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.

Conclusion

     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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