Upright standing after stroke: How loading-unloading mechanism participates to the postural stabilization

Postural strategies employed by hemiparetic stroke patients need to be better understood to guide rehabilitation. Of the two complementary mechanisms used to stabilize the standing posture, loading-unloading (LU) and pressure distribution (PD), it is hypothesized that the former would be predominantly used. To this aim, posturographic assessments, through a dual force-platform, were performed in 30 Hemiparetics tested 3 months after a unilateral stroke, and 30 matched healthy Controls. Original indices (from 0 to 1) were calculated to assess LU and PD contributions. The results show that along the mediolateral axis, the LU contribution was very high and similar in Hemiparetics and in Controls (0.80 ± 0.07 vs 0.76 ± 0.09 a.u; p > 0.05), indicating a predominant hip involvement. Along the anteroposterior axis, the PD contribution was very close to 1 in controls (0.96 ± 0.03 a.u.) indicating an exclusive ankle involvement. Despite a lower contribution in Hemiparetics (0.88 ± 0.11 a.u.; p < 0.01), the indices were surprisingly always above 0.5, meaning that ankle movements remain predominant for controlling postural sways along the anteroposterior axis in all patients even those with severe clinical deficits. However the PD contribution appeared larger in patients with light or moderate deficits of the sensitivity (r = -0.532; p < 0.01) or the motor command (r = -0.513; p < 0.01). These results indicate that postural stabilization of hemiparetic persons remains controlled by a PD mechanism along the anteroposterior axis, even in those combining poor distal motor command and deep sensory loss. This ankle control, piloted by the more-loaded non-paretic limb, would therefore be preferred to a hip control through lateral trunk motion. This should be considered when defining the objectives of the postural rehabilitation after stroke.