Specifications for a standardized stabilometric platform

Pierre-Marie GAGEY

Institut de Posturologie, Paris

[I am very sorry, but I'm too lazy to translate into English all the comments, all the discussions made about each paragraph of this founding text, but these comments, often very interesting, are published in the French version of this document >http://ada-posturologie.fr/Specifications.htm< ]

§1. The interest of stabilometry in clinical practice is recognized. More, in France the “Haute autorité de la santé” recommends that stabilometric parameters can be compared to standard values. This wished standardization is a complex process, which first implies that be specified what is a stabilometric platform for clinical practice.

§2. With the assistance of the “Laboratoire National de Métrologie et d’Essai”, a comity organized by the “Association Posture et Équilibre”, tried to formulate the specifications for such a platform, which is a compromise between clinicians wishes and current technological possibilities.

§3. As many as possible distances between two sampled successive positions of the center of pressure have to be measured, for approximately one minute, under such conditions that even for a child weighing 20 kg, maximum permissible error is one tenth of millimeter, with a resolution of five hundredth of millimeter. Evaluating the conformity of the device error with maximum permissible error must include uncertainty of this error determination.

§4 The maximum permissible error must be guaranteed for an excitation frequency of the platform at least equal to 4 Hertz.

§5. The measuring chain must be able to correctly transmit a mechanic vibration on the platform whose frequency equals the signal sampling frequency, which must be at least 40 Hz.

§6. Clinicians wish being able to observe as accurately as possible movements of the "resultant center of pressure "of the body of a subject standing upright on the measuring apparatus. It is better if centers of pressure of each foot are distinguished; in this case the relative position of the feet can be free or standardized.

§7. Clinicians agree Takagi and coll. conclusions (1985); recording time duration must be at least thirty seconds, better one minute, but no more not to tire patients and to be able to make several recordings.

§8. Some clinicians want to record children.

§9. Although the recording of the vertical forces, according to the Z axis, is not yet currently used in clinical stabilometry, some clinicians, still very few, would wish that it becomes possible, but after more fundamental researches.

§10. The accuracy (ability of a measuring instrument to give responses close to a true value [VIM, 5.18]) of a stabilometric platform depends on the uncertainty of measurement (parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand [VIM 3.9]) of its sensors (element of a measuring instrument or measuring chain that is directly affected by the measurand [VIM 4.14]).

§11. When a manufacturer will put on the market a stabilometric platform for clinical practice made in conformity with these specifications not only it will be up to him to find technological means to manufacture and develop its measuring chain so that it presents performances required by the specifications, but still he will belong to him to provide a certificate of conformity to these specifications.

§12. The manufacturer has to define the mean time of good functioning, which specifices the mean temporal interval between two successive calibrations of the measuring chain. The stabilometric platform is supposed to respect all its metrological characteristics during this period.

§13. Within this legal framework, clinicians will be able to observe accurately and safely movements of the "resultant center of pressure "of the subject standing upright and quiet on the platform. They will need only to verify absence of unpredictable breakdowns, for example by looking at the values given by the strain gauges when the platform is ‘empty’.

H.A.S. (2007) Analyse de la posture statique et/ou dynamique sur plate-forme de force (posturographie) statokinésigraphie informatisée (http://www.has-sante.fr)
VIM (2007) Vocabulaire international de métrologie. ISO-IEC-BIPM,