Is the total vertical ground reaction force time-amplitude profile an invariant during sit-to-stand movements following stroke?

Abstract

Weight-bearing asymmetry is a common characteristic of post-stroke individuals when executing sit-to-stand (STS) movements. Spontaneously, these individuals put less weight on the affected limb during STS. In healthy subjects, it has been shown that events of the total vertical ground reaction force (VGRF) occurred invariably in sequential order for every individual for different STS conditions. This study aimed to determine whether the total VGRF time-amplitude profile remains similar between healthy and post-stroke individuals executing STS and to examine the bilateral coordination of VGRF in both groups. Eleven individuals with sub-acute stroke (mean (±SD) time post-stroke: 80.4 (±46) days) and nine age-matched healthy volunteers participated in the study. Both groups executed three STS movements from a standard chair, without back support, height adjusted to obtain 90° of knee flexion. They were instructed to stand up at natural speed with their arms raised straight ahead and hands folded. A force plate positioned directly beneath each participant's bare feet allowed recording the VGRF at 50-Hz while kinematics data at the trunk and lower limbs were recorded with a VICON system. The hip angular velocity signal was used to determine STS duration. Non-parametric statistics and intercorrelation functions were used to compare total VGRF characteristics between the two groups and coherence of the VGRF between sides. The stroke group took more time to execute STS (3273 vs. 2467 ms) and their weight distribution was asymmetrical (67.6% vs. 32.4%) compared to healthy group (50.1% vs. 49.9%). The maximal values of total VGRF did not differ between groups (stroke: 123.8% of total body weight (BW) vs. 120.2% BW for healthy group, P = 0.79) as their occurrence (at 35% vs. 36% of STS duration; P = 0.52). The intercorrelation analyses revealed that VGRF were still coordinated in stroke subjects (Correlation coefficient (CC) = 0.83) but lesser than in healthy subjects (CC = 0.94, P = 0.007) with corresponding temporal delays of 100 ms and 78 ms, respectively. As healthy subjects, individuals post-stroke show invariant in STS task realization. Mainly, the maximal total VGRF seems to be an important control parameter regardless of the weight-bearing asymmetry.