Effect of loss of balance on biomechanics platform measures of sway: Influence of stance and a method for adjustment

This paper describes a method for adjusting biomechanics platform measures of sway for loss of balance. Area and velocity measures of sway were determined in forty-seven elderly women, in double and single leg stance, first with their eyes open, then closed. Subjects were rarely able to complete 10 s trials during single leg stances. Therefore, a method was developed for eliminating data associated with loss of balance. Monitoring changes in vertical force and velocity by computer, those points exceeding trial specific thresholds associated with loss of balance were truncated. In double leg stances, loss of balance increased area measures by 0-3%, but did not effect velocity measures. In contrast, the loss of balance experienced by most subjects in single leg stance exaggerated area measures by 16-38%, and velocity measures by up to 10%. In double leg stances the correlations between unadjusted area measures and area measures adjusted for loss of balance ranged from 0.98 to 1.00. In single leg stances, the correlations for the area measures ranged from 0.69 to 0.89. The correlations between adjusted and unadjusted velocity measures were 1.00 and 0.93 for the double and single leg stances respectively. Although the question of which sway measure is best remains unanswered, this study provides useful data for future research. First, it demonstrates a method for modifying area representations of the center of pressure excursions for the effects of loss of balance. Second, it quantifies the effects of loss of balance in different stances; for the population studied the effects are minimal in double leg stances, but may be substantial when sway is measured in single leg stances.