As the central nervous system, with its complex sensory and motor functions, evolved over millions of years and became absolutely essential to the survival of early humans, it required the corresponding development of one of nature’s most elegant and intricate solutions to the dual demands of sthira and sukha: the spine. In order to understand how the human spine came to be what it is, we must first go back to studying the simple cell.
PHYLOGENY: A BRIEF HISTORY OF THE SPINE
Imagine the cell floating around in a primordial sea of fluid, surrounded by nutrients ready to be assimilated across its membrane (figure 1.1, page 2). Now imagine that the nutrients become less concentrated in some areas and more concentrated in others. The more successful organisms are the ones that develop the ability to reach the nutrients by changing their shape. This was probably the first form of locomotion; the pseudopod in figure 2.1 is an example of a simple cell with that ability. Shape changing as a survival method is an important principle to remember later on.
It is not too difficult to see how moving around becomes more and more valuable to these organisms, so the pseudopod eventually refines itself into a specialized organ, such as the flagella pictured in this bacterium (figure 2.2).
Now, rather than passively floating around in their environment, these primitive forms of life actively seek out nutrients that are necessary to their survival. An added benefit of mobility is that in addition to seeking out food, they can avoid becoming food for other organisms. Thus, we see the early biological basis for the yogic principles of raga and dvesha (attraction and repulsion). Seeking out the desirable and avoiding the undesirable is a fundamental activity of all living things, and another window into the concepts of prana and apana.
Life forms respond to this pressure of seeking what’s desirable and avoiding what’s undesirable with ever more complex adaptations. As an organism’s sensitivity and response to its surroundings become more complicated, it reaches a point where these activities require central organization and guidance.
Figure 2.3 shows a parasitic worm with a flattened body, called a platyhelminth, and in it we see the development of a rudimentary central nervous system. It exhibits a cluster of primitive nerve cells at the top and two nerve cords running down its length. Worms are invertebrates, but in their descendants, these rudimentary nerve cells have evolved into the brain, the spinal cord, and the dual trunks of the autonomic nervous system. They all require the corresponding development of a structure that allows for free movement but is stable enough to offer protection to these vital yet delicate tissues—in other words, a skeletal spine.
A central nervous system allows for an enormous amount of flexibility in a vertebrate’s survival activities, and the spine must thoroughly protect it while still allowing free movement. In sea creatures such as the fish (figure 2.4), the shape of the spine is consistent with its environment: water surrounding on all sides, exerting an equal amount of mechanical pressure from top to bottom and side to side. As the fish employs its head, tail, and fins to propel itself through the water, the spinal movements are oriented in the side-to-side dimension.
This lateral undulation in the spine was preserved even when aquatic creatures made the enormous evolutionary leap to terrestrial life. Figure 2.5 demonstrates that pattern in the amphibious salamander. Even though its limbs (evolved from fins) are assisting in locomotion, they are not supporting the weight of the spine off the ground. That development, probably resulting from a need to orient the eyes to ever more distant food or threats, requires a dramatic reorientation of the spinal structures.
A straight spine, such as that of the fish if it were supported on four limbs, would be subjected to gravity’s maximum destabilizing force at its very weakest point: the center of the span between the two supported ends (figure 2.6). Once raised up on its limbs, the most successful newly terrestrial creatures would be those that arched their spines in response to gravitational stress in order to direct that stress toward the supported ends, rather than the unsupported middle.1 This is the development of the primary curve of the terrestrial spine—what we know as our thoracic curve. It is primary in the sense that it is the first anterioposterior (front–back) curve to emerge, and also in the sense that it is the first curve that a human spine exhibits prenatally.
The curve of the neck was the next to evolve. Our fish ancestors had no real necks; their heads and bodies moved as a single unit with the gills placed directly behind the brain. The gradual downward shift of the breathing structures allowed for the development of a highly mobile neck that was capable of producing quick, precise movements of the head and sensory organs, offering an ever further look into its environment and tremendous survival advantages. This orientation of the cervical region signaled the first development of a secondary, or lordotic curve in the spine, which can be seen in the cat (figure 2.7).
When creatures began to use their forelimbs to interact with their environment, the ability to bear weight on the lower extremities became more necessary, and this signaled the beginning of the uniquely human second lordotic curve—the lumbar. At first, it was just a flattening out of the primary curve at the base of the spine, in order to allow animals such as the yellow-bellied marmots pictured in figure 2.8 to support their center of gravity above their base of support for longer periods of time.
The presence of a tail also helped in that feat of balance, but as the tail gradually disappeared, the shape of the spine had to change in order to bring the center of gravity fully above the base of support. When this occurred in human evolution, the hip, sacral, and leg structures essentially remained stationary in their quadruped relationship to the earth, and the torso pushed its way up and back, forming the lumbar curve.
Figure 2.9a illustrates the difference in shape between a chimpanzee spine and a human spine. Notice the absence of a lumbar curve in the chimp. This is why in order to move across the ground, primates walk on their knuckles (figure 2.9b), and when they run on their hind legs, they must throw their long arms back. Without a lumbar curve, that is the only way they can get their weight over their feet.
The human spine is unique among all mammals in that it exhibits a full complement of both primary (thoracic and sacral) and secondary (cervical and lumbar) curves (figure 2.10). Only a true biped requires both pairs of curves; our tree-swinging and knuckle-walking cousins have some cervical curve but no lumbar curve, which is why they are not true bipeds.
If we view our evolution from quadruped to biped in yogic terms, we could say that the lower body developed more sthira for weight bearing and locomotion, and the upper body more sukha for breathing, reaching, and grasping. One way to describe it is that the lower body moves us out into the environment, while the upper body brings our environment in to us.
ONTOGENY: AN EVEN BRIEFER HISTORY OF OUR OWN SPINE
After understanding the evolution of our species (phylogeny), it is useful to study the developmental stages experienced by each individual human (ontogeny).
Although the developing fetus exhibits—and then loses— certain characteristics that we share with our ancient ancestors, such as gills and a tail, the theory that ontogeny recapitulates phylogeny has long since been discredited. There is, however, at least one sense in which this is true: how the phyolgenetic and ontogenetic development of our spines mirror each other. Consider how our fetal spines exhibit only the primary curve along their entire length; this remains the case for our entire intrauterine existence (figure 2.11).
The first time our spine moves out of that primary curve is when our heads negotiate the hairpin curve of the birth canal, and the neck experiences its secondary (lordotic) curve for the very first time (figure 2.12).
As our postural development proceeds from the head downward, the cervical curve continues to emerge after we learn to hold up the weight of our head at about 3 to 4 months and then fully forms at around 9 months, when we learn to sit upright.
After crawling and creeping like our quadruped ancestors, in order to bring our weight over our feet we must acquire a lumbar curve. So, at 12 to 18 months, as we begin to walk, the lumbar spine straightens out from its primary, kyphotic curve. By 3 years of age, the lumbar spine begins to become concave forward (lordotic), although this is not outwardly visible until 6 to 8 years of age. After the age of 10, the lumbar curve fully assumes its adult shape (figure 2.13).
The full glory of nature’s ingenuity is apparent in the human spine, perhaps even more so than in other vertebrates. From an engineering perspective it is clear that we have the smallest base of support, the highest center of gravity, and the heaviest cranium (proportional to our total body weight) of any other mammal. As the only true bipeds on the planet, we are also earth’s least mechanically stable creatures. Fortunately, the disadvantage of having a cranium as heavy as a bowling ball balancing on top of the whole system is offset by the advantage of having that big brain; it can figure out how to make it all work efficiently, and that’s where yoga can help.
Our human form in general, and our spines in particular, exhibit an extraordinary resolution between the contradictory requirements of rigidity and plasticity. As we shall see in the next section, the structural balancing of the forces of sthira and sukha in our living bodies relates to the principle of intrinsic equilibrium, the deep source of support that can be uncovered through yoga practice.
ELEMENTS OF LINKAGE BETWEEN THE VERTEBRAE
The spinal column as a whole is ideally constructed to neutralize the combination of compressive and tensile forces to which it is constantly subjected by gravity and movement. The 24 vertebrae are bound to each other with intervening zones of cartilaginous discs, capsular joints, and spinal ligaments (shown schematically in blue in figure 2.14). This alternation of bony and soft tissue structure represents a distinction between passive and active elements; the vertebrae are the passive, stable elements (sthira), and the active, moving elements (sukha) are the intervertebral discs, facet (capsular) joints, and a network of ligaments that connect the arches of adjacent vertebrae (figure 2.15). The intrinsic equilibrium of the spinal column can be found in the integration and interaction of these passive and active elements.
To understand the overall architecture of the spine, it is useful to view it as two separate columns. In the schematic side view in figure 2.16, its front-to-back dimension can be roughly divided in half between a column of vertebral bodies and a column of arches. Functionally, this arrangement very clearly evolved to contend with the dual requirements of stability and plasticity. The anterior column of vertebral bodies deals with weight-bearing, compressive forces, whereas the posterior column of arches deals with the tensile forces generated by movement. Within each column, the dynamic relationship of bone to soft tissue exhibits a balance of sthira and sukha. The vertebral bodies transmit compressive forces to the discs, which resist compression by pushing back. The column of arches transmits tension forces to all the attached ligaments (figure 2.17), which resist stretching by pulling back. In short, the structural elements of the spinal column are involved in an intricate dance that protects the central nervous system by neutralizing the forces of tension and compression.
Discs and Ligaments
If you look deeper, you can also see how sthira and sukha are revealed in the components of an intervertebral disc: The tough, fibrous layers of the annulus fibrosis tightly enclose the soft, spherical nucleus pulposus. In a healthy disc, the nucleus is completely contained all around by the annulus fibrosis and the vertebra (see figure 2.18). The annulus fibrosis is itself contained front and back by the anterior and posterior longitudinal ligaments, with which it is closely bonded (see figure 2.17 on page 29).
This tightly contained arrangement results in a strong tendency for the nucleus to always return to the center of the disc, no matter in which direction the body’s movements propel it.
From the top of the cervical spine to the base of the lumbar spine, individual vertebrae are dramatically different in shape based on the functional demands of the varying regions of the spine (figure 2.19). There are, however, common elements to all vertebral structures, as illustrated by the schematic representation in figure 2.20.
Weight-bearing activities in general, as well as axial rotation (twisting movements), produce symmetrical (axial) compressive forces that flatten the nucleus into the annulus, which pushes back, resulting in a decompressive reaction (see figure 2.21). If the compressive force is high enough, rather than rupture, the nucleus loses some of its moisture to the porous bone of the vertebral body. When the weight is taken off the spine, the hydrophilic nucleus draws the water back in, and the disc returns to its original thickness. That is why humans are a bit taller right after getting out of bed.
Push–Counterpush
The movements of flexion, extension, and lateral flexion produce asymmetrical movements of the nucleus, but the result is the same: Wherever the vertebral bodies move toward each other, the nucleus is pushed in the opposite direction, where it meets the counterpush of the annulus, which causes the nucleus to push the vertebral bodies back to neutral (see figure 2.22).
Assisting in this counterpush are the long ligaments that run the entire length of the spine, front and back. The anterior longitudinal ligament runs all the way from the upper front of the sacrum to the front of the occiput, and it is fixed tightly to the front surface of each intervertebral disc. When it is stretched during backward bending, not only does it tend to spring the body back to neutral, but the increased tension at its attachment to the disc also helps to propel the nucleus back to neutral. The opposite action occurs in the posterior longitudinal ligament when it is stretched in a forward bend. It runs from the back of the sacrum to the back of the occiput.
Every movement that produces disc compression in the anterior column necessarily results in tension to corresponding ligaments attached to the posterior column. The recoiling of these ligaments out of their stretched state adds to the other forces of intrinsic equilibrium, which combine to return the spine to neutral.
Note that all this activity occurs in tissues that behave independently of the circulatory, muscular, and voluntary nervous systems. In other words, their actions do not impose an energy demand on these other systems.
TYPES OF SPINAL MOVEMENT
There are generally thought to be four possible movements of the spine: flexion, extension, axial rotation (twisting), and lateral flexion (side bending). These four movements occur more or less spontaneously in the course of daily life: bending over to tie your shoes (flexion; see figure 2.23), reaching for something on a high shelf (extension; see figure 2.23), grabbing a bag in the car seat behind you (axial rotation; see figure 2.24 on page 34), or reaching your arm into the sleeve of an overcoat (lateral flexion; see figures 2.25 and 2.26 on pages 35 and 36). There are, of course, yoga postures that emphasize these movements as well. What follows is a detailed analysis of these ranges of motion. Please note that these ranges are averages established by measuring a wide variety of people. Any given individual will exhibit significant variations at both ends of the spectrum of flexibility and in differing regions of their spines. The numbers given for degrees of range of motion are approximate, as are the angles pictured, with a five degree variation in either direction.
A more thorough look into the nature of the four ranges of motion of the spine shows that a fifth possibility called axial extension exists. This motion does not happen spontaneously in the normal course of daily movements. You have to learn how to make it happen intentionally because it is somewhat “unnatural” (see page 42).
Flexion and Extension, the Primary and Secondary Curves, and Inhalation and Exhalation
The most basic movement of the spine emphasizes its primary curve: flexion. As discussed previously, the primary curve is the curve present primarily in the thoracic spine, but it is also obvious in the shape of the sacrum. It is no accident that the yoga pose that most commonly exemplifies spinal flexion is called child’s pose (see figure 2.27)—it replicates the primary curve of the unborn child. From a certain perspective, all the curves of the body that are posteriorly convex can be seen as reflections of the primary curve. A simple way to identify all the primary curves is to notice all the parts of the body that contact the floor in savasana, or corpse pose (see figures 2.28 and 2.29): the curve of the back of the head, the upper back, the sacrum, and the backs of the thighs, the calves, and the heels. Consequently, the secondary curves are present in all the body parts that are off the floor in this position: the cervical and lumbar spine, the backs of the knees, and the space posterior to the Achilles tendons.
From this perspective, spinal flexion can be defined as an increase in the primary spinal curves and a decrease in the secondary spinal curves. A reversal of this definition would define spinal extension as an increase in the secondary curves and a decrease in the primary curves.
Note that as far as movement is concerned, the relationship between the primary and secondary curves is reciprocal: The more you increase or decrease one, the more the other wants to do the opposite. For example, an increase in the thoracic curve automatically produces a decrease in the cervical and lumbar curves.
A classic yoga exercise that explores this reciprocal relationship of the primary and secondary curves is cat–cow, or chakravakasana (see figure 2.30).
Supported at both ends by the arms and thighs, the spine’s curves can move freely in both directions, producing the shape changes of flexion and extension. Although it is common for instructors to teach this movement by telling the student to exhale on spinal flexion and inhale on spinal extension, it is more accurate to say that the shape change of spinal flexion is an exhalation and the shape change of spinal extension is an inhalation. As our definition of breathing shows, spinal shape change is synonymous with breathing shape change.
MOVEMENT EXPLORATION
From a comfortable sitting position, try increasing your thoracic curve by dropping your chest forward. Notice how your neck and lower back flatten. Now, try the same movement, but initiate it with your head; if you drop your head forward, you’ll notice how the chest and lower spine will follow. The same will occur if you initiate this movement with your lower spine. You may also notice that these flexion movements of the spine generally tend to create an exhalation.
Going in the opposite direction, try decreasing your thoracic curve by lifting your chest. Notice how your neck and lower back increase their curves. If you try initiating the movement with your head or lower spine, the results will be the same. Did you notice if these extension movements of the spine tend to create an inhalation?
Spatial Versus Spinal Perspectives in Forward- and Backward-Bending Poses
Spinal extension is not necessarily the same thing as bending backward, and spinal flexion is not necessarily the same thing as bending forward. To avoid confusion, it is important to keep these distinctions clear. Flexion and extension refer to the relationship of the spinal curves to each other, while forward bending and backward bending are terms that refer to movements of the body in space. The terms are not necessarily interchangeable. By way of illustration, consider the following contrasting examples of how two different body types might appear in some standard yoga movements.
· 1. A stiff, sedentary office worker, whose stooped posture doesn’t change as his hips move forward and his arms reach overhead in an attempt to do a standing back bend: His spine remains in flexion while his body moves backward in space (figure 2.31a).
· 2. A flexible dancer, who hyperextends her spinal curves in the overhead reach and keeps her spine extended as she flexes forward at the hip joints to move into uttanasana (standing forward bend): Her spine remains in extension while her body bends forward in space (figure 2.31b).
The valuable skill in observing movements this way is the ability to distinguish movement of the spinal curves in relation to each other from the movements of the torso in space.
Figure 2.32 shows more of an integrated orientation to a standing back bend. Here, the secondary curves are kept under control, and the pelvis is kept firmly over the feet. As a result, there is much less movement backward in space, but a greater emphasis on thoracic extension (reduction of the primary curve). Although this is not a dramatic movement spatially, it actually provides a safe and effective stretch to the thoracic and rib structures and is less disturbing to the process of breathing than either the dancer’s or the office worker’s movements.
Spatial Versus Spinal Perspectives in Lateral and Twisting Movements
When looking at yoga poses that involve lateral and twisting movements, it is also important to distinguish spatial from spinal perspectives. Trikonasana, or triangle pose, is often referred to as a lateral stretch, and this is true insofar as it lengthens the connective tissue pathway that runs along the side of the body (see figure 2.33).
However, it is possible to lengthen the lateral line of the body without any appreciable lateral flexion of the spine, so again, it must be clear what exactly is meant by the term lateral bend.
In trikonasana, more of a lateral line stretch would result from a wide spacing of the feet, and an intention to initiate the movement primarily from the pelvis while maintaining the spine in neutral extension. This also turns the pose into more of a hip opener.
Lateral flexion of the spine could be emphasized by a closer spacing of the feet. This allows for more stabilization of the relationship between the pelvis and thighs, which would require the movement to come from the lateral bending of the spine.
When we look at parivrtta trikonasana, the revolved variation of triangle pose (figure 2.34a), we can apply the same perspective to the twisting action of the spine. The lumbar spine is almost entirely incapable of axial rotation (only 5 degrees; see figure 2.34b), which, in this pose, means that it will go wherever the sacrum leads it. Consequently, for the lower spine to twist in the direction of this pose, the pelvis would have to turn in the same direction.
If the pelvis is free to rotate around the hip joints, this pose exhibits a more evenly distributed twist throughout the spine rather than an overloading of T11 and T12—the first joints above the sacrum that can freely rotate (see figure 2.35). The lumbar spine fully participates because the pelvis and sacrum are also turning; the neck and shoulders are free and the rib cage, upper back, and neck are open, along with the breathing.
If the hips are restricted, the lumbar spine appears to be moving in the opposite direction of the rib cage and shoulder girdle rotation. When this is the case, most of the twisting originates from T11 to T12 and above. In addition, the twisting of the shoulder girdle around the rib cage can create the illusion that the spine is twisting more than it really is. So, the body can indeed be twisting in space, but a careful observation of the spine may tell where exactly the twisting is coming from.
Axial Extension, Bandhas, and Mahamudra
The fifth spinal movement, axial extension, is defined as a simultaneous reduction of both the primary and secondary curves of the spine (see figure 2.36). In other words, the cervical, thoracic, and lumbar curves are all reduced, and the result is that the overall length of the spine is increased.
Because the primary and secondary curves have a reciprocal relationship, which is expressed in the natural movements of flexion and extension, axial extension is “unnatural” in the sense that it bypasses this reciprocal relationship by reducing all three curves at once. In other words, axial extension generally doesn’t happen all on its own; it usually requires conscious effort and training to accomplish.
The action that produces axial extension involves a shift in the tone and orientation of the breathing structures known as the bandhas. The three diaphragms (pelvic, respiratory, and vocal) and their surrounding musculature become more sthira (stable). As a result, the shape-changing ability of the thoracic and abdominal cavities is more limited in axial extension. The overall effect is a reduction of breath volume but an increase in length. The overall yogic term that describes this state of the spine and breath is mahamudra, or great seal, which always involves axial extension and the bandhas. It is possible to do mahamudra from many positions, including seated, standing, supine, and in arm support.
A seated posture named mahamudra (figure 2.37) adds a twisting action to axial extension. It is considered a supreme accomplishment to do this practice with all three bandhas executed correctly, because it represents a complete merging of asana and pranayama practice.
INTRINSIC EQUILIBRIUM: THE SPINE, RIB CAGE, AND PELVIS
If you were to remove all the muscles that attach to the spine, it still would not collapse. Why? Intrinsic equilibrium is the concept that explains not only why the spine is a self-supporting structure but also why any spinal movement produces potential energy that returns the spine to neutral. The same arrangement exists in the rib cage and pelvis, which, like the spine, are bound together under mechanical tension. Intrinsic equilibrium is also exhibited by the pressure zone differentials described in the previous chapter (page 20).
These facts about the core structures of the axial body reveal a deeper truth about how yoga practice appears to liberate potential energy from the body.
True to the principles of yoga and yoga therapy, the most profound changes occur when the forces obstructing that change are reduced. In the case of intrinsic equilibrium, a deep level of built-in support for the core of the body is involved. This built-in support does not depend on muscular effort because it is derived from the relationships among the noncontractile tissues of cartilage, ligament, and bone. Consequently, when this support asserts itself, it is always because some extraneous muscular effort has ceased to obstruct it.
It takes a lot of energy to fuel our constant, unconscious muscular exertions against gravity, and that is why the release of that effort is associated with a feeling of liberated energy. Thus, it is tempting to refer to intrinsic equilibrium as a source of energy because its discovery is always accompanied by a profound sensation of increased vitality in the body. In short, yoga can help to release the stored potential energy of the axial skeleton by identifying and releasing the less efficient extraneous muscular effort that can obstruct the expression of those deeper forces.
CONCLUSION
As noted at the end of chapter 1, a balance of both will and surrender is needed in order to honor the true nature of the breath and the spine in yoga practice. Without this perspective, the deeper, intrinsic support within the system is forever overshadowed by a futile attempt to reproduce through effort what nature has already placed at the core of the body.
1 Think of the difference between Greek and Roman architecture. Far more of the Romans’ buildings are still standing because they built with arches and the Greeks didn’t.