Validity of functional stability limits as a measure of balance in adults aged 23-73 years.

Functional stability limits (FSLs) are the percentage of the base of support that individuals are willing to extend their centre of pressure. The objective of this study was to provide construct validation of FSLs as a measure of balance by comparing FSLs across ages and with clinical balance measures. A total of 52 participants volunteered. FSLs significantly decreased with age (p < 0.004). Correlations between FSLs and age (-0.56 < R < -0.73), multi-directional reach test (0.35 < R < 0.75) and anteroposterior and mediolateral centre of pressure excursions during static stance (-0.29 < R < -0.72) were generally moderate to good. Cronbach's alpha (0.75) indicated that these measures were internally consistent, i.e. measuring similar aspects of the balance construct. FSLs appear to be valid indicators of balance ability. They may be used in posture prediction models to determine when a step is required when reaching or lifting objects and in biomechanical models as a means of incorporating stability constraints.