Postural stability in stroke patients: Vectorial expression of asymmetry, sway activity and relative sequence of reactive forces

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Abstract
Bilateral force measurements on the supporting limbs in postural sway while standing still were made to evaluate post-cerebral-vascular accident (CVA) patients during rehabilitation. Normal subjects of the same age group were tested as controls. From the force tracings obtained, three oscillation frequencies were identified, with orders of magnitudes of 7, 1 and 0·1 Hz, respectively, of which the middle frequency, i.e. that corresponding to 1 Hz, was selected for subsequent processing and analysis. These included the determination of relative sequence of the force vectors on both feet and evaluation of timings and amplitudes of the waveforms. Weight-bearing imbalance was defined in the vertical direction to express the difference between the average forces supported by each of the legs. In the horizontal plane, two parameters were defined: sway total activity (SA), to represent the vector summation of the absolute values of the horizontal force components acting on both legs; and asymmetry (ASYM) to express the difference in activities between the two legs. The results presented disclose the reactive force patterns acting on each of the legs of post-CVA hemiplegic individuals, in comparison with normal individuals. Although these forces were shown to act synchronously on both legs, they appeared to be asymmetrical in nature, with a typical vectorial pattern for every individual, which generally differed from that of normal subjects. Sway activity was found to be significantly higher in hemiplegics compared with the normal controls.