The History of Posturology

I. The emergence of a certain logic of time


     The history of Posturology relates the emergence of a certain logic of time in the approach to reality. Not the time of Science of the XIXth century which «counted the minutes, noted the simultaneousness of events but offered no purchase as to what occurs in the intervals», as BERGSON said. But a time that includes the duration, the «continuation of what no longer is into what is.» (Bergson H., 1922) A time like that of TAKENS, which does not hesitate to mix successive instants in time, assured that the logic of the continuity would allow us to better comprehend reality than that of succession.

The epistemological error

     We can date the beginning of this history to 1865 with the Introduction à l'Étude de la Médecine Expérimentale de Claude BERNARD.

     Why did the author choose such an overly subtle title? He was not naïve. He knew very well that, since the beginning of time, physicians'thinking was nourished by clinical experience; there is no question of his trying to pretend to 'introduce' them to what they had been practicing since the dawn of time! But, he invited them to partake in a moment of reflection, a 'study of the study' of diseases.

     So, what did he understand? What was he in such a hurry to say in the middle of the XIXth century? Claude BERNARD does not state it explicitly anywhere. And yet, the answer seeps out of every pore of his text: it is urgent to contemplate the conditions of our knowledge. If you could only know what wealth of understanding this idea brought me!

     To know is to establish a line of communication between reason and reality, is essentially what he said: «The philosopher imposes his idea as an absolute truth. In contrast, the experimenter, more modest, advances his idea as a question that he repeats at every instant in the face of the reality measured by means of his experiments.» (Introduction à l'Étude de la Médecine Expérimentale, first part, chapter 2, Introduction).

     It is difficult to be clearer.

     Nevertheless, nothing justifies, a priori, the possibility of such a dialogue: Indeed, preliminary evidence would rather support that a sharp divide separates the human mind from reality, as so beautifully related by this old Chinese fable:

Two men, leaning against the railing of a bridge, are both looking at the river's flowing water. It is clear and full of fish.
"I think the fish are happy!" exclaims the first.
"How can you think that the fish are happy? You are not in their scales?" retorts the second.
"And you, you are not in my head - responds the first - how can you think that I cannot think that the fish are happy?"

     Indeed, things are what they are, independently of what man thinks of them. That man's thinking might, nonetheless, join the logic of things, this is the wonder that is so important to understanding the intellectual process of reasoning, so as to better advance knowledge, Claude BERNARD tells us.

     However, there remains in the author's thinking, a certain odor of Kantian rationalism. Of course, for Claude BERNARD, it is not reason that structures reality at the will of his demands! But, if the experimenter must submit his idea to the observations, it is not, according to Claude BERNARD, just any observation; he specified that reason maintains control over the operations, «An observation whose determinism is barely rational should be excluded from Science.» (Introduction à l'Étude de la Médecine Expérimentale, third part, chapter 2, ¶2). Obviously!... Science is not a collection of indeterminate and irrational observations. But, when the wise man's a priori reasoning helps him decide whether the observations that he made are scientific or not, his science runs the risk of being merely the reflection of his a priori hypotheses!

     The history of medicine, and not only Posturology, will run head on into this petitio principii (a logical fallacy in which a premise is assumed to be true without warrant or in which what is to be proved is implicitly taken for granted), this epistemological error, this forgetfulness that the logic of the world is not our logic. At certain times, physicians lose their balance; shaken by a conflict between their rationality and reality, they no longer know where to set the limits of practicing their art. Indeed, it is not so simple to enter the interactive game between the world and intelligence, to allow the world to educate our reason in order to learn how to better understand.

Before 1870: the triumph of clinical anatomy

     At the time that Claude BERNARD was publishing his Introduction à l'Étude de la Médecine Expérimentale, the group at the Salpêtrière Hospital, under the leadership of Jean-Martin CHARCOT, was trying to structure the classification of diseases of the central nervous system around the concept of clinical anatomy. They discovered that the rational representation in space of this part of the body enabled them to advance a coherent explanation linking their organization, their function and clinical observations. In this way, the essential elements of Neurology were described in less than 10 years.
Spatial logic and temporal logic

     The fulgurant success of the clinical­anatomical approach reflects the ease with which man's intellect manipulates spatial logic. And there are good reasons that it functions as such. Not only is space controlled by the senses but man has also understood for several centuries how to calculate the area of a field or the volume of a barrel, how to formulate the equation describing the trajectories of stars. What can be seen, what can be touched, what can be manipulated in space, these represent the reality upon which reason was constructed during the centuries of strong complicity.

     The same did not hold true for time, which appeared to be an assumption, without reality, simply destined to place a series of events in order: Time is the «number of movements, according to before or after», as has been said since the days of ARISTOTLE. As a result, the reality of this assumption is questionable and questioned, and the latter even more because time is not regulated by the senses. The past, the present and the future cannot be perceived simultaneously, like space. This time 'lapse' renders the present, in an instant, the past, and the future is merely an act of intellectualization, with the domain of history being the only link between the past and the future in the moment called the present. Did KANT not write (Critique de la raison pure), «Time is not something in and of itself, not even an objective determination inherent in things.»?

     Yet, certain thinkers started to realize the poverty of the concepts of time. BERGSON, for example, wrote to a friend, «I realize, to my great surprise, that scientific time does not last, that nothing of our scientific understanding of things would change if all reality were to be deployed all at once, instantaneously, and that science essentially consists of eliminating the duration.» (Letter to Papini; écrits et paroles 2) This conceptual poverty came to be at the heart of the questions that physicians would start asking themselves.

After 1870: the problem of diseases without lesions

     Indeed, after 1870, the situation changed for the first group of neurologists; the project to describe new anatomical­clinical entities passed to the back burner, behind the burning question of the moment that haunted and stimulated all the intellects: What are these diseases of the central nervous system for which we find no anatomical lesions?

     A first response was provided by hysteria. Because a hysterical episode can be triggered by hypnosis, CHARCOT saw proof that these diseases without lesions could reflect psychological involvement. FREUD, who was in Paris at the time, attended the grand rounds on hysteria conducted by CHARCOT. He participated in the development of this school of thought; he helped it progress and this led him to think of time in such a new way that he no longer knew if it were appropriate to still call it 'time': «The processes of the unconscious are timeless» - he wrote in The Unconscious (1915) - «that is, they are not ordered in time, they are not modified by the time that passes, in short, they have no relationship with time.» No relationship, that is, with time as defined by PARMÉNIDE which carried all in its wake: «One never bathes twice in the same river» stated the philosopher. Using this image, the new logic of time specified: never twice in the same river, if one considers the water that flows, but can the river be reduced to water that flows at an instant t? The river's time, like that of the unconscious, negates the principle of non-superposition that characterizes chronology and spreads itself over a duration that accumulates the effects of the events. Thus, it is around and by history that we organize our search for pleasure, at the mercy of what happens within and around us, without necessity.

     FREUD discovered that a new rational representation of time would enable a coherent explanation of neuroses, without making the slightest allusion to any topological organization of the central nervous system.

     Doctors did not recognize themselves in this new approach to reality, the proof being that, even today, anyone can treat these patients with psychoanalysis, without ever being accused of illegally practicing medicine, at least in France. But if neurotics are not sick, what is disease? What is medicine?

1916: First recognition of a postural disease

     While Sigmund FREUD was studying the unconscious in Vienna, Joseph BABINSKI, in Paris, was pursuing another route that he would not carry to term. In addition to the neurological diseases that he knew so well because he participated in their description, in addition to the psychological illnesses that he had extensively studied with CHARCOT, he suspected the existence of another type of central nervous system disease that did not belong to either hysteria or neurology, as he explained in Hystérie­Pithiatisme & Troubles Nerveux d'Ordre Réflexe, which he wrote with FROMENT (1918). Did his intuition include postural disorders? Nothing could be less clear.

     What is certain, however, is that Pierre MARIE was the moderator of the first meeting of neurologists that recognized postural disease. That meeting of the Société de Neurologie was held during the First World War - in 1916 - in the presence of the French Minister of the Armies, because the field physicians were encountering severe problems with soldiers with head trauma. Most of these wounded complained of subjective symptoms (vertiginous sensations, visual disorders, instability, headaches, etc.) for which no anatomical­clinical explanation could be found. So, what was wrong? Was it a subterfuge to avoid returning to the front? To a nightmarish fear of the executioner's stake?

     Pierre MARIE oriented the discussion: «What is the nature of these subjective disorders? How severe are they? How should we respond to these wounded once the their head wounds have healed?» (Marie P., 1916) All the big names of neurology were present at this meeting and each had his turn to express his opinion. All hypotheses were advanced A consensus emerged, singular, and quite extraordinary, that Pierre MARIE reported in the following terms: «All these subjects described the exact same disorders that they were experiencing, using absolutely identical terms. Obviously, this cannot be the repetition of a learned lesson.» Intersubjectivity is the basis of objectivity! A philosophical principle led the assembled neurologists to recognize the reality of this illness without anatomical­clinical support... Astonishing!

     The disease was recognized as such, but was not given a name, at least not exactly, Pierre BONNIER attended the meeting and suspected the involvement of a postural disease - meaning bearing, as they said at the time - but he dared not say so: «I did not want to ask to present a kind of technical observation and I later regretted it.» A secretary accepted to add several of BONNIER's comments at the end of the final printed text.

Awaiting chaos

     Indeed, Pierre BONNIER held his tongue, because - other than his intuition - he did not have a great deal to say! (Bonnier P., 1904) How could he have offered a coherent concept in the face of the so little known about postural control? The fundamental question, "How does man stand erect?", had been asked by Charles BELL several decades earlier (1837), but responses proffered by physiologists at the time were disconcerting. According to the topographical logic of sensory organs - one organ for one sense - one had been looking for the sense of equilibrium and, lo and behold, a profusion was found: the eye (Romberg M.H., 1853), the vestibule (Flourens P., 1829), cervical muscles (Longet R.A., 1845), the foot (Heyd, cited by von Vierordt K., 1860) and even the oculomotor muscles (de Cyon E., 1911). Karl VON VIERORDT (1860) had been trying to explain that perhaps all these organs could participate in the same function. Obviously, his voice was not heard.

     This unnamed disorder, which did not fit into the anatomical­clinical categories of the neurologists, for which no advanced hypothesis afforded an explanation, would meet its chaotic destiny during the course of the XXth century. Very quickly, the physicians started to forget the subtle reasoning that reigned at the consensus conference of 1916. And, because the title of Pierre MARIE's report included the term 'subjective', they referred to 'Pierre Marie's subjective syndrome of head-trauma patients', even though Pierre MARIE never assigned a name and certainly not this one! Unable to treat the affection, the doctors focused their attention only on how the patients lived with their disease. This subjective approach - justified - to the patient seemed to earn the use of the epithet 'subjective' to qualify not only the therapeutic strategy but also the syndrome itself. As the years passed, the disorder remained hidden behind this label and the syndrome indeed became more-and-more subjective, to such an extent that its 'reality' would again be contested, spreading confusion throughout the medical community. Depending on the hat worn - specialist in forensic medicine, health insurance medical advisor, specialist in occupational medicine, etc. - contradictory decisions were made that no logic could reconcile. What a mess!

     Indeed, it was a shambles for all diseases involving posture. In the absence of an underlying concept, a firm and consensual operating principle, nothing was organized for the patients with a postural disorder - no courses for doctors, no scientific society, no administrative and/or hospital structure - while all patients who could be classified and assigned to an anatomical­clinical category were referred to the corresponding specialized unit and benefited from its scientific and administrative arms.

     This rat's nest confirms that topographical logic is not applicable to postural disease, and cannot organize its management or treatment. As a result of being unconditionally faithful to the logic of anatomical­clinical relationships, the medical community sank further into its epistemological error, already indelibly recorded by Claude BERNARD's pen. It is not rational to complain of a multitude of symptoms, as head-trauma victims do, when the most sensitive medical imaging examinations detect, and just barely, only minute lesions of the cerebral trunk. But neurosurgeons know that they cause rather large brain lesions without ever observing even the most minor post-concussion syndrome during the aftermath of their interventions.

     What is not rational must be eliminated from scientific medicine!

1970: NASHNER brings in the engineers

     The situation only started to be untangled in the 1970s, when a young student at the Massachusetts Institute of Technology in Cambridge, Massachusetts, decided to do his doctoral thesis on the system subjected to postural control (Nashner L.M., 1970). To open the feedback loops of vision and pedal proprioception, he constructed a masterpiece of technology for the era, susceptible of being subjected to the movements of the center of gravity of the individual being examined, the movements of the cabin in which and/or the platform on which he was standing. This apparatus, designed by an engineer, had the immense merit of showing physicians that VON VIERORDT (1860) had been right! The posture of a human standing erect, at rest, is indeed controlled by a system that integrates the information from a series of entries from the postural system within a feedback loop intended to correct any straying of the body from its equilibrium so as to stabilize it.

     The logical impact of this assertion was not immediately appreciated. Obviously, it was already known that the temporal series of positions of the body in its environment was not merely a succession of different states but that it consisted of a chain of events, a continuity. Obviously, the difference between x = f(t) and x[t] = f(x[t ­ 1]) was known, but it was not known how to logically manipulate this difference, and it was not known that it could explain the colossal efficacy of a butterfly's batting wings (Lorenz E.N., 1993). As Henri POINCARÉ had written in 1908 concerning this type of system: «Minute differences in the initial conditions lead to very large ones in the final phenomena.»

1955: Jean-Bernard BARON's doctoral thesis

     And yet, almost 20 years earlier, Jean-Bernard BARON (1955) had already told us so. But he expressed himself so poorly that nobody could understand what he was saying. Neither mathematician nor philosopher, and hostile to any formalization, he barricaded himself in an ivory tower he constructed of pure 'facts', without willing to realize that, alone, words give meaning to the fact.

     In addition, his discovery did not appear very informative at the time: asymmetrical postural hypertonia of an animal's (fish and mice) paravertebral muscles occurs after tenotomy (cutting) of an oculomotor muscle, if and only if the tenotomy was extremely minor, i.e., unable to cause a deviation of the visual axes by more than 4°!  Indeed, it had to be observed hundreds of times to dare to repeat it continuously and in an atmosphere of general indifference. Furthermore, nobody, or almost nobody, had seen these animals and thus no one could imagine to what extent they were twisted around themselves, unable to swim or walk a straight line. Truly, it had to be seen to be believed. And I had the incredible luck to do so.

     But even seen, even believed, the observation had to be integrated into a logical and coherent explanation. My mathematics courses had not gone beyond integral and differential calculus, and, in my head, I could not find a model that would enable me to understand this absence of proportionality between the cause and the effect. All my concepts were 'linear', they tolerated addition and multiplication, but they could not explain the weird things happening to BARON's animals: minor cause, major effect; major cause, non-existent effect!

     It was Élie DE CYON's studies (1911) that enabled BARON to make the connection between oculomotor muscles and postural control, a connection that was made even more evident by his own experiments. These findings brought us closer to the logic of a cascade of temporal events that characterize postural control; indeed, we manipulate all these ideas all day long, but without elucidating what might bring them together.

Logistic equation

     Thus, it was at a laboratory at the Salpêtrière Hospital, thanks to Jacques MARTINERIE, that I at last came face to face with the logistic equation:


     It immediately suggested to me the idea that it might be part of the postural control system, because of their many common points (Martinerie J. & Gagey P.M., 1992).

     This equation describes a cascade of temporal events: what happens at time t depends on what happened at t-1, exactly like the functioning of postural control, in a feedback loop. There is no proportionality between the variations of the parameter Q and variations of the solutions of the equations that they induce, exactly what BARON had discovered in the functioning of the postural system: a minor tenotomy provoked severe hypertonia of the paravertebral muscles, completely out of proportion with the damage caused to the oculomotor muscle.

     This absence of proportionality, this non-linearity, as the mathematicians say, has since proven to be a general rule of the functioning of the postural system. BARON demonstrated it at the level of the oculomotor muscles in the 1950s; BOURDIOL found it in the sole of the foot in the 1970s: a minimal lifting of the sole of the foot, by inserting a very thin prosthesis into the shoe, could be more effective than a conventional small but thick wedge (Bourdiol R.J. et al., 1980); FOURNIER located it in dental cusps in the 1980s: indeed, in this test, a thin plastic film between the teeth induced a greater modification of tonus than a large, dental, saliva-absorbing cotton roll (personal communication). Today, all posturologists know that, the finer the manipulations of postural system input, the more chance they have of being effective, an efficacy that has nothing in common with their intensity.

     In the meantime, posturologists have learned to collect and analyze a physical signal from the postural system and the dynamic non-linear analyses of this stabilometric signal confirmed that the dynamics of the postural system are indeed non-linear (Gagey P.M. et al., 1998).

Duration formulated by TAKENS' state-of-the-system vector

     The studies of these dynamic non-linear systems by POINCARÉ and DULAC (1934) and today by TAKENS have made a liar out of BERGSON. Thanks to them, natural sciences no longer consist essentially of 'the elimination of the duration'; indeed, on the contrary, the POINCARÉ­DULAC theorem, modified and improved by TAKENS (1981), introduced the formula describing of the duration, which renders the concept operative. This theorem proves that the state of a system, at a 'reference time' ti , can be represented in phase space by the vector:

 Knowing that
in this equation designates a time lag, one can easily read the duration of this 'reference time' that goes from .


     At the instant t of conventional kinetics, nothing happens, but at the 'reference time' ti of TAKENS' state-of-the-system vector, something is happening, because one is within the duration, and the duration carries an inherent dynamic element.

     For one chess move, DE SAUSSURE (2002) said that is was not useful to know what moves had been made previously to respond, and it is indeed true that the displacement of one chessman on the board creates a totally new state, whose structure is perfectly determined by itself and for itself. But this example - destined to clearly differentiate the diachronic from the synchronic - blurs the role of the opponent's strategy of dynamics which becomes unmasked with the successive moves of the temporal cascade.

     Of course, time has a spatial relationship, defined in and by a phase space with n dimensions, but this spatialization, generally used to think of time, here enables us to think and operate in the context of the 'continuation of what is no longer and what is'. And this is new.


     Thanks to this new element, we state that it is now possible to explain coherently the non-linear deregulations of standing erect. This declaration, logically deduced from the structure of the postural system, presents a certain obviousness. But like all scientific statements, it does not claim to be true; it can merely hope to suffer the critics' slings and arrows, and possibly resist for some time against its "falsification" (Popper K.R., 1935). However, it must be kept in mind that this declaration has already been corroborated by 100 years of history because, since 1870, it has been said to be impossible to coherently describe the non-linear deregulations of standing erect based only on anatomical logic.

     Furthermore, this declaration can be corroborated by its therapeutic efficacy. From the non-linear nature of the fine postural control system, it is possible to deduce practical conclusions and it is relatively easy to show experimentally whether these conclusions are false or not. But clinical posturology is still in its formative years and much work remains to be done to fully confirm the declaration. Let us simply state that those studies merit being done.


     Neither posturologists nor physicians have been responsible for the emergence of this new logic of time, of instantaneous dynamics, in the approach to reality; but they have been and continue to be affected by it.

     As long as we did not know the whims of non-linear systems, physicians had to get by as best they could, to interpret what they saw with their clinical and philosophical intuition.

     Today, posturologists hypothesize that the logic of postural disorders is a dynamic in the process of becoming that they hope to be able to drive out of its hiding place and eventually reroute before it bears fruit in the form of irreversible lesions.

     It remains for physicians to decide whether or not postural disorders enter within the framework of medicine.


II. Coherence of the biomechanical discourse: stabilometry

The physicians’ incoherence

     Why oh why did it take so long for physicians to adopt the precision of the language of physics to talk about equilibrium? Indeed, it had been understood, as of the middle of the XVIIth century, that the concepts of Newtonian mechanics did not apply exclusively to celestial bodies but also to the human body; biomechanics had easily adopted the notions of force, mass, acceleration, moment of inertia, etc.; and BORELLI had drawn, as early as 1680, in his De Motu Animalium, the first representation of the vertical of gravity of the human body … Yet, for centuries, physicians continued to look for a definition of equilibrium specific to the human body!

     In 1940, the admirable neurologist, André THOMAS, still defined equilibrium in his superb book, Équilibre et Équilibration, as follows: “Equilibrium can no longer be considered a state of rest when all the parts of the body in question are able to move. If the term equilibrium is confusing, it would be better to delete it and change the definition.” How did things become so complicated? It would have sufficed to adopt the definition of equilibrium rigorously formulated by physicists so long ago: “[In a state of equilibrium ...] the body is positioned between two aligned, equal and opposite forces – i.e., one the action of its gravity and the other the reaction of the surface on which it rests (BRISSON, 1803).” It would have been obvious right away that the human body is never balanced ... Because its center of gravity is situated above its center of pressure on the ground, as soon as the resultants of the forces of gravity and reaction are no longer strictly aligned, a torque appears that tends to precipitate a fall. Because the human body cannot continually maintain these two resultants perfectly aligned, it is never balanced, but it is ‘stabilized’ – that is to say, it possesses the ability to return close to its balanced position whenever it strays from it.

     Three hundred years ... to start enforcing the obvious truth that the human body is never really balanced but that it is stabilized … it has taken three hundred years.

Discovering stability

     The right question, which led physicians to correct their ideas, was asked by Charles BELL, in 1837: “How does a man maintain a standing or bending posture against the wind that blows against him? He obviously has a sense through which he knows his body’s degree of bending and he has the ability to readjust it and to rectify any deviation from the vertical. What sense is that?” The question was remarkably well-formulated. Even so, it still had to be oriented in the right direction, towards the ‘how’ of phenomena, i.e., towards the observation of man standing erect, and not towards the ‘how’ of explanations which, at the time, could only look for the – phantom – sense of balance, because the concepts of set, system and cybernetics had not yet been mastered by physiologists, even though some of them had already begun to foresee the need for them.

     The first recordings of man standing at rest seem to have been done by VON VIERORDT (1860). The equipment was rudimentary – a feather attached to the peak of a helmet scratched a lampblack-covered sheet stuck to the ceiling. Nonetheless, the results were very encouraging and already indicated the role of vision and of plantar sole sensitivity in postural control. VON VIERORDT had many emulators – the most famous of them were: MITCHELL and LEWIS (1886), HINSDALE (1887), BULLARD and BRACKETT (1888), HANCOCK (1894), BOLTON (1903), MILES (1922), FEARING (1924), HELLEBRANDT (1938) and TOULON (1956). Unfortunately, all the instruments that they used, including VON VIERORDT’s, modified the phenomenon they were observing, and only gave a raw signal that was very difficult to interpret.

     The first force platform built in France by SCHERRER (RANQUET, 1953), freed the subject of all the paraphernalia – strings, belts, feathers, etc., which, in order to visually track the movements of the body, attached it to its environment. On the platform, the subject is free, liberated at last from the all interfering sources of complementary information on the positional changes of his/her body relative to the environment.

     However, in the 1950’s, analogical signal analysis remained rudimentary and one had to wait for the further development of computers for the force platforms to reveal all their potential, thanks to more powerful signal analysis.

     The error then became manifest: normal man standing still upright is able to maintain his vertical of gravity not roughly within his support base, but much more precisely within a cylinder of barely one square centimeter in surface area! This extraordinary accuracy is achieved by anyone without any training whatsoever as a tightrope walker.

     When physicians accorded the sway of a ‘balanced’ subject’s vertical of gravity the same limits as those of his support base, they totally ignored not only the body’s intrinsic stabilization system and the astonishing fine-tuning of its control but, most of all, the possibility of its pathological dysfunction. When the vertical of gravity exceeds the limits of normality – one square centimeter – it is still far, very far, very very far, from a loss of ‘balance’ induced by going off the limits of the support base, which is tens of thousands of square millimeters. This huge range of ‘abnormality’ remained completely ignored by physicians ... thereby explaining why they could not make the connection between all the disorders of the fine stabilization system and the pathologies they nevertheless knew well because they saw them frequently … but never understood them.

     A hundred years. It took a hundred years of posturographic observations and the invention of force platforms for the stability of the human body to achieve the almost palpable physical density of the experimental realities that oblige the international scientific community to accept their existence. Stability is a fact ... regardless of what we may think of it. Its existence imposes itself, with all its mysteries and its implications that remain to be studied and measured.

Center of gravity, center of pressure: how are they connected?

     To introduce these measurements of stability, let us be very technical, at least for a short while. The force platforms record the position of the center of pressure of the subject being examined. But what the physicians want to know – no doubt referring to their traditional ideas of equilibrium – is the position of his center of gravity ... So what do we have to do?

     All the posturographists admitted the existence of a relationship – obviously – between the position of the center of pressure and the position of the center of gravity, but what is this link? That question has been the theme of many of the International Society of Posturography’s debates since the 1970’s (see Congress).

     Now, we know. After theoretical physical studies (GURFINKEL, 1973; BIZZO, 1993), after spectral and stochastic analyses of the signal (GAGEY et al., 1985; COLLINS and DE LUCA, 1993), after simultaneous recordings of the center of pressure and of the center of gravity (SCHIEPPATI et al., 1994; WINTER et al., 1998), we know. We know that the center of pressure behaves somewhat like a sheepdog with regard to the center of gravity, it runs faster and further, to the right, to the left, forwards and backwards, as if to bring the center of gravity back, to keep it close to its mean position. That is to say, the movements of the center of pressure stabilize the center of gravity. The center of pressure applies exactly the same tactics as one uses when trying to stabilize a broom held upside down on the tip of a finger – that finger is, in rapid succession, jerked to the right, to the left, forwards and backwards, faster and further than the center of gravity of the broom, in order to bring the latter back to its mean position. Ninety-five percent of the observed stabilization phenomena of a ‘normal’ man standing at rest would, according to WINTER et al. (1997), correspond to these center of pressure strategies.

     Thus today, physicians can easily understand that to study this standing-erect stability of man, we must first observe the movements of his center of pressure, because stability is achieved through its movements, at least as long as he is young and healthy. The force platforms are therefore the instruments of choice to elucidate if and how a human normally achieves stability, precisely because they record the position of the center of pressure.

Measuring stability

     It is impossible to measure stability directly ... it is not a magnitude, but merely an aptitude, enabling the body to return close to its position of equilibrium whenever strays from it. Yet stability has characteristics that indeed can be measured – using force platforms, of course.

     The mean position of equilibrium, first. It is normally centered between right and left. If it is more than one centimeter away from the median sagittal plane, it is statistically as well as effectively abnormal because the subject who is ‘out of balance’, even if only very slightly, subjects his joints to abnormal strains and soon suffers more and more.

     The deviations of the vertical of gravity from its mean position that the fine stabilization system tolerates can be more or less large. These deviations are measurable distances that tell us something about the accuracy of the system.

     The distance traveled by the center of pressure – to the right, to the left, forwards and backwards – in order to stabilize the center of gravity is a measurable magnitude that gives us an idea of the energy spent by the stabilization mechanisms (GAGEY and WEBER, 1999).

     The center of pressure sometimes saunters, sometimes rushes, in order to accomplish its stabilization task, and the rhythm and amplitude of such acceleration changes provide information on the dynamics of the system (BARATTO et al., 2002; SASAKI et al., 2002).

     The ventilatory movements of the rib cage can impose their rhythm on the movements of the center of pressure, when the subtle play between vertebrae and ribs that normally counterbalances the postural effects of ventilation is perturbed (GURFINKEL and ELNER, 1968). The stabilometric signal must then be subjected to frequency analysis, which is able to demonstrate the amplitude of the sway of the center of pressure according to its frequencies, particularly but not exclusively to determine whether the ventilatory rhythm perturbs the subject’s stabilometric signal (GAGEY and TOUPET, 1997; Hamaoui et al., 2002).

     Progressively, the stabilometric signal – a series of successive sampled positions of the center of pressure – became the object of more-and-more sophisticated techniques of mathematical analysis: chaotic (MARTINERIE and GAGEY, 1992), then stochastic (COLLINS and DE LUCA, 1993), and each time in forms more-and-more specific to their objective (FOURNIER, 2002; BARATTO et al., 2002). Thus, a considerable number of stabilometric parameters could be proposed. MORASSO and his collaborators, for instance, have studied about forty parameters that describe more-or-less different characteristics of the stability of man standing at rest (BARATTO et al., 2002) ... and the data harvested from the stabilometric signals are so rich in information that they have not yet exhausted their analyses of the subject.

For what?

     Such precision obtained by using the language of physics, so many state-of-the-art mathematical calculations, so much data derived from stabilometry ... But to do what? To what do they apply? What end do they serve?

     These questions call for different levels of answers.

     The man in the street does not need in-depth explanations to understand that the more thorough the studies on stability are, the better they help to understand the disorder and cure the patients who have difficulties standing up! It is self-evident.

     And some physicians and therapists did not require lengthy explanations to understand that these studies could be highly lucrative! They had hardly begun to use this new knowledge that their waiting rooms filled with so many patients, informed by the highly efficient communications grapevine and waiting for someone to know at long-last how to treat their postural deficiency syndrome (DA CUNHA, 1987). The financial aspect is very important for patients because, obviously, posturologists will only be willing to take the time to diagnose and treat patients if posturology in general, and stabilometry in particular, enables them to make a living!

     Ironically, those asking “what’s the use?” are physicians! “No one ever told us for what it could be used", states the American Academy of Neurology (1992). And the French Academy of Medicine simply said that it would not be of any use: “The stabilometer is a research instrument useful to physiologists or pharmacologists. Its use in medical practice ... does not in any way shed light on the cause.” (CAMBIER, 1993). But why do physicians, and not the least talented among them, have such difficulties understanding the repercussions of thorough studies on human stability? Of course, as we have already seen, the posturologists’ discourse does not comply with the physicians’ traditional way of thinking, or with their dated conception of equilibrium, or with their century-old reference to anatomical–clinical reciprocity. But this new approach merely reveals the underlying intellectual rigidity, it does not explain it. Somewhere there must be some reasons why physicians are so attached to their antiquated traditional way of thinking ... Stabilometry can also serve to pose this kind of critical question, whose interest probably escapes the patients, even though it is far from negligible.

For whom?

     At present, stabilometry is practically of no use to any patient – compared to what it is capable of! There are only a few thousand of stabilometry platforms in the world, and many fewer physicians or therapists to use them ...

     Stabilometry will probably not be of much use to patients with disorders, even if they are postural, that correspond to a known pathology, such as Parkinson’s disease, whose cause and evolution have been elucidated by traditional medicine. Physicians did not need the platforms to make those diagnoses and specifically treat those afflictions, so it is most likely that they will only use the platforms to improve, even slightly, their management ...

     In addition to these quite well-known diseases, how many patients suffer from still ‘mysterious’ disorders, whose diagnoses are nothing but labels without any indication of the true causes. Without pretending to be MOLIÈRE, we can nevertheless point out that making the diagnosis of ‘lumbago’ amounts to telling the patient that he suffers from lumbar or lower back pain – which he already knew – without adding even a grain of intelligibility. And such ‘mysterious’ afflictions can be found in many medical fields. Eighty percent of the patients seeking medical advice for vertiginous sensations cannot be classified in a precise nosological category, according to some otoneurologists. Yet these patients, whether suffering from body axis pain or being unstable/prone to vertigo, have a point in common: they have difficulties standing up, whether they stagger or suffer in that posture. Therefore, it is perfectly logical to question the functioning of the system that maintains them in a standing position – the fine postural control system – and to take advantage of any tool available to elucidate it.

     Experience teaches us that this concept of intelligibility – indeed quite simplistic – works wonders in the hands of the few therapists who have started using it. So, for the time being, we think that stabilometry could be useful for many patients with ‘mysterious’ disorders who have any type of difficulty standing up; and they are very numerous.


     Thanks to the evolution of ideas over almost two centuries, a few rare physicians and therapists have, for no more than twenty-five years, understood the logical errors that prevented, and continue to prevent, traditional medicine from specifically treating patients who have difficulties standing up, whether they stagger or suffer in that posture. It will certainly take more time for people and administrations to integrate this new assessment of postural patients. But the logical coherence of the biomechanical discourse of stabilometry is corroborated by enough scientific data and therapeutic successes to become able to progressively impose that new organization


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