## 4 Discussion
The difference between the groups in the parallel EC task could imply that older adults show a larger variation in CoP frequency across the trial and need more time to stabilize their postural control after closing their eyes. However, the CoP frequency seems to be the least sensitive among the CoP variables for assessing the risk of falling in older adults [3], making further inferences questionable. On the other hand, older adults tended to exhibit different transient behavior for CoP ML velocity in the parallel EO task (note that this task was preceded with 15 s of standing with closed eyes). Namely, both indexes of transient behavior were closer to 100% in young adults (94-99%) than in older adults (85-88%), indicating that transient behavior is larger in older adults after visual information is restored. Indeed, sensory reweighing also needs to arise when a sensory source is re-established and not only after its removal [8; 9]. As mentioned before, [PERSON] et al. [7] reported that the indexes of transient behavior after closing the eyes (such as in the parallel EC task in our study) distinguished between older and young adults with moderate to large effect sizes ([PERSON]'s d = 0.45-0.71). We did not observe statistically significant differences in terms of the transient behavior between the age groups in parallel EC task, although some outcomes (CoP ML velocity and CoP ML amplitude) approached statistical significance (\(p\) = 0.064-0.078). The older adults in our study were relatively fit and physically active, as reflected in high Berg balance scale scores (51-54 points). Therefore, it is reasonable to expect that we would observe even larger transient behavior in older adults at a high risk of falling; however, this needs further investigation. Larger responses to the removal/restoration of vision are perhaps expected, given the increased weight placed on visual information for postural control in older compared to young adults [24]. As the sensory weights for the vestibular and somatosensory systems change with visual manipulations, the transient behavior of postural sway could reflect the deterioration of these two systems, which is frequently observed in older adults [24; 25; 26]. Regarding the transition to the single-leg stance, it was found that transient behavior is different between ballet dancers and the general population [11] but not between athleteswith and without ankle sprain history [14]. Tasks involving postural change (e.g., transition from parallel to single-leg stance) are methodologically problematic, because it is difficult to distinguish between the transient effects of the postural change itself (i.e., the CoP moving under the stance leg) and the supposed subsequent effects of sensory reweighting. As in previous studies [11; 14; 15], we started the data acquisition \(\sim\)1 s after the participant assumed the new position; however, the effect of CoP movements arising from the change in body position itself remains unknown. Due to these issues, we suggest that investigating transient behavior after the removal or restoration of vision (parallel EC and EO tasks) is methodologically a sounder option. In addition, a transient response in postural sway was also observed after the addition of a cognitive task [27], which is another promising avenue, especially in light of the well-known increased cognitive cost in older adults [28] and possible links of dual-task ability to the risk of falling [29; 30]. Finally, extending the application of analyzing the transient behavior of postural sway to patients with neurological diseases is also intriguing. For instance, when vision is removed or disturbed, patients with Parkinson's disease seem to be able to reweigh sensory information appropriately but not as quickly as age-matched controls [31]. Indeed, [PERSON] et al. [13] reported a larger transient increase in postural sway in Parkinson's disease patients (compared to age-matched controls) after closing the eyes.