## 4 Discussion
### Frontal Plane
Increased pelvic rotation, combined with greater head and trunk movements, disrupts the smooth transfer of forces required for ML stability [155; 156]. Although these adjustments may temporarily enhance CoP stability, they also compromise biomechanical alignment, increase energy cost, and hinder optimal LL orientation and forward progression during GI [92]. Non-linear results revealed additional deficits in dynamic stability. Less negative LyE values for pelvis POS indicate impaired stabilization during lateral weight transfers, while trends in pelvis MDISP point to rigid control strategies with limited adaptability. Significant MSE reductions across S1, S2, and CI for head, trunk, and pelvis POS and DISP reflect constrained adaptability and reliance on stereotyped motor responses to maintain stability during perturbations. These limitations in force distribution between the trunk and lower limbs during dynamic transitions further contribute to postural instability. Figure 15: Significant results of linear and non-linear joints analysis in transverse plane between healthy and stroke groups. LR: left rotation; RR: right rotation; ER: external rotation; IR: internal rotation; SwL: swing limb; StL: stance limb; \({}^{*}\): significant linear differences; \({}^{*}\): significant LyE differences; \({}^{*}\): significant MSE differences. In the LLs, exaggerated rotational displacements at the hip and knee, evidenced by increased TDISPL, MDISP, and AMPL, reflect compensatory strategies to counteract reduced trunk and pelvic control. These findings are consistent with [PERSON] et al. [41], who reported reliance on rotational adjustments in stroke survivors to compensate for proximal instability during GI. Non-linear measures further reveal deficits in dynamic control [36; 135], with reduced LyE convergence for hip, knee, and ankle POS and DISP, particularly in the swing hip and in both ankles. At the knee, LyE variability--converging during the swing and diverging in stance--suggests heterogeneous strategies that balance proximal stabilization and distal compensation [157]. MSE findings reinforce the following observations: significant reductions in swing hip POS and DISP at CI indicate limited adaptability during rotational weight shifts. Similarly, decreased ankle POS MSE across S1, S2, and CI highlights its limited capacity to ensure stability during stance and to adapt during swing [158]. Significant POS differences in the shoulder (swing limb) and wrist (both limbs) highlight the active role of both proximal and distal UL segments in managing postural instability. Increased MDISP and AMPL in the swing elbow and wrist reflect compensatory adjustments to redistribute the COM. However, these amplified movements also point to coordination inefficiencies, likely related to impaired trunk integration [159; 160]. Non-linear findings reinforce these deficits, with reduced LyE convergence in POS and DISP across the shoulder, elbow, and wrist and with divergent LyE values for the swing and stance--indicative of poor segmental coordination and distal control [6]. MSE reductions in swing elbow POS across S1, S2, and CI further reflect constrained adaptability, limiting synchronization with other segments and reducing responsiveness to rotational demands [161]. These results reinforce the interdependence of the head, trunk, pelvis, and limb segments, underscoring the systemic nature of post-stroke impairments. Neuromotor deficits disrupt the natural biomechanical interplay, leading to asymmetry and exaggerated compensatory movements [162]. Together, these findings underline the value of combining linear and non-linear analyses to capture the cascading impact of such strategies on stability and motor control, offering critical insights for developing targeted, effective rehabilitation approaches [6].