Introduction: In addition to standing on both feet, it is important for people to successfully maintain posture and balance on one leg as well. Since, in quiet stand, the projection of the center of pressure is never still, the measurement of the various variables of center of pressure movement enables the evaluation of the ability for single leg stance without support. Purpose: The purpose of this study was to determine the reliability of the modified sensory interaction single leg stance test using a force platform in young and healthy subjects. Methods: The study involved 12 young (18-23 years old) and healthy females. The test was performed standing on the dominant leg on the force platform for three sensory conditions: standing on a firm surface with eyes open and closed and on compliant surface with eyes open. Each measurement lasted 60 seconds at a sampling frequency of 200 Hz. To determine the reliability, the measurements were repeated after seven days, 30 seconds of each measurement was used for the analysis. Nine variables of the center of pressure movement were observed: medio-lateral and antero-posterior displacements, total path lenght, velocity, medio-lateral and antero-posterior path length, sway area from principal components, sway area calculated by Fourier coefficients and sway area calculated by Fourier coefficients divided by sway area from principal components. Results: The most reliable variable for standing on a firm surface with eyes open was medio-lateral path length (intraclass correlation coefficient = 0,848). For eyes closed, the most reliable variable was sway area calculated by Fourier coefficients divided by sway area from principal components (intraclass correlation coefficient = 0,644). As well as for standing on the firm surface with eyes open, medio-lateral path length was the most reliable variable also for standing on the compliant surface with eyes open (intraclass correlation coefficient = 0,877). Discussion and conclusion: When applied the modified sensory interaction single leg stance test on the force platform with healthy, young subjects the results varied from excellent reliability to not being reliable. Although some variables have shown poor reliability, those with excellent reliability can still be used for evaluating the abilities to maintain posture and balance. The most (six out of nine) observed variables proved excellent reliability during single leg stance on a compliant surface with eyes open.
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