Directional Differences in Passive Balance Abilities among Middle-aged and Older Adults Revealed by a New Support-base Variation Device

Passive balance ability (PBA) maintains postural stability on unstable ground. The efficacy of this compensatory mechanism appears to differ among individuals and may also differ between directions (back–forth vs. left–right) and sexes, but variations in PBA during ground instability have not been assessed comprehensively under controlled laboratory conditions. The current study examined both the reproducibility and inter-relations among center of gravity sway (COGS) variables as measured on a new support-base variation device fluctuating downward slightly in the back–forth or left–right directions, and further if these variables differ between sexes and directions. Thirty-one middle-aged and older Japanese individuals participated in this study, including 17 males (mean age ± standard deviation, 61.8 ± 8.3 years) and 14 females (56.9 ± 9.6 years). Participants were instructed to maintain a stable standing posture for 1 min on the support-base variation device without the edges contacting an underlying sensor plate while the device measured COGS variables. Center of gravity variables were measured during two, one-minute trials separated by a one-min intertrial interval following one practice trial in either the back–forth or left–right device variation condition. The x-axis and y-axis trajectory lengths, total trajectory length, outer peripheral area, and rectangular area were selected as COGS variables. There were no significant differences in COGS variables between trials, but intraclass correlation coefficients (ICCs) were generally higher for the left–right device condition compared to the back–forth device condition (≥0.7 vs. 0.7). A two-way analysis of variance with main factors, sex and direction, revealed no significant direction × sex interaction but a significant main effect of direction on COGS distance variables. Post-hoc tests also revealed that in both sexes, the x-axis and total trajectory lengths were higher during left–right device variation than back–forth device variation, whereas the y-axis trajectory length was higher during back–forth device variation than left–right device variation. There were moderate to strong significant correlations among almost all COGS variables during back–forth device variation (r values ranging from 0.50 to 0.99). The x-axis trajectory during back–forth device variation also correlated moderately with COGS variables measured during left–right device variation (r values from 0.41 to 0.73), except for rectangular and peripheral areas. In conclusion, the COGS variables during left–right ground variation and the y-axis trajectory length during back–forth ground variation can be measured with high reliability. The strong associations among COGS variables for the same direction but weaker associations between back–forth and left–right COGS variables suggest direction differences in PBA. In contrast, no significant sex differences in PBA were observed.