Validity and variability of center of pressure measures to quantify trunk control in stroke patients during quiet sitting and reaching tasks

Background: In the immediate period following stroke, sitting balance is one of the most important predictors of functional recovery at discharge after rehabilitation. Thus, sitting balance determines the content of the early phase of stroke rehabilitation and an appropriate measurement tool is important. Research Question: The aim of this study is to investigate the concurrent validity of center of pressure (CoP) excursions of patients seated on a force plate, as well as to examine the daily variability of trunk control after stroke. Methods: Twenty stroke patients at an inpatient rehabilitation clinic underwent two assessment sessions, on average eight hours apart. Each session comprised two trials: quiet sitting for 30 s; extended reaching in forward, backward, left and right directions. The Trunk Impairment Scale (TIS) was measured during the first session. CoP excursions were measured to determine the outcomes of sway area and sway velocity during stable sitting and the maximal excursions in frontal and sagittal planes during the reaching tasks. Results: High Spearman's correlations (0.72, 0.79) were found between the TIS and the frontal and sagittal excursions. However, only low correlations between the TIS and the sway area and sway velocity were observed. Within sessions, all CoP outcomes showed high ICCs (0.73-1.00). Between sessions, high ICCs (0.86-0.93) were found except for sway velocity (ICC 0.51). Sway velocity increased significantly between sessions. Significance: Frontal and sagittal CoP excursions during reaching tasks appear to be valid measurement parameters to evaluate trunk control in patients after stroke. Only small variability was observed and no significant differences between consecutive days.