There is a «posture-ventilation» mystery: sometimes the ventilatory rhythm appears on posturographic recordings, sometimes it does not, and we do not know why!...
I believe that mystery could hide things of interest not only to the fundamentalists, but also to the therapists - we must pay attention to it. Unluckily, the new parameter of 0.2 Hertz normalised amplitude is but a mean to study that problem, it does not solve it.
Man breathes, even when on a stabilometry platform. And such ventilation movements may affect the control of the orthostatic posture. We know, indeed, that the range of the ventilatory movements is far more important than the range of postural sway - the latter are in the order of the millimeter, whereas the former are in the order of the centimeter. Moreover, the mass of the body parts engaged by the ventilation movements is far from negligible: the liver, the stomach and, through its intermediary, part of the digestive tract, the lungs, the heart, the ribcage and its osteo-musculo-ligamentary complex. We can put forward the hypothesis that the ventilatory rhythms may change the position of the center of gravity of the body and consequently affect postural control.
As a matter of fact, many teams of researchers, in France with Bouisset & Duchêne (1994), Vuillerme & Rougier (1997), in the USA with Hunter & Kearny (1981), in Japan with Watanabe (1976), showed that the sign of the ventilatory rhythm could always be found in posturographic recordings... But with the use of artifices!
Watanabe and Vuillerme compare the surfaces of subjects recorded in two different situations, with and without ventilation.
Hunter and Bouisset perform mathematical alchemy: a mean of all the positions of the center of pressure sampled at the same moment of the ventilatory cycle.
Without such artifices, the ventilatory rhythm remains mysterious: sometimes it appears on stabilometric drawings, and sometimes it does not.
More mysterious still, and more exciting: the ventilatory rhythm does not appear in the normal subject, it only appears in some patients.
Gurfinkel was the first to show that it does not appear in the normal subject (fig. 1).
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FIG. 1 - Ventilatory rhythm and stabilogram from a normal subject. 1: Recording of the ventilatory movements 2: Goniometric recording from the hip 3: Forward-backward stabilogram 4: Left-Right stabilogram |
 (From Gurfinkel et al., 1971) |
Bouisset's work confirms it in a particularly interesting way: on a same stabilometric signal, he shows the presence of the ventilatory rhythm thanks to his artifice, but he also shows its absence on conventional recordings and even on frequential analyses (fig. 2 & 3).
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FIG. 2 Mean of the stabilometric signal according to the ventilatory rhythm. If the mean is calculated from all the center of pressure positions that were sampled at the same period of the ventilatory rhythm, then this rhythm appears for all human beings, normal or abnormal.
(From Bouisset et Duchêne, 1994) |
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FIG. 3 Power Spectrum of the stabilometric signal from a normal subject. The ventilatory rhythm (± 0.2 Hz) does not appear on conventional analyses of stabilometric signal, even on spectral analysis. The signal used for this picture is the same as for figure 2.
(From Bouisset et Duchêne, 1994) |
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In 1986, a statistical study of the spectral analysis from 100 normal subjects, allowed us to show the absence of any frequency peak in the band of the ventilatory rhythm, 0,2 Hz (fig. 4).
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FIG. 4 - Means and 90th decils of the amplitude of postural sway according to their frequencies. Statistical analysis from 100 normal subjects.
(From Gagey, 1986) |
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But this rhythm appears on the recordings of some patients. Gurfinkel was the first to show that, as far as we know (fig. 5).
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FIG. 5 - Stabilogram and recording of the ventilatory rhythm from a patient.
(From Gurfinkel et al., 1971) |
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In 1986, a statistical study of the amplitude spectrum of 32 lumbalgic patients allowed us to show that this population was distinguishable from the normal population by a superb peak in the 0.2 Hz frequency band (fig. 6), just the ventilatory rhythm frequency.
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FIG. 6 - Means and 90th decils of the amplitude of postural sway according to their frequencies. Statistical analysis from 32 patients suffering from low back pain.
(From Gagey, 1986) |
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That fundamental peak at 0.2
Hz was found again by Guillemot and Duplan in the group of lumbalgic
subjects they have studied (1995). We also found it in hundreds
of recordings of patients examined by Toupet.
So it now seems an ascertained fact: the ventilatory rhythm does not appear on the stabilometric recordings of normal subjects, but it does appear on the recordings of some patients.
To express that phenomenon, Gurfinkel put forward the hypothesis that a «synergy» reduced the importance of postural sway relatively to ventilation in the normal subject, and that such synergy could be broken by some illnesses. That is exactly what Kohen-Raz observes: the range of postural sway in the 0.2 Hz frequency band increases in some patients (fig. 7 & 8).
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FIG. 7 - Amplitude of postural sway according to their frequencies. Bar graph from 21 normal subjects.
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FIG. 8 - Amplitude of postural sway according to their frequencies. Bar graph from 9 patients suffering from diabetes mellitus .
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Is that «synergy» a coxo-femoral one, as Gurfinkel said, or much more complex, «respi-static», as Tardy (1994, 1997) proposes?
The problem still has to be studied and the parameter of 0.2 Hz normalised amplitude does not solve it or explain anything. It is simply proposed in order to answer a precise question of the clinician: in that specific patient I am examining, is the amplitude of postural sway in the ventilatory frequency band normal or not? (fig. 9)
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FIG. 9 - Distribution of the NAØ2 parameter Gaussian curve of the normal theoretical distribution of the NAØ2 parameter (Left-Right postural sway, eyes open; m = 11,39% ± 6,95; Confidence limit at 95%: 25%). Bar graph of the distribution of the same parameter in a population of 483 patients with abnormal functional vestibular tests.
(From Gagey & Toupet, 1997) |
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That parameter is a tool, not an explanation. It sharpens our observation power (fig. 10).
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FIG. 10 - Follow-up of low back pain population through ANØ2 parameter. Means and S-D of the distribution of this parameter (ordinate) according to the time of the follow-up (abscissa). Means comparisons by Student's t-test with the normal distribution.(*: p<0,05; **: p<0,01; ***: p<0,001; m: mean; s: S-D; LC: confidence limit at 95%). Vertical arrows: time of osteopathic thrust. (From Scheibel, personal communication ) |
To find the clue to the mystery
of the posture-ventilation relations, rather than parameter NAØ2,
I would trust the therapists' hands, who have to palpate their
patients' bodies and be guided by their reactions.
As for NAØ2 parameter (ANØ2 in French), it must be just a servant in that research (see Amplitude of postural sway in the 0.2 Hertz frequency band ).
Bibliography