## 4 Discussion  
### The CoP Displacement after the Sequence of Asymmetric Perturbations  
The body weight shifting capacity during the stance can be restored in patients by different rehabilitation procedures, not directly focusing on the operation of proprioceptive processes [43; 44]. Here, we tested a novel, simple and unpretentious protocol in healthy subjects; whereby asymmetric balance perturbations were administered with the purpose of inducing a post-effect consisting of a temporary shift in the position of the CoP during quiet stance. In keeping with our initial hypothesis, we found that the administration of a prolonged sequence of periodic _asymmetric_ translations of a supporting platform in the horizontal plane changes the post-stimulation CoP position in the frontal plane of the participants standing on it. This pre-/post- CoP displacement has a small but significant amplitude and is consistent in direction across subjects. It takes place toward the direction of the fast half-cycle of the platform translations. The effect is unrecognized by the participants, and is immediate, being obvious at the very end of the sequence of perturbation cycles. The duration of the effect is variable within a limited range across participants and on average amounts to a couple of minutes, before the CoP recovers its initial pre-stimulation position. Before the beginning of the sequence of translations, the mean CoP rested midway between the feet in all the participants. Its displacement after the continuous asymmetric translations amounted to about 4% of the distance between the heels (set equal to the distance between the anterior superior iliac spines) and corresponded to about 7% with EC and about 5% with the EO of the maximum functional CoP displacement produced by voluntarily leaning to the right or to the left. The post-stimulation mean displacement entails a non-negligible difference in the distribution of weight on each foot (about 5%, see [45; 46]). Although less than 1 cm may seem negligible, one may note that such a distance during the random sway of the CoP is an uncommon occurrence in healthy subjects [1; 47], even when standing in tandem condition [9]. Here, the mean SD of the ML CoP sway during the pre-stimulation period was about 0.2 cm and displacements of 0.5 cm from the mean were never observed. Moreover, we noted that the symmetric platform translations did not produce any consistent or significant post-effects on the average position of the CoP. These findings allow to trace the post-stimulation displacement to the asymmetric profile of the perturbation sequence. In spite of a considerable variability across subjects, there was a significant relationship between the amplitude of the displacement reached during the fast and the slow phases of the translations and the amplitude of the post-stimulation displacement. Furthermore, the ML CoP shift was not accompanied by a major increase in the ML sway around the mean position, and there was no relationship between the pre-/post-stimulation difference in the SD of the ML sway and the ML displacement of the mean CoP position. Hence, the post-stimulation displacement would not be a non-specific effect of an excessive body sway. The duration of the post-effect was relatively short. It was just longer than a couple of minutes on the average. However, it is unexpected that the CoP can drift about 0.5-1 cm out of its normal standing position for this stretch of time. During clinical tests it is customary to record the CoP position during a quiet stance for 1 min or 30 s or even less (see [48; 49]), assuming that no major drift occurs in this time-interval. In some cases, the effect vanished slowly, whilst in other cases it stopped in coincidence with a rapid redistribution of the body weight, resetting the postural attitude as often occurs during quiet stance [50; 51]. The duration of the present post-effect is similar to that observed by [PERSON] et al. [52] in subjects who stood on an inclined surface for 3 min.