## 4 Discussion  
In this study, the impact of aging and Tai Chi practice on the adaptation to sensory and motor perturbations while standing was observed, using frequency-domain features of the COP and rambling and trembling decompositions. We hypothesized that older Tai Chi practitioners would exhibit a decreased reliance on visual processes as sensory and motor perturbations increased, relative to naive healthy older adults. However, no significant condition-by-group interactions were observed in the low-frequency components between these groups. Additionally, we hypothesized that healthy older adults, relative to healthy younger adults, would exhibit decreased adaptation to sensory and motor perturbations across repeated trials. Partially supporting this hypothesis, a significant trial-by-group interaction was observed in the high-frequency rambling-y component, although this effect was not clearly linked to specific sensory or motor perturbation conditions. Contrary to our first hypothesis, Tai Chi practitioners did not exhibit a measurable reduction in visual dependence across increasing perturbation levels. This suggests that Tai Chi practice may not lead to enhanced down-weighting of visual input in challenging sensory and motor contexts. Prior studies have reported mixed findings regarding visual dependence in Tai Chi practitioners. Some have shown reduced visual reliance, such as improved balance with eyes closed [15], while others reported increased dependence [33] or no difference at all [34]. Nonetheless, these studies consistently indicate that Tai Chi improves overall balance by enhancing sensory re-weighting [15; 33; 34]. In line with this, our results suggest that the effects of Tai Chi may reflect broader enhancements in multisensory integration rather than selective changes in reliance on a specific modality. In partial support of our second hypothesis, younger adults exhibited greater reductions in high-frequency power over repeated trials in the rambling-y component, suggesting more effective proprioceptive adaptation. Notably, older adults showed lower overall high-frequency power in this component, as reflected by a significant main effect of group. This pattern aligns with prior findings that aging is associated with diminished proprioceptive adaptation in postural control [35]. However, the lack of a significant three-way interaction (condition-by-trial-by-group) suggests that this adaptation was not strongly modulated by specific perturbation types. To evaluate whether these adaptation patterns were associated with postural performance, we examined correlations between high-frequency power and SOT scores. Among older adults, proprioceptive reliance showed a weak-negative correlation with stability in Trial 1 (r = \(-\)0.144), which diminished by Trial 3 (r = \(-\)0.0299). In contrast, younger adults showed a slightly increasing correlation across trials (Trial 1: r = \(-\)0.0746; Trial 3: r = \(-\)0.137), although still weak. This suggests that reduced proprioceptive reliance over time may be modestly associated with improved postural stability in younger adults, potentially reflecting more effective proprioceptive adaptation. For older adults, however, this relationship was minimal, possibly reflecting age-related declines in proprioceptive sensitivity and integration that limit the benefits of sensory adaptation [36]. These adaptation effects were observed primarily in the rambling-x component, suggesting that age-related differences may reflect disparities in cortical-level sensory integration. This interpretation is supported by evidence linking rambling to sensory re-weighting processes [37] and EEG studies implicating cortical activity during postural control under SOT conditions [38]. The observed effects in the mediolateral direction further suggest that such integration differences are particularly evident in mediolateral balance control, a known predictor of fall risk in older adults [39].