48The Hip Axis

The one-joint model of the body is nearly self-explanatory (Notes on S.I. 90/1). It appears simple enough and corresponds directly to human experience. Nevertheless, it seems hard for many people to bodily sense the relevance and significance of the device. It helps sometimes to ask them to imitate the model, “to model the model of the body”.

The one-joint model consists of two sticks which are joined by a hinge. The hinge permits the sticks to rotate to either side. The axis of rotation represents the hip axis, which is defined by the straight line through the centers of the two hip joints. The sticks stand for the legs and the upper body. The lower stick would be a little shorter and weigh less than the upper one; its lower end should be thought to be point-like.

If the two sticks are vertical, one on top of the other, they form one vertical line. The hip axis is also on this line as well as the gravitational centers of the two sticks. Therefore, the overall center of gravity is on this vertical, too. The sticks being of solid material, the force of gravity acting downward is cancelled out exactly at each level of height by the normal force of the earth acting upward.

Equilibrium of the labile or unstable type prevails. No energy is needed for the model to stay as it is as long as it is not disturbed. If it is disturbed, even microscopically only, the model collapses. Since in reality disturbances cannot be avoided absolutely, labile equilibrium is a purely theoretical matter and no real model would stand that way.

With the two sticks not vertical but slanted, the model will always collapse at the hinge joint. If this is held fixed, however, so that no movement is possible, there exists a place of labile equilibrium for the model. The condition for this is the same which holds for any kind of equilibrium: The center of gravity and the point of support, which forms the point of action for the normal force, must be on the same vertical line, the Line.

The center of gravity of the whole system is found in the middle between the centers of gravity of the two sticks. A slight correction would have to be made to account for the difference in weight. If the lower stick is slanted forward, its center of gravity is in front of the point of support. Therefore, the upper stick must be slanted backwards at exactly the correct angle which brings the overall center of gravity on top of the point of support. If the lower stick is slanted back, the upper one must slant forward accordingly.

The amount of force by which the hinge joint must be fixed depends on the degree of slant. The distance between the joint and the vertical line through the center of gravity and the point of support, the Line, provides a measure for the quantity needed. The closer to the Line the joint is, the less force is necessary. It approaches zero for the exactly vertical arrangement.

Any possible arrangement of the model in the field of gravity could be recreated with the sole information of the distance between the joint and the Line. In addition, the force could be calculated which would be needed to hold the joint immobile. It is assumed that weight and length of the sticks are known.

If we give the model a “front” and a “back”, however, two “solutions” would always be possible. They would be symmetrical, and the force needed for fixing the joint would be equal. A qualitative kind of information is necessary which tells whether the hip axis of the body is anterior or posterior. The reference line would be the Line. A slightly different way of saying this would be that the hip axis is either found in front or in back of the frontal plane through the gravitational center of the body.

This qualitative piece of information would also provide the key to knowing where at the hip axis the immobilizing force would have to be applied. In the body, the “upper stick” is held similar to the way a cranejib is stabilized. Instead of cables running around the convex side of the joint, tensed fascia does the job. With the hip axis anterior, tension would have to be in front; with the hip axis posterior, fascia in back would have to be tensed.

As always, a passive and an active component have to be kept apart. Passive tension of fascia on the convex side resists further bending in proportion to how much it is distended. This stretched fascia exerts an elastic force. But active tension in the form of muscles tensing fascia would also be necessary, of course. The passive component is probably low as long as the hip axis is not strongly in front or in back. It can be considered to be equal on both sides(1). The remainder would have to be provided by muscles.

While the degree by which muscles tense fascia would be equal in front and in back, the energy they consume would not be the same. The muscles in front have to spend a considerably larger amount of energy for holding the hip axis stable than do those in back. The reason for this is their different degree of efficiency.

The efficiency of a muscle indicates how much of the energy it consumes is actually put to use. The larger part dissipates uselessly, only the smaller part is converted into mechanical tension. If the tension a muscle needs to develop is given, the energy it consumes to do the job is distinctly larger, and its amount varies greatly according to the efficiency of the muscle. The muscles by themselves are not much different in their efficiency, but the efficiency changes drastically in dependence of the angle of action.

At the hip joints, the muscles in back have an excellent angle of action and the loss of energy is minimal when they hold the posterior hip axis. In contrast to this, the angle of action is not favorable for the muscles in front when holding the anterior hip axis. They waste a lot of energy.

This difference becomes even larger when the actual mechanics are examined. To hold the anterior hip axis, fascia must be tensed from the knees up to the chest. The quadriceps femoris, the tensor, and the rectus abdominis, which are mainly involved in this, strongly disrupt the arrangement of the whole body. This makes it necessary that other muscles join the action to hold the body stable, which magnifies energy expenditure. The gluteals which hold the posterior hip axis possess an excellent angle of action and perform their duty with ease and without disrupting the body.

In the anatomical view, it makes sense that the hip extensors hold and extend the flexed hip easily. It is their physiological design to do so. The hip flexors in front are not really built for efficient holding and “de-hyperextending” of the hyperextended hip.

From the economical point of view it is preferable for the body to function in an arrangement which features the posterior hip axis. It is less economical to function with the hip axis anterior.

The point becomes obvious when “bending” is considered. It is impossible to lower the gravitational center of the body significantly with the hip axis anterior. The “objection” suggested by reality that flexing the knees instead of the hips will do that better is not valid because this entails a clear anterior shift of the body which is highly uneconomical.

It could be argued that for a certain measure of time it might be economical to stand with the hip axis anterior. And indeed, if standing completely still for a prolonged period of time is the “test situation”, the argument might not be false. This is rare in reality, however. Usually one moves and shifts around the weight of the body all the time, the arms move, or at least the head turns. In all these instances, an impulse travels down the body and bends it forward or back, depending on whether the hip axis is anterior or posterior. And every time, the bending out backwards is checked more economically than that forward.

The Zigzag Line

The hip axis corresponds tp the “pelvic fold”. In Folding, the body “folds” the pelvis back out and turns around the hip axis. The movement is anatomically “flexion of the hip joints”, but several requirements must be met by it to be normal functionally.

Any normal movement is started by muscle relaxation, and nearly always this happens first and primarily in the abdominals(2). This could be called the “unspecific component” of the initial tonus reduction which is characteristic for normal function. A “specific component” would be a relaxation in that part of the body where the movement is desired.

As an example, “lifting the arms in front” illustrates the case. First, the abdominals relax, which lets the pelvis start to go back. The pelvic fold is accentuated without any part of the body sinking. This is only possible in the extension mode, in which the body initially lengthens. Right after this, all the shoulder muscles relax. The shoulders and the elbows sink while the trunk does not change its position in space. The anterior convex midline in the upper part of the trunk is momentarily accentuated as the weigth of the shoulder girdle drops. Then, the extensors of the shoulders, mainly the triceps, relax even more which leads to the elbows swinging forward horizontally. The weight of the elbows stretches the extensor fascia of the shoulders, and when it reaches its limit, they begin to rise. The shoulders are pushed down strongly once more by this because of Newton’s Third Law(3). The trunk reacts the same way as it did to the initial sinking of the shoulders.

Tonus reduction starts at the abdominals and the pelvic fold, readying the bulk of the body for the movement to come. Relaxation then “spreads” more specifically to the area which needs to let go, and from there to the fascia which must be released most. This is generally the fascia of the “antagonists” to the “agonists” which would produce the movement in the flexion mode. The sequential steps follow each other very fast. In quick movements they would perhaps only be hundredths of seconds apart.

The hip axis begins to recede horizontally after relaxation of the abdominals. The pelvis is at first not moved but only “drawn long”, lengthened along its long axis which is part of the midline of the trunk. Then, it begins to slide back along its axis initially without any rotation. The path of its movement is back and down. Almost immediately, the legs below, which act as a buttress, redirect the course the pelvis takes closer to horizontal. The pelvis now slides back, but down only minimally. By this time, the chest has begun to go forward horizontally, its base, the pelvis, having been pulled away from under it. The midline of the trunk is considerably longer than before Folding started. It is also a little more anterior convex at the same time. Only then does the pelvis pull back the thighs. It rests with its weight far back in the tensed extensor sling of the hip, being held up and refrained from falling by the passive resistance of the legs below.

“Flexion” of the pelvic fold resembles what happens if one attempts to cut tough leather with scissors which are too small. Instead of being cut, the piece of leather pushes back the scissors. But the actual movement is still more sophisticated than that. First, the hip axis starts to go back before the “blades” begin to converge. Secondly, the “blades” are lengthened first and pulled slightly convex before they begin to come together. Thirdly, the upper “blade” representing the pelvis moves and starts to “close” first and ahead of the lower “blade”.

For any movement where a part of the body goes forward, the initial shifting back of the pelvis assures that the overall center of gravity stays where it is. But the pelvis starts the movement also if the arms are to swing back. It is of course quickly overtaken by the upper body going forward which keeps the center of gravity in its place. Only as the third step do the elbows swing back. But even if the center of gravity moves forward, the initiation of the movement is in the same direction: back.

The common bracket for this uniform way of inducing movement in any direction is the zigzag line. The zigzag line is the normal arrangement of the midline of the body in a “four-part model” consisting of lower legs, thighs, trunk, and neck/head. The parts are joined by transverse axes of rotation. The hip axis links the thighs to the trunk. The knee axis is between lower legs and thighs. The ankle axis allows rotation of the lower leg with respect to the foot, which remains stable and fixed to the ground.

The shoulder axis connects the neck/head to the trunk. Its name is a misnomer because it has nothing to do with the shoulders. It derives partly from Ida Rolfs contention that ankle, knee, hip, and shoulder should be on the same vertical line. Another reason is that the shoulders can be seen well from the side. This is especially welcome if someone does not have ease and experience in seeing axes of rotation and folds.

The shoulder axis in reality designates that transverse axis of rotation around which neck and head rotate back in Folding with respect to the trunk. It is given by the “dent” which shows in the midline of the body. It is not defined as sharply as the other axes because many joints are involved and not just one. In addition, the anterior convexity of the midline of the trunk extends into the neck segment, which makes it hard to exactly pinpoint the shoulder axis. The knee axis is the same as the “knee fold”, and the ankle and shoulder axes correspond to what has been termed the ankle and shoulder “semifolds”.

The four transverse axes can be arranged in four different ways with regard to gravity. They constitute four definable patterns which are affected differently by gravity. The functional norm is the pattern with hip and ankle axis posterior, knee and shoulder axis anterior to the Line. If the structure has adapted to this normal zigzag line, the regular internal or the symmetrical external type result.

The zigzag line can be reversed with knee and shoulder axis posterior, hip and ankle axis anterior to the Line. When structure fits this arrangement, the locked-knee type is the result.

In the “anterior convex banana”, the hip and knee axes are anterior to the Line, shoulder and ankle axes posterior. Structural adaptation produces the regular external type.

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The “posterior convex banana” is something of a curiosity. Knee and hip axis would be posterior, shoulder and ankle axis anterior. It is possible that structure might adapt to this, but certainly such types would be rare.

A gross argument for the functionally normal zigzag line is that it is the only arrangement which permits bending far down by relaxing alone as in picking up something from the ground or tying one’s shoes. If exotic and uneconomical contortions are disregarded, any of the other three arrangements necessitates creating the normal zigzag line first. This always requires that energy be spent. Folds must “be heaved over” to the other side.

The more subtle argument recognizes that in movement the actual weight of the body, the force by which it is pressed against the ground, is constantly modified. There exists no problem when it is momentarily lessened as in downward acceleration in an elevator. This is the case when muscles relax momentarily or when the center of gravity sinks.

This is different when the body is heavier for a moment as in an elevator starting to go up. The most obvious example is walking where at every step the weight of the body comes down on the leg in front which was free of load before. This happens all the time and in quick succession in walking. The impulse travelling down the body presses it to the ground more strongly for a moment. It also bends the body out where it can do that: at the folds. This constant “bending out” at the folds is checked most economically if they are arranged in the normal zigzag line. This is not a surprise because the body’s anatomical design demands it. The “extensor muscles” easily check induced flexion in a nearly extended joint.

There is an added benefit to doing that as “lazily” as possible. If the “extensors” work minimally, their fasciae are distended by the impulse. They store elastic energy and they exert an elastic force which further reduces the work necessary by the muscles. This effect is entirely dependent on minimal tonus, which lets the body become maximally long. Under this condition however, it could be said that “laziness breeds economy”.

The Anterior Convex Midline of the Trunk

The trunk needs additional consideration. The other sections of the zigzag line are not very variable in shape. The trunk is so, however, thanks to the many joints of the spine and the ribcage.

Ideally, the midline of the trunk would be straight. It could then be vertical in erect posture with the weight distributed evenly all around. But nothing is ever exactly straight, and therefore the direction of deviation must be examined. In a first approach, the midline can be anterior or posterior convex. Functionally, the anterior convex midline of the trunk – or “acmott” – is normal; the posterior convex midline is not. Put differently, it is favorable economically to approach the ideal of the straight midline from the anterior convex deviation, not from the posterior convex one.

Acmott is generally difficult to produce, and for many bodies it is not possible for structural reasons. Only in very rare cases is acmott too pronounced, therefore. A secondary principle must be regarded then, which states that the degree of deviation should be small in order for function to be economical.

Only acmott and the posterior convex midline are considered. “S-forms” and “double-S-forms” are not economical for the posterior convex part of the midline they contain. In extreme movements posterior convexity cannot be avoided. A “lower reversal” and an “upper reversal” can then be distinguished (Notes on S.I. 91/1, p.17). In instances of posterior convexity, whether functional or structural, the principle holds that the degree of convexity should be minimal; the midline should be as close to straight as possible.

The midline of the trunk in the sideview is given by the line which runs between the front and the back contour keeping exactly the same distance to both(4). It is a very good indicator for the behaviour of the trunk in the gravity field.

Acmott as the functional norm for the trunk is usually not easily accepted. It runs counter to most beliefs, rules, and views. Because they generally consider only the back, and because they call for a “straight back” in not very specific ways, the trunk as a whole invariably shows a posterior convex midline in part or over its whole length. Substantiating the claim for acmott to be normal makes it necessary to go through a little series of models of the trunk which explain its various aspects.

A very simple but unrealistic model is given by a cylindrical column made of some solid material. If it stands exactly vertical, the weight of each segment – or horizontal “slice” – is transmitted down vertically. Pressure on the segment below is distributed evenly.

When such a column carries a vertical load, e.g. the stone columns of a Greek temple supporting a roof, it is compressed additionally. It is a little shorter and wider. The change is “microscopic” of course because the material resists compression. If one forms a “toy column” using clay and compresses it vertically, the effect is immediately visible because the material has little resistance to compression.

In order to understand the situation in bending, the stone column as an example is best replaced by a wooden beam. When bent, compression accumulates on the concave side. The convex side is distended (“tensile stress”). With increased bending, pressure is concentrated more and more on the concave side, and the area which is distended extends more and more in its direction (Fig.2).

The deformation produced by the weight and the normal force is elastic if the material of which the column is made is elastic. Stone, metal, and wood are all highly elastic so that the column regains its original shape once the deforming force has ceased to act.

A more sophisticated column has the form of a pipe or a hollow cylinder and is made of metal. When bent, the wall which is concave is compressed and bears the weight.

The convex side is distended. Steel is extremely well suited for the task because it resists pressure as well as tension.

The hydrostatic balloon is a more realistic model for the body. If it has the form of a column and stands vertical, it will also bear weight. Of course, neither its liquid content nor its walls are pressure-resistant. The pressure in the liquid causes it to go in all directions: sideways, up, and down. The molecules are pushed against the walls inducing tension in it. Weight cannot deform the “water column” inside greatly because the tensed walls of the balloon prevent that. If the fabric of the walls is tough and strong, and if the inside initially is under high pressure, the column will be able to bear a lot of weight without marked deformation.

The fabric of the wall is best imagined to be made up of a cross-work of f hers running vertical or longitudinal and horizontal or circular. When the “column” is bent, compression is not concentrated on the concave side. The liquid content still distributes it evenly in all directions. There is no need for anything resisting concentrated compression. There appears a difference in the tension of the longitudinal fibers, however. Those on the convex side are tensed more, those on the concave side less.

<img src=’https://novo.pedroprado.com.br/imgs/1993/1046-2.jpg’>
Fig.2 – In a solid column standing vertically, pressure is distributed evenly. Gravity and Normal force cancel each other out at any height level. In a bent column, compression is on the concave side, the convex side is distended.

Bending of the “hydrostatic balloon column” is also elastic. When the force effecting the deformation ceases to act, the “column” goes back to vertical on its own. In the more detailed description, the model at the outset bears its own weight without having to spend energy. If the load increases momentarily, bending which is always present at least microscopically is accentuated. At the new equilibrium of forces the “column” is stable again. When it is relieved of the load, it assumes its original shape. The “system” is mechanically “self-regulating over a wide range of different loads.

The model is extremely satisfying from the economical point of view. It functions and conserves its shape “flexibly” without consuming any energy. It doesn?t need “muscles” at all. Unfortunately, the trunk of a real body does not comply very well with it. It is deficient in both the conditions necessary which are essential for the model’s functioning. First, its content is not purely liquid. There is gas in the trunk which is easily compressed in volume and so compromises the integrity of the system which depends on the non-compressible liquid tensing the walls. Of course, the damage is limited in the thorax which is protected by the ribcage from collapse. There is also some gas in the intestines, though. All this weakens the features the model displays. It certainly renders it useless to try to improve the performance by raising the pressure inside. The trunk as a hydrostatic balloon functions at low pressure gradients only.

The second incongruity concerns the walls of the balloon. Those of the trunk of a real body are not and should not be tight and taut. Neither should they be soft, of course. They are “resilient” in the ideal case. This property of fascia is understood intuitively more or less, but it is actually very hard to define in a more exact way. Anyway, it means that the walls of the trunk give in more to pressure than would be desirable for a weight-bearing “hydrostatic balloon”.

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Fig.3 – In a “hydrostatic balloon column” pressure is in all directions and even. When bent, tension in the longitudinal fibers of the wall becomes asymmetrical.

This model does not show a difference in its behaviour under load according to the direction of bending. It is indifferent in that respect and does not explain the difference we obviously see in bodies. But still, with acmott the trunk behaves like a very well functioning “hydrostatic balloon”. With the midline posterior, it immediately collapses and shows all the limitations just enumerated.

Going back to the “pipe model”, an improvement suggests itself when the bending is permanently to one side. Then, one side is always “the concave side”, bears weight, and must resist compression. The other side, which is always “the convex side”, is permanently distended. For holding tension, it can be replaced by a string. We arrive at something like the bow of a violin. The rigid stick is convex to the side of the string. It “carries weight” and resists compression. When the load increases, it is bent more: its convexity increases, too. This automatically stretches and tenses the string more. The added tension of the string checks and limits the bending of the stick. This model is also mechanically self-regulating. Under increased vertical pressure it will change its form slightly to a new one in which the forces are again in equilibrium. When pressure is reduced it will regain its original shape. No energy needs to be added to the system. Another feature of this model becomes obvious: its property of being stabilized by weight. With increasing vertical pressure, it becomes more and more rigidly fixed and can less and less easily be disturbed.

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Fig.4 – The violin bow model. Under axial pressure it “self-stabilizes”. The bow is bent more, which tenses the string, which in turn limits the bending of the bow.

The violin bow model explains the asymmetry of the behaviour of the trunk nicely. The stick corresponds to the posterior wall of the trunk which contains the “spinal column” as a compression- resisting element. The front wall of the trunk, from clavicles to pubes, is represented by the string. Because the front wall of the trunk does not possess a compression-resisting element, the trunk with a posterior convex midline is not stable and self- stabilizing under pressure like a violin. Instead, it collapses freely.

The stick of a violin bow is much more rigid than the “spinal column” which is highly flexible due to the many joints it possesses. However, rigidity as resistance against bending is not necessary in the model at all. All that is required is that the compression-resisting element resists axial pressure, that it doesn’t shorten along its long axis under load. A very soft stick would not shorten under load, although it would bend outwards freely. In the model, this is not checked and limited by the rigidity of its material as in a real violin bow but by the tension of the string(5).

<img src=’https://novo.pedroprado.com.br/imgs/1993/1046-5.jpg’>
Fig.5 – A balloon with a compression-resisting element in its concave wall. Under axial pressure, it also “self stabilizes”. The cross-bars are kept out and pushed apart more by the rising pressure inside the balloon

When the soft stick bends out the other way – in the direction away from the string-, nothing limits the bending. The system gives in to the weight with hardly any resistance. The trunk does the same when its midline is posterior convex. The front then collapses freely and the back goes out wide. It always takes considerable effort and energy consuming work by the back muscles to check and resist the bending backward of the back by vertical pressure and weight. With acmott present, the posterior wall of the trunk as a “soft stick” is bent forward by axial pressure. This bending is checked by the fascia of the front wall of the trunk. It forms the “string” to the bow. No muscles are necessary to stabilize the trunk against gravity. On the contrary: weight and therefore gravity stabilizes the trunk in its form if it possesses acmott! Muscles only disturb the arrangement. Those in front refrain the front wall from doing its job properly. In fact, in most bodies it takes only very little active tension of the muscles of the front wall for reversing the convexity of the midline. All the attention is on relaxing the muscles of the front maximally in order to even just produce acmott. Even then, this is often not possible for structural reasons.

The back muscles are also completely relaxed. It is gravity which bends the back wall forward, and there is no use nor gain for the back musculature adding to that. They only jam the back and shorten the body when “working”, and all they possibly achieve is impeding on the length of the “string” in front which is essential for the self-stabilizing mechanism!

The different behaviour of the trunk according to the direction of the convexity of its midline can easily be felt. In simple words, when acmott is present, the back wall bends under load but does not shorten as the front wall lengthens. Therefore, the midline also lengthens. When the midline is posterior convex, the front wall shortens. The back wall does not lengthen but only bends. Therefore, the midline shortens.

<img src=’https://novo.pedroprado.com.br/imgs/1993/1046-6.jpg’>
Fig.6 – The upper and lower part of the trunk under axial pressure. The upper biconvex balloon widens in both directions. The lower anterior convex balloon is bent forward more, and its top begins to slide forward. When put on top of each other to simulate the trunk, the upper biconvex balloon approaches the form of the lower balloon because of the necessity of maintaining the continuity of the walls and because the overall center of gravity is displaced forward. The LDH slides up and down under load and its release.

The violin bow model depends on two cross-bars – at the top and at the bottom -, which hold the string away from the stick. In the trunk, the bony pelvis and the ribcage fulfill this function.

The most obvious and probably the most important one of the fasciae which make up the “string” in front is the fascia of the rectus abdominis. In such a way, the string can be considered to attach to the pubic bone. The pubes form the most anterior part of a solid or “bony” ring which is anchored in back in the posterior wall to the spine. It can be called the “pelvic ring”, and it is fairly stable since the bony pelvis is only minimally variable in shape as far as the front-to-back dimension is concerned.

The pelvic ring seems to follow closely the pelvic floor, but it should not be considered an anatomical entity but rather a structural abstraction. The ring seems to pass around the back at the level of the lower sacrum. Its plane seems to be slightly inclined forward with respect to the plane given by the line between the tip of the pubes and the tip of the coccyx.

As for the structural norm, it can perhaps be assumed that the ring is horizontal and therefore at right angle to the midline which would ideally be vertical. With acmott as the functional norm, and if the angle between midline and posterior wall on one side and the plane formed by the ring on the other remains the same – 90° -, the ring is normally slanted forward. This is congruent with normal function which states that the anterior pelvic tilt is normal and not the posterior tilt. When the trunk is “loaded” and acmott increases, the anterior pelvic tilt should also increase. In that case, the “string” in front of the trunk is lengthened effectively. If the pelvis tilts posterior however, the “string” is not tensed and the system collapses.

The abdominal muscles are very effective in pulling the pubes in the cranial direction and tilting the pelvis posteriorly. The aspect of the Pelvic rings as the lower cross-bar confirms again how important and essential maximal relaxation of the abdominal muscles is in order to be able to function normally.

The upper cross-bar is given approximately by the bony ring which forms the upper thoracic aperture in anatomy The manubrium can be considered the part most in front of the ring otherwise made up of the first ribs and the gust thoracic vertebra. The plane formed by the ring would ideally also be horizontal, prependicular to the vertical midline. With acmott, it would even be a little higher in front than in back if the angle between the plane of the ring and the midline remained at 90°. This is rarely the case, and it’s even dubious if that would have to be considered functionally normal.

Instead, the direction of its change when the trunk comes under load becomes of primary importance. And indeed, the manubrium easily goes up if two conditions are met. The first is the presence of acmott. The second is a “reactive tonus reduction” of the abdominals. This means that they relax additionally and maximally when the trunk is “loaded”. One senses clearly that the pubes go down and the manubrium goes up, both as the result of the abdominals relaxing.

Gravity is not that much of a problem in the neck as it is farther down. The weight of neck and head is considerably less than that of the trunk. And the “pressure-resisting element”, the cervical spine, is fairly central so that the weight is easily carried in the concave back wall of the neck segment.

Experientially, if the conditions for normal function are met, the lower and the upper ring function like “lids” on a pipe. They are fixed to the pipe in back and they can “flap” up and down. They are contained in the “body stocking” of course and not completely free in front. When pressure rises inside the “pipe”, which represents the trunk, they are pushed apart and they go in their respective normal directions. The fact can tentatively be explained by a combination of the hydrostatic balloon model and the violin bow model. Under load, pressure inside rises. The posterior wall is bent forward more, reducing the space in the sagittal dimension between front and back. This tenses the “string” in front which so resists being pushed out in front. Therefore, pressure acts most noticeably on the relatively soft top and bottom “lids”, pushing them out. It is easy to understand that the “bottom lid” flaps down. Pressure inside the trunk and gravity act in unison. It is harder to understand why the “top lid” flaps up. It does so freely, however, if the conditions for normal function are met. Although the thorax contains mostly air, not liquid, it acts like a hydrostatic balloon and transmits the pressure upwards reliably. It is interesting that this function is impaired immediately if the lower ring is not left free to “flap down” because the abdomen is not completely relaxed. The trunk in its functionally normal shape seems to form a unified system which functions only as a whole.

A possible answer is found if the thoracic wall is considered all around continuous with the hydrostatic balloon. The space inside the balloon would then comprise the abdominal cavity, the space between the body fascia and the endothoracic fascia, and that of the mediastinum. Pressure would spread easily in this space. Of course, the “hydrostatic wall” of the thorax is held open and prevented from collapsing inside by the ribs. But one has the impression that the thoracic walls are stable in form like well-filled balloons if the ribcage is left to float above the main bulk of the “hydrostatic balloon” below.

Acmott is easy to understand in the lower part of the body. Within the pelvic segment, the midline of the trunk is identical with the long axis of the pelvis. It is anterior convex if the pelvis is tilted anteriorly. It is posterior convex if the pelvis is tilted posteriorly. In the second case, this is equivalent to saying that the pelvis is pushed down in back and that the midline is bent out backwards.

In the lumbar segment, the front contour is convex, the back contour concave. The midline is anterior convex, therefore.

Above, in the thorax, the situation is different. Front and back contour are both convex. The thorax is “biconvex”. Acmott is only present if the convexity of the front contour is marked more strongly than that of the back contour. Acmott as part of the functional norm calls for a fairly flat upper back and a rounded chest. Obviously, this is not possible for many bodies for structural reasons. It must also be remembered that an anterior convex midline which is produced and held by muscles has nothing to do with normal function. Acmott is only present in the upper body if it is the result of relaxation of all muscles of the trunk and due to the influence of gravity.

Under axial pressure, both the front and the back contour of the lower part of the trunk bend forward more. Acmott is more marked. In the upper part, the front is pushed out in front more, the back is pushed backwards more. The lower part behaves like a bent column which is compressed more. The upper part behaves like a balloon on which one pushes down vertically: all walls move out. Since the front wall, the chest, is softer in normal structure than the back wall, it gives in to the pressure more and moves out more than the back. Acmott is asserted.

If the back wall is regarded only, a problem appears at the level of the LDH. Above, the wall moves out backwards. Below, it moves forward. A sagittal shearing is generated (Fig.6). In this situation, either of two things can happen: the upper part takes the LDH and the lower one back, or vice versa.

The two possibilities seem to be neatly assigned to the curvature of the thoracic midline. If it is anterior convex to begin with, the lower back takes the posterior wall above with it. The concavity travels up the back and can go up all the way to about the upper margins of the shoulder blades. This is felt and seen very well in the standard forward movement in sitting (Notes on S.I. 90/ 1, p.20).

If the thoracic midline was posterior convex before, the convexity of the upper back spreads down into the lumbar area. The middle back bends out backwards and the chest collapses. The midline of the trunk has an S-form, and the upper posterior convexity will pull the whole trunk into the posterior convex midline arrangement if left to do so.

The phenomenon is so distinct and clear that it forms the most reliable means for testing the curvature of the midline in the thorax. If upper back, chest, and shoulder girdle are released vertically, the midline of the thorax will in either case be accentuated. It will bend more in the direction in which it is bent already.

The practice of experimenting and teaching is very subtle in the thoracic area. There is a wide variety of devices which by muscle action imitate acmott, and which therefore are not acmott. The shoulders can be held back just a little bit, or the muscles of the middle back are slightly contracted just holding the thorax up and pushing the chest out a little.

Up to this point the trunk has been considered in a vertically upright position. Weight and additional vertical pressure acted exactly in the axial direction – with the “ideal arrangement” as the reference line – where top and bottom of the midline were on the same vertical line. With the zigzag line, the trunk is inclined forward a little, however. The top is in front of the bottom, the shoulder axis is forward with respect to the hip axis. Weight and any additional vertical load no longer act along the long axis of the trunk but at an angle to it. The angle is small in a nearly straight zigzag line but increases in proportion to a stronger inclination of the trunk. It is convenient to divide the vertical force vector into two vectors which act at right angles. One component acts axially. The consequences are the same as those discussed before. The other acts perpendicular to it, “pushing” at the long axis from behind.

<img src=’https://novo.pedroprado.com.br/imgs/1993/1046-7.jpg’>
Fig.7 – The behaviour of an anterior concave and an anterior convex beam or balloon under vertical load when inclined forward.

It is intuitively clear that this improves the situation for acmott. The midline is more securely anterior convex.

The situation is similar to that when one makes a bow for a child to shoot arrows with. If one holds a hazel stick vertically and pushes down, it will bend out in the direction in which it was already bent a little. If one holds the stick at an angle, fixing the lower end with one foot, and pushes down at the top, it will bend much more easily.

The situation is also favorable for the biconvex upper part of the trunk. The component “pushing from behind” tends to flatten the posterior countercurve and reinforces the anterior convexity of the chest. But a still more sophisticated feature turns up which derives from the countercurve of the “compression-resisting element” in the posterior wall.

It is best to visualize a hydrostatic balloon which has the form of a banana and which stands slanted at an angle to the horizontal ground. The base of the balloon is firmly fixed. The “banana” can have a posterior convex midline, in which case its hollow side faces toward the ground. Or it can have acmott. It rises up and the convex side is the lower side.

When a load is placed somewhere above the middle of the balloon, the curvature of the midline is accentuated. In the case of the posterior convex midline, the balloon will be pulled down toward the ground more. In the case of acmott, the lower part will be pulled down, its curve flattened, but the upper part will rise up steeply.

The posterior wall of the balloon contains the “compression-resisting element”(6). Its countercurve at first impinges on the performance of the balloon which has acmott. But if the load is placed at the apex of the countercurve, the disturbance turns into a clear advantage. Now that component of the load which pushes straight forward against the back flattens the countercurve. The adjoining anterior convex parts of the posterior wall corresponding to the “cervical” and the “lumbar lordosis” are pushed away up and down. The posterior wall lengthens and approaches a more homogenous concavity at the same time.

The situation is exemplified by a blister in a linoleum carpet. When one stomps down on it, it is flattened; the linoleum is pushed sideways. It widens. Usually the walls are in the way, of course, and the blister just changes its location.

In the trunk, the posterior wall lengthens freely. Since it is also bent forward more, and because this tenses the front side, the trunk lengthens up- and downwards. The midline lengthens distinctly while the convexity of the anterior wall is augmented at the same time.

This explains the strange phenomenon that the trunk lengthens most distinctly when one lifts weight. It helps if the trunk is clearly inclined forward instead of “being held straight”. It also becomes very clear that this is a purely passive mechanism which muscles can only disturb. All muscles of the trunk – front, side, and back must be completely relaxed to optimize the effect.

The weight one lifts is transferred to the back by the arms. These act as ropes which are tied across the back. If they do that with the shoulders completely relaxed and therefore far distal and lateral, the line across the back where the weight is put on it runs exactly across the apex of the countercurve. If the shoulders are held up or back just a little, this line runs across the back more cranially. It then reinforces the “cervical lordosis” and this bends the thoracic countercurve back out more. Everything is different: hard and difficult.

A good way to sense this is sitting at a table and lifting it. One sits far back, sensing the tuberosities far behind and wide apart. The trunk is strongly inclined forward so that the chest is far forward above the table and low. The hands are placed under the table so that the palms touch the surface of the plate underneath.

<img src=’https://novo.pedroprado.com.br/imgs/1993/1046-8.jpg’>
Fig.8 – The trunk because of the countercurve is not only bent more under load but is also lengthened. Back and front contour and therefore the midline all lengthen.

Acmott must first be asserted, the whole front of the trunk must be relaxed as well as all back muscles. It is best to check the “normal arrangement” by turning the head left and right and tilting the pelvis slightly forward and back. The neck and the hip joints must be free.

When beginning to push up with the palms of the hands at the table-plate, the first thing one notices is that the shoulders are pulled forcefully distal and lateral, off the ribcage. The elbows are far down and back.

With increasing upward pressure of the palms, the thoracic countercurve is pushed flat. The manubrium sterni goes forward and up a little, the pubes go far back. The trunk lengthens to a great extent, and it is pushed more into its anterior convexity.

If one continues pushing up, the tuberosities are lifted back and up. The maximum length is reached when the feet extend against the floor. This eventually lifts the table if one wants to go that far.

Four Errors and two Examples

Functionally normal erect posture is in minimal Folding, which means that the zigzag line including acmott is present although nearly straight. As long as the body stays in this arrangement – without any fold ever crossing the Line -, movement is economical. The body going up and down resembles a bedspring being pushed down vertically and released again.

There are a number of errors which are committed almost “habitually” and which result in the movement not being Folding and erect posture not being normal because it is not in minimal Folding. An error of a more quantitative nature should be addressed first because it defeats the purpose of normal function: economy of movement. Mechanically, economical function rests entirely on the principle of the body extending maximally as expressed by the lengthening of the midline. Only then are the fasciae around the convex sides of the folds loaded with elastic energy so they are able to hold the body passively and give it lift in Unfolding. It takes very little active muscle tension to negate this effect and render function uneconomical and not normal.

Movements often look as if they are normal although they are not. They are nearly indistinguishable if one only looks at the visual clues, which indicate the spatial change of the form of the body. However, with experience one easily discerns the quality of a movement which is normal function because it is produced by weight tensing fascia as opposed to muscles working. The shoulder fold can be used to exemplify this fact. One stands with the legs straight and wide apart, the trunk with acmott inclined forward about 45°. If posture is normal, the chest in front hangs forward and is suspended in front as if resting with its weight in a passively tensed net. If one retracts the chest only minimally, perhaps by a few millimeters, the effect immediately disappears and one is now holding the chest muscularly with quite an effort. If the weight is released again into the body fascia in front, everything returns to “easy”. The head serves as an indicator in this “experiment”. If the chest rests fully in front and is supported by fascia and not held by muscles, the back of the head and its weight can be felt as it hangs down in back. When holding the chest however slightly, the sense of the weight of the back of the head immediately vanishes, too.

This “quantitative error” is best prevented by concentrating fully on “letting the folds out” maximally. In order to achieve this, one senses exclusively the horizontal movement of the folds and lets them go out as far as possible. This requires that all of the musculature be maximally relaxed, the effect of which is produced by sensing the weight of the folds and how the body is pulled long in the sagittal dimension, forward and back. Focussing one’s intent on the horizontal movement, one will hardly notice that the folds will also eventually sink down in order to be able to go out more.

The body always more or less resists this letting go. It takes some preparation, getting ready in one’s mind, in order to really let the body be pulled long in both directions by gravity. With practice, it becomes easier over time, but it should not be expected to become fully automatic. This would probably not even be a goal we should attempt to achieve.

“Letting the body go out in the folds” is essential for normal function. This becomes obvious when considering the physics of movement. Weight, which is a force vector pointing vertically down in the body, is not constant. Weight as a force changes all the time to some degree with movement. Any movement upward, which physically contains the element of the acceleration of a mass, adds to the weight vector momentarily a certain amount of force, proportional to “mass times acceleration”. When part of the body or the whole body moves down – accelerates downward – the weight vector is momentarily reduced. The matter is illustrated by an elevator starting to go up or down, respectively.

The momentary additional force presses the body more strongly down to the ground. This happens with every step in walking, or even more obviously in running or hopping /jumping. But for reasons of economy the center of gravity should not go down, or it should at least do so only minimally. With the zigzag line present, the downward impulse is converted to horizontal, pushing out the folds more. The additional force “loads” fasciae with elastic energy, and they then exert an elastic force which limits the degree by which the folds are pushed out horizontally and restores the former state of matters when the phase of acceleration has passed. With the zigzag line being accentuated as fascia catches the downward impulse, the center of gravity would sink for obvious geometrical reasons if the midline did not lengthen. It does that in normal function but is able to do so only if muscles are maximally relaxed. If anywhere tonus is higher than necessary, it will restrict the body’s capacity for being lengthened by the vertical impulse. Consequently, holding up the body with muscle force against the downward impulse will not only be uneconomical. Because it prevents the body’s lengthening effectively, it will actually cause the center of gravity to sink a little, and it will have to be brought up again by additional muscular force.

There are many ways of doing Folding the wrong way – which in effect means that the movement is not Folding however closely it resembles normal function. Four typical mistakes turn up regularly and therefore deserve special consideration. They pop up even after years of practice if one does not concentrate and take the short amount of time necessary to check the arrangement of the body in the gravity field and sense the tensional pattern of the fascial net.

Error number one

This error would be trivial if it didn’t turn up time and again! It consists in beginning “on the wrong side”, with the hip axis anterior. The error is understandable since nearly everybody stands in an anterior hip axis stance when “standing to stand”. A problem arises only if one is not aware of this and tries to begin “Folding” this way. It is easy to see and sense that with muscle relaxation alone the pelvis will sink forward, not back. It is also easy, however, to deceive oneself and push back the pelvis “over the hill” without noticing what one is doing. The problem is that even afterwards the movement never turns to normal unless done consciously. A movement that starts out with holding (and pushing) will continue that way until consciousness intervenes.

For dealing with this error it is favorable to remember that in reality normal function is not the objective, but being conscious of what one is doing and knowing whether one moves normally or not.

A good way to develop that is to commit the error on purpose and in as subtle a way as possible. One tries to deceive oneself so that one almost “believes” that the movement is Folding while in fact one performs error number one as smoothly as possible! A good way to be sure that the hip axis is on the right side to begin with – posterior – is to start out in clear Folding. One then unfolds until the zigzag line is nearly straight and the transverse axes are “just one millimeter” short of reaching the Line. Then, by reducing tonus, the body will fold deeper into the zigzag line again with gravity the “propelling force”.

Error number one automatically induces further errors.

<img src=’https://novo.pedroprado.com.br/imgs/1993/1046-11.jpg’>
Fig.9 – Folding against the wall. For the mechanics refer to the text.

Error number two

The shoulder axis goes forward before the hip axis goes back.

This error is related to the physiology of movement in the flexion mode. Muscle contraction starts “automatically” as soon as one thinks of a movement – and before becoming fully aware of it. It prepares the body “for action”. There seems to exist a physiological link between the beginning of a conception of moving to raising the tonus of all the musculature.

An opposite mechanism also exists, but it seems to be hardly developed in Western civilization. Upon a stimulus – and a “beginning conception of a movement” is such a stimulus in a broad sense – the nervous system “automatically” inhibits muscles so that the overall tonus drops initially as a response. This mechanism is rarely in evidence. An example would be the reaction to suddenly being frightened.

The “flexion mode mechanism”, leading to unspecific and general muscle contraction, seems to start at head and neck and to spread downwards from there. Typically, when one asks someone to turn, the head turns first and pulls the rest of the body along. This is in clear contrast to normal function which considers all physical forces and not just physiology. When muscles relax, movement begins far down at the ground and spreads upward. An example is given by slipping on thin ice. The feet are accelerated first, strongest, and they are slung away most. “Movement” spreads from the feet up.

In standing, the feet are fixed to the ground by friction and the knees are not yet allowed to go forward. Therefore, the pelvis starts the movement, “to be accelerated”, by being pushed backwards on top of the legs. The pelvis actually “settles” in the first fraction of a second down on the legs and then slides straight back out because of the resistance from the legs below. Only then does the chest begin to sink forward horizontally. It is helpful to sense how the pelvis forms the foundation for the upper body above it, and because this foundation is pulled away from under it the chest will eventually – as late as possible – go forward.

Trying to deceive oneself is again a good method for becoming conscious of this error. If one starts with moving the chest forward slightly – which constitutes flexion mode! -, one is then able to push the pelvis back from above so that a difference can hardly be noticed. However, when the capability for sensing the tensional pattern in the fascial net has been sharpened, the difference to normal is hugely in evidence. The qualitative difference between flexion mode and extension mode appears in full contrast.

<img src=’https://novo.pedroprado.com.br/imgs/1993/1046-9.jpg’>

<img src=’https://novo.pedroprado.com.br/imgs/1993/1046-10.jpg’>
Fig.10 – Different ways to produce and experience acmott. Error number three must consciously be avoided.

Error number three

This error is typical for internals. It consists of the pelvis tilting anteriorly but without shifting back first. The pelvis sinks down in front of the thighs and pushes them back from in front instead of being on top of them.

The movement of the pelvis in Folding is subtle. It can be demonstrated by an egg standing on its rounded end on a slippery table. It is kept from falling by pressing down on the pointed end with a finger. If the pressure of the finger is changed slightly in the “forward” direction, the egg immediately slides back while “tilting” anteriorly. One may also place a bony pelvis upright on the table and push down vertically on the pubes to get the same effect. The pushing down simulates the additional effect of the weight when the abdominals and the hip extensors which hold the pubes forward and up relax.

A practical way of getting familiar with this error is the following. The fingers of one hand are placed on the pubes. The back of the other hand lies on the sacrum. In Folding, both hands move back (and down a little), the hand in front faster than that in back. As soon as the hand in back initially goes forward and down, error number three is being committed. The border between Folding and error number three can be approached from both sides in order to feel the difference clearly. It becomes especially clear that gravity can only make Folding happen, but not error number three, which is a muscular event.

As a further difference it can also be sensed distinctly that in Folding the pelvis stays high and on top of the legs while with error number three its center of gravity sinks forward and down.

<img src=’https://novo.pedroprado.com.br/imgs/1993/1046-12.jpg’>

<img src=’https://novo.pedroprado.com.br/imgs/1993/1046-13.jpg’>
Fig.11 – Folding on the toes.

Error number four

This is the opposite of error number three. The pelvis tilts posteriorly relative to the trunk. Externals tend towards this error. The hand in back feels that the sacrum pushes against it instead of sliding back and away under it. As in error number three, the pelvis does not stay on top of the legs but sinks down behind them in error number four.

Folding against the Wall

Folding in free stance is often difficult. The wall makes it easier because it offers a point of reference which allows to orient oneself in the sagittal dimension.

One stands with the feet parallel, facing the wall, and the hands are put on the wall fairly high and parallel. The arms should be fully extended. The long axis of the body, which can be imagined as a straight line through ankle and shoulder axes, is slanted forward a little. The difference between two mechanical regimes can clearly be felt. With the hip axis anterior to the long axis, the extended arms holding the thorax away from the wall, and imitating the one-joint model, the body “hangs through” forward. The lower back is jammed; the quality of balance is low as the body is not well supported from the ground. With the hip axis posterior to the long axis, the system is stabilized by gravity. All muscles of the trunk can be fully relaxed, acmott is present. Support can be felt and balance prevails.

The hands should be imagined to be glued to the wall, the weight of the trunk partly hanging down from there. The arms staying extended keep the pelvis away from the wall passively and at the same time pull the thorax up and forward, helping to establish and maintain acmott. Persons who have a structural upper reversal of acmott flat chest and rounded back – are often able to produce and experience acmott in this position. The contour of the chest is rounded because of gravity and the contour of the upper back is flattened by the pull from the arms.

In this position, acmott extends factually from the pelvic floor to the hands on the wall. The front – elbows, chest, belly- must be sensed as heavy and felt to rest completely in the body stocking. This supports the body passively and without muscle tension. The extended legs which are slanted forward slightly exert a certain degree of axial pressure on the trunk from behind. Acmott is accentuated a little by this, and the lower back may even feel a little compressed.

In this version of Folding, nothing happens when the abdominals relax maximally. The arrangement is stable. Instead, the knees are now allowed to go forward slowly. Care should be taken that the pelvis does not also go forward in the direction of the wall. It should stay well back, kept there by the trunk and the arms. The pelvis should not sink, either, but stay as high as possible, the pelvic floor pointing more back than down.

One looks for the moment in which the thighs “pass vertical” and go into a posterior slant. The knees stop and no longer go forward. Now the mechanical system changes abruptly. The pelvis, instead of being pushed forward a little, is suddenly free and wants to slide back down. Its weight should be felt very distinctly in order to allow it to go backwards horizontally as forcefully as possible. The trunk is pulled very long by the weight of the pelvis drawing back. The trunk hangs down more and should be permitted to do that freely while the pelvis slides back more and more.

The pain in the fascia being stretched can be excruciating and therefore limits the degree of how much the pelvis is left to go back and the trunk is allowed to hang. The pain is usually located in the upper arms and the shoulder girdle. It can be imagined that the “flesh is being pulled off the bones” and that the shoulder girdle is drawn away from the ribs in front, back, and on the sides. It helps to let the breath flow down along the inside of the front wall of the trunk to the pubes.

In coming up, care must be taken to do it normally. The heels are imagined to be pushed back: extension against the ground brings the tuberosities back up first. Only when the legs are extended again does the sternum come up parallel to the wall.

Folding against the wall is structurally active. This means that structure is changed. It is in the direction of integration if all parameters of normal function are regarded.

Folding on the Toes

Standing on the toes puts much stronger demands on balance than standing on the feet. Folding must be precise as this allows the relevant aspects to be sensed clearly and distinctly. Especially Unfolding is sensed very clearly and exactly.

As always, one begins by making certain that the zigzag line including acmott is present. In order to be precise, the first objective is to let the center of gravity go forward horizontally but not yet up. The movement is initiated by relaxing the abdominals; the pelvis starts to slide back. This brings the center of gravity back first although it should go forward. Therefore, the sternum is let go forward much earlier than in Folding, reversing the direction in which the center of gravity has begun to move to forward. Then the knees are also left to go forward. The folds should only move horizontally, not down, which is only possible if muscles relax so the midline can lengthen. Now the center of gravity is over the balls of the feet, the zigzag line is accentuated.

The balls of the feet are pressed against the floor. As a reaction, the center of gravity rises vertically. The toes should spread both medially and laterally like a fan. First the knees extend, then the heels lift off the ground, and only then the body opens in front at the level of the hip axis, the sternum going forward and up. There should not be a sense of pulling up the heels. They should rather be felt to be suspended back and down. The feet must not be averted; the heels point outwards slightly.

The zigzag line is sensed again, and balance should be secure with shoulder girdle, chest, and belly all completely relaxed. Folding happens in the usual way until one sits on the heels. Balance should not deteriorate while Folding. The forearms are placed on the knees and can be used for support. Acmott should still be present. The center of gravity tends to be too far back and should therefore be brought far forward. The weight of the trunk rests via the ilia on the thighs. These rest on the lower legs which in turn are on top of the feet which transmit the whole weight down to the ground. Balance should be exceptionally good, no effort should be felt, and one should be able to comfortably stay in this position for a long period of time.

Extension against the ground broadens and flattens the balls of the feet and the toes even more. As a reaction to the increasing downward pressure, the trunk first lengthens again. The tuberosities are pushed back, the sternum goes forward. The middle back is felt to sink down a little more! Only when no more length is possible do the tuberosities begin to rise back up. The trunk still rests on the thighs and so tilts forward on them, the sternum sinking almost to the knees while these are being extended. The trunk can be left hanging completely until there is full extension of the legs and until one reaches something like “one-jointed” Folding on toes (Notes on S.I. 91/1, p.18). Acmott is then reversed.

Or, one begins extension around the hip axis earlier, when the knees have been extended about halfway. Acmott is preserved, of course. The sternum should be felt to be pushed up nearly vertically and forward; the chest must not tilt back. Care must be taken that the pelvis is not pushed forward by the hip extensors nor pulled forward by the abdominals. It should feel to be drawn forward from the sternum and only then, when the front wall of the trunk is not capable of getting any longer.

One stands in minimal Folding on the toes again. During all of the Unfolding one has always focussed on the ground and never thought of going up at all. The sign for Unfolding is that one always feels grounded very much and that balance is never impaired during the movement but improved by it.

To go back to standing, only the heels are left to go back and down first. Then, the pelvis slides back into place, taking the upper body and the knees along.

Notes

1. “Sleeve-support” is disregarded (Notes on S.I. 91/1, p.30).

2. This is different in a rare number of movements where the hip axis has to be anterior, as in some gymnastics exercises or the tennis serve.

3. actio = reactio

4. Since the midline is really the “line of gravity” some modifications would have to be made for the pattern of the specific weight in front and in back. These would be minor, however.

5. Since only the front-to-back situation is regarded here bending sideways is assumed to be absent.

6. An even more fanciful model would state that the “compression-resisting element” is contained in a languish balloon of its own which adjoins the posterior wall ventrally, as a “bag-within-the-bag”