Introduction
     During millennia, man stood upright without asking questions about the machinery that allows this posture. The idea of an animal machine was unknown, moreover. But when he dared notice that celestial bodies were submitted to the laws of the mechanics, then he began thinking his own body in terms of masses, accelerations, forces, balance (Borelli, 1680) (fig. 1)...

FIG. 1 - Vertical line of gravity. As far as we know this is one of the first drawings of the vertical line of gravity of the standing man, printed in the book of Borelli «De motu animalium» (1680).

    The spiral of questions about upright posture was in being then; actually it was initiated by Charles Bell at the beginning of XIX-th century: «How does man maintain a posture upright or tilted against the wind?» He wrote «It is evident that he possesses 'a' sense by which he knows the tilting of his body and that he possesses the capacity to right it and to correct any deviation from the vertical. But what sense is it?» (Bell Ch., 1837).
Charles Bell set the problem clearly. But he set it inside the problematic of his century! At this time it was impossible to imagine that a function is not assumed by an organ - the function creates its organ - and an sense organ in this case: «What sense is it?». The vestibular apparatus put forward Flourens (Flourens P., 1829), and gradually his proposal was winning. Not only the vestibule is an organ that gathers in a definite, visible, place parts of the body in connection with balance, but moreover this organ is structured according to the Cartesian concept of the three dimensions space. Nothing more was needed to seduce all the spirits... All, except one at least: Vierordt.
    Tübingen's physiologist was aware that things were not so simple (Vierordt K., 1860, 1862, 1864, 1871, 1877). He had to recognise that looking for «THE» sense of balance, in fact several senses had been found. He knew that vision (Romberg M.H., 1853), sensibility of soles (Heyd, undated), proprioception - muscles sense as it was said then - (Longet, 1845) act also for keeping upright posture. Then how to understand this bizarre function of the upright position, manifestly impossible to summarise in a single organ of senses? The concepts of system, control, cybernetics were still far from being available. If it could not yet be understood how does man maintain an upright posture, at least that could be observed. This remark, brilliant of simplicity, led Vierordt to become the first posturograph in the world. He, the first, invented a system which allowed recording the postural sway of a man standing quiet (fig. 2), so much he was sure that these small oscillations would introduce us to the secrets of this endless struggle that man's body leads with gravity forces to stabilise itself.

FIG. 2 - First stabilometric recordings. Vierordt kept only the envelope of the drawing made through the feather worn by the standing subject, in the eyes open (a) and closed condition (a'); in the «position hanchée», the right leg being the support (b); sitting eyes open (c) and closed (c'); standing on the only right foot (d).

    One hundred and fifty years passed by since these first posturographic recordings and gradually it was noticed that Vierordt was right, things are not as simple as Flourens thought. Today all the basic researchers admit that upright posture is controlled by a split system, which integrates sensitive and sensory information, coming from multiple canals: visual, oculomotor, vestibular, somatosensory, plantar exteroceptif. And we no longer have difficulties handling these experimental data from posturography by means of models arisen from system theories. Vierordt was right... Flourens swindled us!
    From this evolution of ideas on postural control physicians have to draw the necessary conclusion: we must criticise our interpretations of all the diseases, which, by one way or another, concern standing upright quiet. The notion of system dominates the postural physiology, let us impose it to our reflections about the diseases of standing man. Posturographic recordings gave evidence of their efficiency in postural physiology, let us give evidence of their necessity for every patient who has trouble standing upright quiet.

Clinical stabilometry
    Observing the quality of the postural control of a patient begins inevitably with a comparison of his performances to those of a reference population composed of healthy subjects. Is the postural control of this patient normal or not? Such is the first question that physician must be able to answer, and it is not evident...

    For lack of consensus within the International Society of Posturography about the standardisation of posturographic recordings (Kapteyn et al., 1983; 7-th international Symposium, Houston, Texas, December 2, 1983) the Association Française de Posturologie decided to settle precise standards for clinical stabilometry (A.F.P., 1984) concerning construction of platforms (Bizzo et al., 1985), recording conditions and signal analysis (A.F.P., 1985; 1986). This standardisation allows characterising postural control of a man standing quiet by means of a series of parameters whose statistical distribution is known. It is therefore possible to say, in this context of standardisation, that the postural control of a patient is normal or not. The equipment for clinical stabilometry, normalised and computerised, corresponding to the schedule of the Association Française de Posturologie, is commercially available, proposed by various French or foreign manufacturers (C.I.A. Modena; DYNATRONIC, Céreste; Q.F.P. SYSTEM, Sophia-antipolis; SATEL, Toulouse; MIDICAPTEURS, Toulouse; DUNE, Mulhouse).
    To suspect the interest of stabilometric recordings of patients who have trouble standing upright, an example is better that long speeches. Low back pain was the object of several stabilometric studies of which some are already published (Gagey, 1986; Gagey et al., 1986; Guillemot and Duplan, 1995; Liebenson C., 1996), they will exemplify, but on the condition of saying and saying again that the interest of stabilometry widely extends beyond the field of low back pain.

Stabilometric study of low back pain
    Knowing the position of the gravity line of a low back pain patient allows better understanding of the type of constraint to which his rachis is submitted and stabilometry discovers great variations from a patient to another.

X-Mean
    In the frontal plane, normal mean position of the gravity line (X-Mean) varies in extremely tightened limits; 10 millimetres to the right or to the left of the median line constitute its 95 % confidence limits. Now, curiously, only 20 % of low back pain patients (Guillemot and Duplan, 1995) present a statistically abnormal position of their gravity line in the frontal plane, more, for half of them this statistically abnormal position can be connected with an orthopaedic abnormality.

FIG. 4 ­ Stabilometric recordings in the eyes open and closed conditions. The «spot» between the feet represents the set of the successive positions of the centre of pressure of the subject, sampled during the duration of the recording, here 51,2 seconds. The analysis of this signal supplies a series of parameters for which normal distribution is known (See fig. 3).

    So the hypothesis that low back pain would be due to an abnormal tonic postural asymmetry, isolated, able to divert the gravity line (Créhange, 1950), this hypothesis is not massively supported by this statistics. However, even though there is a few, there are low back pains, which come along with such a tonic asymmetry. Their treatment has to take it into account.

Y-Mean
    In the sagittal plane on the other hand (Y-Mean) a much more important proportion of abnormalities of position of the gravity line is found and always in the same direction: 64 % of patients have a gravity line behind the backwards 95 % confidence limit of normality (Guillemot and Duplan, 1995; Guillamon et al., 1991; Gagey et al., 1986). This transfer of constraints towards articular processes of vertebrae is not a big physiopathologic discovery! It has been known for a long time that these joints may suffer during low back pain. But the 36 % of patients who do not present this backward transfer of their gravity line deserve certainly an adapted treatment.

Area
    Normal subjects stabilise their gravity line inside a cylinder of less than a square centimetre of section (AFP, 1985). But one patient out of two does not manage to stabilise his gravity line inside the 95 % confidence limits of the distribution of this area (Guillemot and Duplan, 1995). Why this difference? Why the precision of the postural control system during low back pain sometimes is disrupted sometimes is not? And is this dysfunction a cause or a consequence of the low back pain? They disappear simultaneously, but what does that mean?
    On the other hand the existence of this dysfunction of the postural control system opens new therapeutic perspectives: since this system works badly, we are incited to tickle its inputs, to manipulate them, to look for the «small» (Gagey et al., 1998c) modification which will restore a correct functioning of the control system: systematic deviation of the visual space in the plane of such or such semicircular canal, change of the topology of cutaneous plantar pressure, systematic modification of the trigeminal impulses, without forgetting the direct manipulations of the rachis and/or its proprioceptive rehabilitation (Gagey and Weber, 1999). When low back pain appears as one, or in connection with one, disorder of the postural control system, we must take advantage of that to widen our therapeutic arsenal; it works!

Normalised amplitude in the frequency band 0,2 Hz (ANØ2)
    The excursions of the gravity centre provoked by the ventilation movements are of the same order of magnitude as postural sway. Therefore logically ventilatory rhythm - around 0,2 Hz - should be seen on stabilometric recordings, spontaneously, without any trick of calculation (Bouisset and Duchêne, 1994), but in fact it does not appear (Gurfinkel et al., 1971; Gagey, 1986; Gagey & Toupet, 1997, 1998b) (fig. 5A); the impact of ventilation on postural sway is compensated by a syncinesia. But during some disorders, as low back pain for instance, the efficiency of this syncinesia is blocked, possibly by a postural tonic phenomenon (?), at least ventilatory rhythm appears massively and spontaneously on stabilometric recordings (Gagey, 1986; Gagey & Toupet, 1998a) (fig. 5B) of 46 % of the patients, according to Guillemot and Duplan (1995).
    The notion of a relationship between rachis and breathing is not new in physical medicine but now, thanks to stabilometry, therapeutist can notice that in fact this relationship is implied or not in the suffering of his patient and that is not without importance for the treatment.

FIG. 5 - Ventilatory rhythm and amplitude of postural sway. Mean and deciles 10 and 90 of the amplitude spectra from 100 normal subjects (A) and 47 low back pain patients (B). In the frequency band that corresponds to the ventilatory rhythm, the amplitude of postural sway is weakened in normal subjects, and it is not in low back pain patients. (Gagey & Toupet, 1998a).

The Romberg's quotient
    The postural control system has to integrate a considerable mass of information from all its inputs, and clinical experience shows that this sensory integration is one of the weak points of the system. Rather often one notices that sometimes normal vestibular information is not used by the postural system - something like a «vestibular omission» - (Freyss et al., 1994), or sometimes normal visual information is not integrated, the patient behaves as a blind person from the postural point of view (Marucchi and Gagey, 1987): he is no more stable eyes open than eyes closed. This integration of visual information is easy to control because it is enough recording the patient in the eyes open and eyes closed conditions and comparing results by a quotient, the Romberg's quotient (Henriksson et al., 1967; Van Parys and Njiokiktjien, 1976).

    Now during a stabilometric study of a series of 126 low back pain, presented at the tenth Symposium of the International Society for Postural and Gait Research (Munich, September 2-6 1990), Guillamon and Gentaz noticed that evolution of this Romberg's quotient and evolution of low back pain are parallel in 90 % of patients. Values of this quotient return towards normal when pain disappears and conversely. This phenomenon is not explained yet, at least we can note that it is coherent with the place of the rachis between the two big sources of postural information. Logically the mutual position of the cephalic and plantar sensors must be known by the system so that information which result from these sensors can be used jointly. This remark of coarse robotics supplies a new track for the therapeutist, and a surprising one, who would have thought of taking charge of the integration of the visual input in the postural control during low back pain? Now this integration can be manipulated (Gagey and Weber, 1999).

Conclusion
    Stabilometric studies of low back pain are reassuring: they consolidate many common ideas (suffering of posterior articular, role of the tonic postural asymmetry, intricacy of the ventilatory and postural kinetics), but these studies also show that patients do not always follow all the common ideas, and the therapeutist in view of the results of stabilometric recordings can modulate his treatments to adapt them to «this» patient. Suffering from his back is only a symptom that covers different realities and stabilometry can help us to identify them.
    As for the new proposals to manipulate the inputs of the postural system and/or their integration, their interest asks to be validated on very great series. Currently they have been used with a certain success only on some thousand of patients; enough to speak about them (Gagey and Weber, 1999), too much little so that asserting was allowed.
    On the other hand, one can - and one must - assert that upright posture is controlled by a split system which integrates sensitive and sensory information coming from multiple canals: visual, oculomotor, vestibular, proprioceptif, plantar exteroceptif. One has therefore to assert that disorders of upright posture must be observed as Vierordt did, and must be thought with concepts of systems towards which also Vierordt led us.
    Vierordt was right, Flourens swindled us.

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