## 4 Discussion
### Relationship between the CoP Motion during the Platform Perturbation Cycles and the Post-Effect  
When the supporting platform translates rapidly toward one side, say to the left, the inertia of the body produces an immediate displacement of the CoP relative to the force platform toward the right side. During the following slow return half-cycle of the platform translation to the right, i.e., to its initial position, the body tends to correct the rightward tilt and recover the original symmetric orientation. After the first few cycles, the CoP moves further right during the fast half cycle. During the slow half-cycle it shows an overshoot to the left that becomes more evident towards the end of the stimulation sequence. This configures an adaptation behaviour whereby the mean CoP displacements during the slow half-cycles tends to remain between the feet. At the end of the stimulation, across the participants, the average amplitude of the CoP shift bears some association with the amplitude of the shift of the CoP position from the fast to the slow phase of the perturbation sequence. The larger the mean value of the CoP during the slow return phase, i.e., the difference between its mean position during the fast and the slow half-cycle, the larger the amplitude of the post-stimulation displacement in the direction of the fast translation. Therefore, the strength of the post-effect seems to be related to the unwitting effort of complying with the continuous medio-lateral translation of the support base, particularly with the recovery of the symmetric posture of the body during the slow return phase of the translating platform. This effort can be viewed as proportionate to the vertical distance (arrows in Figure 4) between the traces of the CoP mean position during the fast and during the slow half-cycle. As a consequence, when the perturbation cycles stop, the post-effect would be driven by the 'tail' of this motion (in search of the new vertical reference built up as to compensate the displacement produced by the fast translation). This process shifts the CoP position on the side of the fast cycles. This new position would be the most appropriate for counteracting a fast displacement of the support base by minimizing the destabilizing effect of the body inertia. This displacement toward the side of the fast platform movement understandably occurs in anticipation of the impending fast phase, in order to more easily counteract the perturbation delivered by the next fast cycle. The behaviour would therefore configure a sequence of compensatory balance correcting reactions to the fast translation phase [53; 54; 55] and of anticipatory postural adjustments (APA) during the slow translation phases [56; 57]. The balance-correcting reaction would be appropriate for halting the ML body tilt induced by the fast platform displacement, a braking action possibly produced by the reflex contraction of the abductor muscles of the side opposite to the fast body displacement; whereas the APA would attenuate the effects of the subsequent ML fast perturbation. Over time, this sequence would be constructed and adjusted by the continuous asymmetric proprioceptive input from the muscles stretched by the fast cycle of the platform translation, and by the effort to return to the initial position during the slow phase and prepare the postural adjustment in anticipation of the following fast translation. In this light, individuals would implicitly learn to anticipate any reflex response by enhancing the anticipatory adjustment (and the post-effect would attest to this phenomenon). At the end of the stimulation, the body would be kept in its new standing position (say to the left) by a tonic activity of the abductor muscles of the right side. This adaptation phenomenon would be produced by a sensory reweighting mechanism, aimed to reduce the postural destabilization [58]. Contrary to many comparable adaptation behaviours described in the literature, the time-course of this phenomenon is longer (see [59]).