Position gains during standing remain slightly above critical values under different loading conditions

Introduction: Stabilization of standing is achieved through the coordination of intrinsic and feedback mechanisms (Fitzpatrick et al., 1996; Peterka, 2002), with their lumped position gains slightly higher than the critical stiffness (Geng et al., 2026), indicating an efficient control. However, it is unclear whether and how the CNS modulates the lumped position gains when the critical gain value changes, for instance due to a change in weight bearing. Here, we changed the critical stiffness by adding extra weight on participants, to study how lumped position gains change.Methods: 16 participants (age: 23.0 ± 4.5 years, height: 1.75 ± 0.08 m, body weight: 68.3 ± 8.3 kg) participated in this study. Participants were asked to stand as still as possible on a force plate under 5 loading conditions: no extra load, 5 %, 10 %, 15 % and 20 % of their body weight as extra load, added at the waist. Each standing trial lasted for 1 minute and was repeated 5 times. Ground reaction forces and center of mass kinematics were collected and used to fit a stabilization model and estimate the effective delay and lumped gains (Geng et al., 2026). To examine how the lumped gains changed under different loading conditions, the raw gain values were normalized by dividing them by their critical values. Repeated-measures ANOVA was used to determine the effect of added load on the lumped gains.Results: Results showed good model fits for all loading conditions with R2 values higher than 0.99. The lumped position gains increased significantly with increasing load added on the body. However, the normalized gains remained slightly higher than their critical values in both AP (-1.05 ± 0.02 ~ -1.04 ± 0.01) and ML directions (-1.03 ± 0.03 ~ -1.03 ± 0.02). The effective delay also increased with increasing load in both AP (175 ± 35 ms ~ 198 ± 39 ms) and ML directions (128 ± 18 ms ~ 160 ± 30 ms).Conclusions: Our results show that lumped position gains changed across different loading conditions, while the normalized values remained slightly above their critical values. This suggests an efficient postural control, in which the CNS maintains stability while minimizing unnecessary muscle activation.