Test-retest reliability of and age-related changes in the subjective postural vertical on the diagonal plane in healthy subjects.

The subjective postural vertical (SPV) reflects gravity perception when the eyes are closed. Changes in the SPV on both the frontal and sagittal planes occur in response to neurological disorders and aging; however, these changes on the diagonal plane are unclear. Here we examined test-retest reliability (n=16) of and age-related changes (n=38) in the SPV on the diagonal plane. Subjects sat on an electrical vertical board (EVB), which was used to measure the SPV on the diagonal plane. An experimenter controlled and moved the EVB seat at a constant speed on the diagonal plane and measured the seat's tilt using a digital inclinometer when subjects verbally reported that they had reached a true vertical position. Measurement was performed for eight trials, and the mean (tilt direction) and standard deviation (variability) were calculated. To determine test-retest reliability, the same experimenter repeatedly measured the SPV 1 week later. To assess age-related changes, tilt direction and variability were compared between the young (n=20) and elderly (n=18) groups. Test-retest reliability on the right and left diagonal planes was 0.61 or more. Moreover, tilt direction on the right diagonal plane - but not on the left diagonal plane - indicated a significant diagonally backward deviation in the elderly group compared with that in the young group. Variability was significantly higher in the elderly group on both planes. SPV measurement on the diagonal plane was indicated, and age-related changes were identified. Thus, future studies should assess the potential clinical applications of SPV in neurological disorders.