a strong functional linkage between these muscles. The superior strength, position, and function of the right hemidiaphragm supports and is supported by the physiological right orientation via right stance [1–4] (see **Figure 3A**).

The respiratory "Zone of Apposition" (ZOA) is the region of interface between the hemidiaphragm and the inner surface of ribs 7–12 [16, 33]. Apposition refers to multiple layers of muscles with differing fiber orientation lying adjacent to one another. The ZOA facilitates inhalation by generating tension between the muscle layers, which promotes external rotation of the ribs, complementing the action of the external intercostals. As the central tendons contract and descend, the hemidiaphragms displace caudally while the ribcage expands and externally rotates. The ZOA diminishes in volume with this activity. Simultaneously, the abdominal viscera are displaced caudally enabling lung expansion [16, 33] (see **Figure 3B**). Exhalation reverses this process. Shortening of the internal intercostals and of the lateral abdominal musculature reduces ribcage dimension. The hemidiaphragms relax and recoil upward returning to their domed configurations. Then, in a position of potential energy, the hemidiaphragms are ready to piston down again, thereby creating a vacuum, which will draw air into the lungs. Additionally, the diminished volume of the pleural cavity aids in expelling depleted air from the lungs [16, 33] (see **Figure 3B**).

**Figure 3.** (A) Functional relationship of diaphragm, psoas, quadratus lumborum, and right stance illustration created by Elizabeth Noble for the PRI copyright. Used with permission from the PRI. Copyright 2017, www.posturalrestoration.com. (B) Respiratory mechanics of inspiration and expiration. www.wikimedia.org

Application of these respiratory mechanics to the biomechanical model of innate human asymmetry gives a more realistic understanding of our functional baseline. The three layers of lateral abdominals: transverse abdominis, internal, and external obliques, taken together, insert cephalically on the costal cartilage of ribs 5–12 and caudally on the ipsilateral iliac crest. These lateral abdominal muscles link the ribcage and pelvis, and they are critical components of posture and respiration [25, 26]. As described previously, shifting of weight to the right leg and orientation of the lumbar spine and pelvis to the right result in anterior rotation of the left hemipelvis. When the left hemipelvis is chronically anteriorly rotated, these lateral abdominal fibers will be adaptively overlengthened and weak. (In some cases, the right hemipelvis will also rotate anteriorly to avoid the strain of this asymmetry, resulting in bilateral compensatory and pathologic anterior pelvic rotation). The weakened, lateral abdominal muscles cannot maintain balance between the anterior ribcage and the pelvis. Without the anchoring action of the lateral abdominals, the anterior ribcage migrates further into elevation and external rotation mimicking thoracic position on inhalation [1–4].

Together, these two muscles span the internal dimension of the body just below the lungs. They insert on the xiphoid process, on the inner surfaces of ribs 7–12, and on the anterior aspect of the spine. The right leaflet is larger in diameter, it has a thicker and larger central tendon, its dome is higher, and it is better supported than the left by the liver beneath it and by strong right eccentric abdominal activity [31]. The right crura anchors to L1–3 on the right, the left crura to L1, 2 on the left [16]. The diaphragm leaflets also insert into the fascia overlying quadratus lumborum and to the psoas muscles via the arcuate ligaments, creating a strong functional linkage between these muscles. The superior strength, position, and function of the right hemidiaphragm supports and is supported by the physiological right orientation via

The respiratory "Zone of Apposition" (ZOA) is the region of interface between the hemidiaphragm and the inner surface of ribs 7–12 [16, 33]. Apposition refers to multiple layers of muscles with differing fiber orientation lying adjacent to one another. The ZOA facilitates inhalation by generating tension between the muscle layers, which promotes external rotation of the ribs, complementing the action of the external intercostals. As the central tendons contract and descend, the hemidiaphragms displace caudally while the ribcage expands and externally rotates. The ZOA diminishes in volume with this activity. Simultaneously, the abdominal viscera are displaced caudally enabling lung expansion [16, 33] (see **Figure 3B**). Exhalation reverses this process. Shortening of the internal intercostals and of the lateral abdominal musculature reduces ribcage dimension. The hemidiaphragms relax and recoil upward returning to their domed configurations. Then, in a position of potential energy, the hemidiaphragms are ready to piston down again, thereby creating a vacuum, which will draw air into the lungs. Additionally, the diminished volume of the pleural cavity aids in

**Figure 3.** (A) Functional relationship of diaphragm, psoas, quadratus lumborum, and right stance illustration created by Elizabeth Noble for the PRI copyright. Used with permission from the PRI. Copyright 2017, www.posturalrestoration.com.

right stance [1–4] (see **Figure 3A**).

140 Innovations in Spinal Deformities and Postural Disorders

expelling depleted air from the lungs [16, 33] (see **Figure 3B**).

(B) Respiratory mechanics of inspiration and expiration. www.wikimedia.org

This positioning has consequences for respiratory mechanics. When the left ribcage is in a chronic state of inhalation (expanded ribcage), the diaphragm is obligatorily in its descended state of inhalation as well. This chronic positioning limits diaphragmatic ascension on exhalation, thereby reducing the left ZOA. Consequently, the diaphragm loses its effectiveness for inspiration. Additionally, as the left anterior ribcage elevates, the diaphragm's domed configuration decreases and its fibers take on a more flattened, diagonal orientation, elevated anteriorly, resulting in further loss of the left ZOA. In this altered state, when the diaphragm contracts, it pulls the lumbar spine forward and reinforces anterior ribcage elevation. Having lost efficiency as a respiratory muscle, the diaphragm now functions more as a postural extensor muscle promoting progressive lumbar lordosis [29] (see **Figure 4**). Left anterior ribcage flares are commonly seen clinically and are exaggerated in patients with scoliosis. These flares indicate hyperinflation of the left lung due to insufficiency of the left lateral abdominals.

**Figure 4.** Positional consequences for respiratory mechanics. Illustration by Erica Bevin for James Anderson and the PRI. Copyright 2017 PRI®.

The right hemipelvis configuration is opposite relative to the left; it is posteriorly rotated. The right lateral abdominals are better positioned to exhale, but are more restrictive to inhale. Compensatory strategies to maximize breathing capacity in order to meet respiratory need will then rely on the accessory muscles of respiration, including the psoas, paraspinals, muscles of the upper back, chest, and anterior neck. With these compensatory changes in breathing mechanics, left anterior ribcage flares and right anterior ribcage restriction may progress along this diagonal trajectory, resulting in the common scoliosis pattern of right posterior ribcage prominence and left posterior ribcage concavity [1–4] (see **Figure 5A** and **B**).

**Figure 5.** (A) EOS of common scoliosis pattern used with permission. (B) Common costal deformity in scoliosis used with permission from The Martindale Press, Three Dimensional Treatment for Scoliosis, 2007 by Lehnert-Schroth, C.

#### **2.4. Right-side dominance, the functional result of physiological asymmetry**

Humans almost universally exhibit right-dominant postural and movement patterns resulting from physiological asymmetry. Preferential standing on the right leg and increased breathing efficiency of the right hemidiaphragm are major contributors to this fundamental bias. Additionally, 90% of the population is right-handed, a defining characteristic of humans [11, 15]. Use of the right upper extremity for manipulative and reach activity dates far back in human history and has been correlated with early human brain asymmetrical development [11]. Right arm swing accompanies right stance phase of gait and coordinates with left leg swing-through. Right arm swing, consistent with right reach activity, promotes left trunk rotation to balance lumbar spine and pelvis right orientation, present in right unilateral stance. However, it is important to emphasize that handedness does not define side dominance [34]. Left-right asymmetry is a fundamental, ancient characteristic of animal development present in the earliest large multicellular organisms according to fossil records [14, 34]. Strong right-hand preference for manipulative and expressive tasks is thought to correspond to the emergence of language. These developments occurred with cerebral cortical lateralization at a much later date [11, 13, 35] and differ from inherent leftright organism asymmetry [34].
