The human body is asymmetric. It is probable that these asymmetries can play a role in the genesis of some pathologies related to orthostatism - but in order to study that hypothesis more thoroughly, it is first necessary to define better the observed asymmetries. To begin with, it is wise, to say the least, to distinguish the asymmetries of the orthostatic posture from the lateralities.

     The analysis of the strains linked to the control of the orthostatic posture lays the bases for a definition of a strictly postural asymmetry, called «pillar foot motor foot».

     The human body is asymmetric. Everybody agrees on this observation even if everybody does not express it similarly. Some, like Bricot (3), put an emphasis on biomechanics and on the strains that such asymmetries put on the joints, and say that the «normal» man is symmetric while acknowledging that 95% of the observed subjects are asymmetric. Others put an emphasis on the neurophysiological realities and/or the statistical approaches and had rather say that asymmetry is the norm...

FIG. 1 - The asymmetries of the human body as seen by sculptor Subirachs (Courtesy of Professor Viladot)

     But these are merely language differences, all the less important as a consensus is appearing on an old notion which is more interesting for the therapist: the asymmetries of the human body can be pathogenic. Alajouanine already said it, and many did after him. In posturology, Boquet and Bricot do insist on that notion and indeed we note a systematic reduction of our patients' symptoms when we manage to lessen the importance of their abnormal - that is to say exaggerated - asymmetries of the orthostatic posture (7).

     But the consensus does not go further. Whenever one studies how these asymmetries of the human body are characterised - by a name or by some criterion - one discovers a real cacophony. Alajouanine referred himself to the patient's standing at ease situation. Créhange (4) - one of his students - used his «orthostatigrams», obtained by the superposition of the plantar impressions: the shield-like area between the isthmuses points, backwards, on the side of «the member that is more strained».

FIG. 2 - Créhange's orthostatigram.

     Da Cunha determined the «supporting foot» by the direction of the look, in the opposite way. Baron spoke of the «pivot foot» around which the patient turns when he is performing the stepping test. Sportsmen make a distinction between the «take-off foot», for impulsions, and the «leading foot», for technical movements (8). And Azémar and Ripoll (2), followed by Jaïs (14), suggest eight combinations of functional asymmetry of the foot depending on whether its laterality is associated to one or the other laterality of the hand and the eye.

     That brief, certainly not exhaustive, review clearly shows that we are far from agreeing on how to characterise the asymmetries of the human body. Yet if we want to push forward the hypothesis of the pathogenic role of the asymmetries, it would first be necessary to define them clearly.

     To begin with, it seems wise to us to distinguish the functional asymmetries related to laterality and the asymmetries of the orthostatic posture. That distinction has probably been already suggested, in another way, by authors like André Thomas, Bergès and Stambak - they spoke of a «structural neurologic laterality» as opposed to the functional instrumental lateralities (1, 9). But whatever the accuracy of that historical observation, we know for sure that the statistical studies of the lateralities and asymmetries of the orthostatic posture lead to such different distributions that we cannot possibly make them coincide. In two studies bearing on 655 adults for one and 134 five years old children for the other, Azémar finds a distribution of 94% of right-handed against 6% of left-handed. Whereas our study of the asymmetries of the orthostatic posture in a population of 103 workers describes 60% of «right» subjects against 40% of «left» subjects (6).

     That difference of distribution between those two kinds of asymmetry is not surprising when we understand that the one and the others observe very independent phenomena. Indeed, laterality can be defined as a functional asymmetry appearing in the body segments, in a prevailing way, for motor conducts. And those motor conducts are of various kinds depending on whether it is a matter of gnosic activity or exploratory activity, of activities that imply complex random situations and adaptative reactions involving decisive choices in the treatment of information, or more simply conducts of intersegmentary coordination in the handling of tools, of spatio-temporal organisation of motricity. The voluntary motor action is at the heart of the observation of the lateralities whereas it is absent from the study of the asymmetries of the orthostatic posture. In that last case we simply note either morphological details on the subject at rest, or orientations of the automatic motricity related to postural control.

     It is not impossible that these observations, very different per se, can coincide at some levels. Indeed, a study by Fecteau (5) allows us to establish a contingency table that reveals a very strong link between manual laterality and lateral inclination of the shoulders (table 1), already pointed out by Bricot (3).

TABLE 1 - Distribution of the inclination of the shoulders depending on manual lateralities. Chi2 = 26.35, p<0.001 (from Fecteau [5]).

     But that link between postural asymmetries and laterality completely disappears at other levels. The same study by Fecteau allows us to establish a table of random contingency between manual laterality and varus tendency of the foot (table 2).

TABLE 2 - Distribution of the varus tendency of the foot depending on manual lateralities. Chi2 = 3.97, non significant (from Fecteau, [5]).

     If the debate on the relations between laterality and asymmetries of the orthostatic posture is still open, it seems cautious to us - at least for the time being - not to mix those two approaches of the asymmetries of the human body. As for us, we will remain faithful to the strictly postural point of view.

The centre of pressure

     Now, during the orthostatic posture, one of the functions devolved to the inferior members in general and more particularly to the feet, is to take part in the variations of the position of the centre of pressure so that the centre of gravity stays near its mean position of equilibrium. And the analysis of the mechanical strains of that function is easy if we use the model of the inverted pendulum. Moreover, it tells us a lot about the postural asymmetry of the inferior members.

The model of the inverted pendulum

     To assimilate the body of man standing up to an inverted pendulum pivoting around the axes of its tibia-tarsal and sub-astragalian joints was considered, during a long time and by many authors, as a hardly acceptable simplification.

FIG. 3 - The mechanical model of the inverted pendulum. Explanations in the text

     But recently Winter and his colleagues showed that this model is in fact very close to reality (13). Experimentally, in man in orthostatism, Winter finds a coefficient of correlation of around 0.90 between the accelerations of the centre of pressure and the distance COP-COM, which validates the model of the inverted pendulum in the study of the orthostatic posture.

     On a mechanical point of view, when the pendulum is in a position of equilibrium (fig. 3, A), the centre of gravity (COM) and the centre of pressure (COP) are aligned on the vertical of the place, the difference COP-COM is nil. When the pendulum deviates from its position of equilibrium (fig. 3, B), the splitting up of the forces reveals an horizontal force that creates a torque around the rotation axis of the pendulum and that torque tends to deviate more and more the centre of gravity from its position of equilibrium. The distance COP-COM is different from zero. To restore equilibrium (fig. 3, C), the centre of pressure has to move beyond the vertical of the centre of gravity, so that the splitting up of the forces reveals an horizontal force that creates a torque around the rotation axis of the pendulum, and that torque has to tend to bring the centre of gravity back to near its position of equilibrium.

FIG. 4 - Movements of the centre of gravity and of the centre of pressure of an inverted pendulum. Luminous points have been set near the centres of gravity and of pressure (right part of the picture) before taking a picture of their respective movements in the dark (left part).

     The equation of the inverted pendulum as chosen by Winter (12):

indicates that the horizontal distance between the centre of pressure and the projection of the centre of gravity is proportional to the acceleration of the displacements of the centre of pressure. In other, more common, words, the quicker the centre of pressure is displaced at the least position change of the centre of gravity, the better the pendulum is stabilised.

Energy saving

     Equation (1) of the inverted pendulum has another interest - it calls our attention on the fact that, if the movements of the centre of pressure have to be quick for the centre of gravity to be correctly stabilised, they have to mobilise a body mass as small as possible to limit the expenditure of energy. Indeed, given that

     If the acceleration g has to be important, the reduction of the force F necessary to the postural control can be obtained only by a reduction of the mass M to which that acceleration is applied.
     It is very probable that the organism uses strategies of mobilisation of the centre of pressure that apply this general principle of energy saving.

The strategy of Henke's axis

     When the subject is standing, in monopodal station, the only possibility he has to mobilise his centre of pressure in the frontal plane consists in making his calcaneum roll under the ankle bone. That strategy complies with the law of energy saving, as the body mass mobilised by these movements is reduced to the mass of the back of the foot.

FIG. 5 - The strategy of Henke's axis. In monopodal station, to move the centre of pressure in the frontal plane, the calcaneum rolls under the ankle bone according to Henke's axis (10).

The strategy of the lever of the plantar arch

     To mobilise the centre of pressure in the sagittal plane, in mono- or bi-podal situations, it is enough to modulate the forces applied on the two extremities of the plantar arch (fig. 6) by adapting the tension of the posterior muscles of the legs - indeed, the vertical of gravity always falls in front of the tibio-tarsals axis, straight above the posterior side of the styloid apophysus of the fifth metatarsal as an average. That strategy, too, obeys the law of energy saving as the corporal mass mobilised by those movements is reduced to part of the mass of the foot.

FIG. 6 - The strategy of the lever of the plantar arch. The position of the point of application of the resultant of the forces acting on the posterior (Fp) and anterior (Fa) parts of the plantar arch varies according to the combination of those forces.

FIG. 7 - The impossible strategy of the pelvis.

     When the subject is standing on both his feet, it is theoretically possible for him to change the position of his centre of pressure in the frontal plane by using a torque acting around the axis of a coxo-femoral (fig. 7). But that strategy would be contrary to the law of energy saving, as the mass on which the acceleration of the movement would apply would then be very important.
     It is probable that the organism uses another, more economical, solution taking place at the level of the foot.
On a plane of both feet (fig. 8), we can notice that the mobilisation of the centre of pressure along the sagittal axis of the foot - under the effect of the strategy of the lever of the plantar arch - has, indeed, an important sagittal component, but also a component in the frontal plane because the two feet are never parallel to the sagittal axis of the body.

FIG. 8 - The strategy of «pillar foot - motor foot». When the centre of pressure moves back on the axis of the right foot from CP to CP', this movement presents a sagittal and frontal component because the axis of the feet is never parallel to the sagittal axis of the body. In order for the frontal component of the movement of the centre of pressure on the left foot to go the same way, the centre of pressure has to move forward on the axis of the left foot from CP to CP'.

     By using that frontal component of the strategy of the lever of the plantar arch, it is obvious that the organism obeys the law of energy saving. But that strategy necessarily reveals an asymmetry in the role of the feet, in order for the frontal component to vary in the same way at the level of each arch. If the centre of pressure withdraws from its position CP to its new position CP' on the right foot, for instance (fig. 8), it has to go forward on the left foot from CP to CP'.
     The results obtained by Kohen-Raz (11) with the tetra-ataxiameter are coherent with that model of strategy - they show a mobilisation of the pressures along a diagonal axis going from an anterior extremity of the arch on one side to the posterior extremity of the arch on the other side.
     As the forces applied at the level of the heel have a more passive connotation than the forces applied at the level of the anterior foot, we suggest to call that strategy of displacement of the centre of pressure in the frontal plane the «pillar foot - motor foot strategy». But whatever the name given to that strategy, it definitely reveals a well defined asymmetry of the postural role of the feet, that first deserves to be confirmed by experimental works before being studied in its relations to pathology.
     The separate recording of the centre of pressure of each foot (fig. 9) should allow to establish a database of those postural asymmetries and to realise some statistical studies that we currently need.

FIG. 9 - Separate recording of the movements of the centre of pressure under the right foot and under the left foot, with reconstruction of the movement of the centre of pressure of the whole body. We clearly observe an asymmetry, correctly expressed by speaking of «pillar» foot on the right and of «motor» foot on the left. Note the position of the vertical of gravity on the side of the «pillar» foot. (Recordings communicated by Maurice OUAKNINE)

     The study of the biomechanical strains inherent to the mobilisation of the centre of pressure reveals the logical necessity of a functional asymmetry of the feet concerning the control of the orthostatic posture. If the reality of that asymmetry is confirmed by experimental works, it could be used as a serious base for a study of the pathogenic role of postural asymmetries.

References

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   Remerciements aux docteurs Bricot et Marignan et à messieurs Faugouin, Fecteau, Helbert et Villeneuve pour leur aide au cours de l'écriture de cet article.