Improving balance assessment: effects of motivation and fear on feet’s base of support

Introduction: 
Balance can be described as how the body is controlled relative to the base of support (BOS) formed by the feet . Recently, a 2D model of the functional BOS (fBOS), above which we can manipulate functional forces (>40% body weight), considerably improved the BOS estimate (Millard+Sloot2025). However, young people showed a large variation in fBOS area (9-36% of the foot), representing differences in either balance ability or task execution. This study examined whether task execution (fear and motivation) influence the measured fBOS area and fBOS repeatability.  
Methods: 
Ten young adults (27±2yrs) leaned maximally in all directions in a circular motion while standing with feet still during five randomized conditions: baseline (with-in day repeated); fear with a safety harness, light fingertip support ; motivation: visual feedback, and verbal encouragement. Force-plates were used to calculate the fBOS area as the convex hull of the centre-of-pressure data (Millard+Sloot2025).Conditions were compared using repeated-measures ANOVA; repeatability was evaluated using Bland–Altman analysis. 
Results: 
There was an effect of condition on the fBOS area (p<0.01), but no post hoc differences were found. Baseline within-day repeatability showed a task effect; a practice effect was suggested, with a larger second baseline (+13.9%), but no statistically significant difference (p = 0.98).
Discussion: 
These findings show that addressing motivation or fear did not change the fBOS area in young adults, suggesting the fBOS variability directly represents difference in balance ability.. To address our limitations, we will next measure older adults and relate the fBOS area with balance performance. The task effect indicates that a practise trial is warranted to achieve a better assessment of the fBOS area, thus improving balance and fall-risk assessments.