Following EMG studies of man's reactions to exogenous destabilisations, Nashner and Mc Collum (1985) proposed to describe the organisation of postural movements around the ankle or around the hip. The «ankle strategy» hypothesised that the body of man standing at rest nearly behaves like an inverted pendulum, an hypothesis validated by Winter and coll. works (1997).
But is it sure that this inverted pendulum really rotates around the ankle axes when a man standing at rest stabilises his posture, threatened only by his endogenous movements and not by outside forces?
Stabilometry, which records the movements of the centre of pressure in these almost static conditions, rather leads us to think that the inverted pendulum rotates around the centre of pressure, like a reversed broom kept in balance on a finger tip. The stabilometric signal, indeed, consists of fast movements of the centre of pressure, as unpredictable as the disturbances to which they oppose (Gagey and al., 1985; Collins and De Luca, 1993) and of slow movements, similar to the centre of gravity movements (Gurfinkel, 1973) and as controlled as them (Gagey and al., 1985; Collins and De Luca, 1993). And comparisons of these two types of movements (fig. 1) show that the centre of pressure moves constantly on both sides of the projection of the centre of gravity (Caron et al., 1997; Winter and al., 1998; Hugon, 1999), apparently as along the tactics of the broom (Gurfinkel et al., 1992).
FIG. 1 - Centre of Gravity and centre of pressure
Simultaneous recordings, for 51,2 secondes, of the centre of pressure by a forces platform (yellow line) and of the centre of gravity by an optical means (black line), sampling at 25 Hertz.
[Recordings by Maurice OUAKNINE]
Therefore it is in no way sure that the inverted pendulum rotates around the axes of the ankles when a man standing at rest stabilises his posture.
Material and Methods
To try and enlighten the debate, this study adopted the hypothesis that a limitation of mobility of the posterior articular complex of the foot limited the mobility of the inverted pendulum.
17 «normal» subjects have been selected because, clinically, they presented a limitation of mobility around their Henke's axis (Henke, 1863).
The test of the pelvipedal quadrilateral
The repercussion of this local limitation of mobility on the body kinematics was verified by the test of the pelvipedal quadrilateral (Villeneuve, 1995). A passive translation of the pelvis in a strictly frontal plane entails a deformation of the pelvipedal quadrangle which can only occur around the joints endowed with a degree of freedom in this plane and/or at the interface between the plantar sole and the ground (fig. 2)
FIG. 2 - Test of the pelvipedal quadrangle.
Henke's axis is not perpendicular to the direction of the passive translational movement (fig. 3), consequently if the deformation takes place around Henke's axis, then inevitably appears a component of rotation in horizontal plane.
FIG. 3 - Henke's axis of subastragalar rotations.
Performances of the postural control have been measured by stabilometric recordings made according to the standards of the Association Française de Posturologie (AFP, 1985), in the four random combinations of visual and plantar situations: opened or closed eyes, hard ground or foam.
These same clinical and stabilometric exams have been repeated, before, then three minutes after, then three weeks after a liberating manoeuvre that restored a normal subastragalar mobility.
The statistical analysis compared the results of clinical (c2 test) and stabilometric (paired Student's t-test) exams before and three minutes after the treatment, before and three weeks after the treatment.
While the treatment modifies in a statistically significant way the kinematics of the pelvipedal quadrilateral (Tab. I), this modification of local and regional mobility does not come along with any statistically significant change of the performances of the postural control.
TAB. I - Comparison of the frequency of
abnormal responses during the test of the pelvipedal quadrilateral
before et 3 minutes after the treatment.
c2: 7,56 ; p<0,01 ; N=17.
Everything happens as if a locking at the level of Henke's axis does not lock postural sway of the inverted pendulum. This independence of postural sway towards the degrees of freedom of the posterior articular complex of the foot is amazing if one accepts the classic model of the inverted pendulum articulated around ankle axes. On the other hand if one admits that the inverted pendulum is articulated around the centre of pressure, the independence of postural sway towards the degrees of freedom of the posterior articular complex of the foot is not surprising any longer. Moreover this model of «the tactics of the broom» is coherent with stabilometric data reminded in the introduction.
The results of these experiments do not bring formal proof, but at least they strengthen doubts on the validity of the model of the «ankle strategy» within the framework of the postural control of man standing upright at rest. Nashner proposed the model of the ankle strategy within the framework of struggles against destabilisation, studied by means of electromyography, but this author has never ever claimed that his model was adapted to the analysis of upright posture at rest.
The contamination of stabilometric analysis by concepts coming from studies of dynamic posturography can easily be explained by similitudes. Nashner described two levels of struggles against destabilisation: around the hip and around the ankle. Now there are also two manners of stabilising an inverted pendulum: mobilising the centre of gravity or mobilising the centre of pressure. But we must pay attention for these similitudes may become traps for our reflection.
The distinction between the «ankle strategy» and «the tactics of the broom» deserves extra studies because, neither phasic contractions of the intrinsic muscles of the feet, nor stiffness of the muscles of the legs, have the same role in both cases.