Afterthought over twenty years of clinical stabilometry
Institute of Posturology, Paris
After Houston congress 1983
During the congress of the International Society for Postural and Gait Research (ISPGR) held in Houston, Texas, in 1983, the Committee of Standardization of the ISPGR gave up continuing its work. Since its creation in Amsterdam in 1969, this society had become less and less "clinical" and more and more a society of basic researchers. However the latter have nothing to do with standardization that attaches their research... But for us, clinicians, this decision was a great problem. It is obvious that our needs are not those of basic researchers. We cannot record our patients on a stabilometric platform before they fall sick, they do not come to see us at this time! We cannot thus compare the results of their recordings as patient with their recordings as healthy subject. We miss comparative data, we need to know the distribution of stabilometric parameters within a normal population. The clinicians need stabilometric "Standards" of the parameters (AFP, 1984). And, obviously, stabilometric data must be collected under identical conditions same recording equipment, same protocol, same environment so that they can be compared, so that they can be the subject of statistics. Since the international society had given up doing this work, lAssociation Française de Posturologie (AFP) decided to do it, in its own field of competence, but letting it known by the international community (Bizzo et al.. 1985; AFP, 1985).
Twenty years latter, which assessment?
The stabilometry was and is a real success. To speak only about France, five companies build and distribute stabilometric platforms, manufactured according to the standards of AFP. We do not have any census of the platforms installed in hospitals and the therapeutists cabinets, we can have only a coarse evaluation of their number, that would be about several thousands. In front of this success, it is possible to wonder: what for all that work?
Stabilometry is not a diagnostic tool
A retrospective study on files of the patients seen at the Institute of Posturology of Paris during its first ten years showed that one of two postural patients presented normal stabilometric recordings (Gagey et al, 2002). It is thus impossible to trust stabilometry when looking for postural disorders, or trying to confirm this diagnosis which remains based only on clinical data: illness history, clinical examination, follow up (Gagey et al, 2005).
Two assumptions make it possible, perhaps, to explain this incapacity of stabilometry to detect all the postural disorders.
On the one hand, stabilometry studies primarily the phasic muscle activity which continuously brings back the centre of gravity of a subject to the vicinity of its mean position, but stabilometry has practically no way for observing and measuring how the postural tonic activity is regulated, possibly it can perceive only a global result, summons of all elementary asymmetries, as an abnormal X-mean position.
In addition, the statistical standards of the stabilometric parameters define not the NORMAL SUBJECT, but only the distribution of these parameters in a normal' population of reference. Many patients of any kind, cardiac, respiratory, urinary, neurotics, etc. present normal stabilometric parameters, but some of these patients react abnormally to disordered postural states. It is the case, for example, of the patients who suffer from obsessional and compulsive disorders, their mental rigidity is expressed even in their postural behavior, whatever arrives, they are held upright rigid, almost completely motionless, it is to say that they express an impeccable precision of their postural control (Floirat and Al, 2005).
Stabilometry as a communication instrument
If stabilometry has no interest for diagnosis, on the other hand it appears a powerful instrument of communication. It allows a common language between all posturologists, a rigorous language founded on biomechanics, which, by its power, imposes itself to the whole medical community, well beyond the limits of posturology.
Communication between posturologists
The fact that stabilometric parameters are standardized implies that they have the same significance for all posturologists. When they speak about them, they speak exactly about the same thing; Area, Length, X-mean, Speed and its Variance, Fast Fourier Transform, correlation fonctions, all these concepts, within the framework of standardized stabilometry, do not leave any place to any interpretation on the nature of the parameter.
These concepts, indeed, are not stupid parameters which only say: "Right" or "Bad", these concepts are true logical operators, which, each one with its manner, allow to apprehend one or several aspect of the postural control of the patient, logically. These various operators can combine in the posturologist mind who plays with them trying to better understand what arrives to his patient. A traditional example of this intellectual play is the parameter LFA which compares length and area to notice that they can or not evolve simultaneously, what has not at all the same significance in one and the other case. This intellectual play of the posturologist is directly communicable because, at the base, the parameters are not interpreted, there is only one level of interpretation, the one where posturologist plays with these parameters and allows himself bringing them closer, combining, comparing them for a better intelligence of the problem.
Communication with the medical community
Any clinical case is always a particular case, comparing it with other clinical cases, which present some analogies with it, always means disregarding factors which count in the history of this particular case; the clinician does not like this work, which disfigures a little each individual patient so that it can cohabit under the same heading with other individuals. But what an aubaine when the patients are apprehended by some physical measurement, as area or X-mean, which lends itself to all fancies of statistics! Certainly one loses a mass of information, but one can compare patients because what is used to compare them is one of their common properties which is indisputably identical. Already stabilometry largely made it possible to draw statistics about postural patients what allowed to submit them to the medical community in an admissible form.
Communication through a language which imposes itself by its rigour
Traditionnally in Medicine, as long as a subject does not fall, he is said in equilibrium. It does not matter that he swings like a drunkard or that it holds on a pin point, like an obsessional... He is always in equilibrium. The medical traditional language thus grants a generous semantic field to the word of equilibrium, which covers with extremely different realities, so different that we do no longer know very well what the word equilibrium' means...
Stabilometry opened our eyes on this limit of the traditional medical language, showing us that man is never in equilibrium if the rigorous physical definition of this term is used: two aligned, equal and of contrary direction forces. When the man is upright quiet, all the parts of his body mass are subjected to gravity from which results this force which applies him on the ground by his feet. The ground resists this push of the various areas of the feet, from where it results a force which is opposed to the penetration of the body in the ground. But the man is quite unable to constantly maintain aligned these two resultants equal and of contrary direction. He is never in equilibrium according to the physical and rigorous meaning of the term, he "stabilizes himself", i.e. that he continuously brings back his vertical of gravity in the vicinity of his position of balance without never managing to keep it there. This rigorous language of "stability" imposes itself now to the medical community and allows it to give more precise contents to "instabilities".
This communication function of stabilometry, that I have been feeling since 1985, and that is now confirmed by twenty years of experiment, validates the intuition of Alain ZARKA: it is time to take again, at an international level, dialogues on the standardization for CLINICAL stabilometry.
A.F.P. (1984) Standards for building a vertical force platform for clinical stabilometry: an immediate need. Agressologie, 25, 9: 1001-1002.
A.F.P. (1985) Normes 85. Editées par l'ADAP (Association pour le Développement et lApplication de la posturologie) 20, rue du rendez-vous 75012 Paris.
Bizzo G., Guillet N., Patat A., Gagey P.M. (1985) Specifications for building a vertical force platform designed for clinical stabilometry. Med. Biol. Eng. Comput., 23: 474-476.
Floirat N., Bares F., Ferrey G., Gaudet E., Kemoun G., Carette P., Gagey P.M. (2005) Aporia of stabilometric standards. Gait & Posture, 21, Supp. 1, 52.
Gagey P.M., Weber B., Scheibel A., Bonnier L. (2002) Le syndrome de déficience posturale : analyse rétrospective dobservations cliniques. In M. Lacour (Ed.) Posture et équilibre. Contrôle postural, pathologies et traitements, innovations et rééducation. Solal, Marseille, 73-80.
Gagey P.M., Weber B., Scheibel A., Bonnier L. (2005) A distinct clinical syndrome defining the postural patient. Gait & Posture. 21, Sup.1, S121.