## 4 Discussion  
The purpose of this study was to compare whole-trial estimates as well as the transient behavior of postural sway across four different postural tasks between young and older adults. Our first hypothesis was confirmed as the postural sway increased with task difficulty, and the increase was more pronounced in older adults than younger adults (as confirmed by statistically significant task \(\times\) group interactions). There were only a few variables related to transient behavior that indicated a difference between the age groups, which were mostly limited to CoP frequency. Interestingly, there were some trends in the easiest task (parallel EO) for both indexes that were closer to 100% in young adults than in older adults for CoP ML velocity. This would indicate the presence of larger transient changes in postural sway in older adults after vision is returned (note again that the parallel EO task was preceded by 15 s of standing with eyes closed). However, the main effects  
Figure 3: Comparison of the DIF_31 index (relative difference between the 3 rd and 1 st interval of the trial) between young and older adults across tasks. CoP—center of pressure; AP—anterior–posterior; ML—medial–lateral; EO—eyes open; EC—eyes closed. The dashed line indicated the value corresponding to no transient response (i.e., the same value of postural sway in the 1 st and 3 rd interval). * Indicates statistically significant difference between the young and older group (\(p<0.05\)). were not significant, making it impossible to draw any conclusion on the significance of these observations. Several previous studies suggested that long trials (e.g., 60 s or more) are needed for a valid assessment of postural sway [6; 20; 21]. However, the transient behavior of postural sway that is masked by whole-trial averaging could also contribute important information regarding the individual's postural control. For instance, falls in older adults could occur during sudden changes in sensory conditions (e.g., lights turning off) or body position. Such conditions entail efficient sensory reweighing [8]. [PERSON] et al. [7] suggested that the difference between CoP outcomes obtained right after closing the eyes and those obtained later in the trial could reflect the efficiency of sensory reweighing. Moreover, they reported that older adults show an exaggerated transient response after closing the eyes compared to young adults [7]. The removal of visual information compels the central nervous system to quickly adjust the relative sensory weights for vestibular and proprioceptive systems [8; 9; 10]. Although purely speculative at this point, it could be that the transient behavior of postural sway after a change in sensory conditions (presumably reflecting sensory reweighing processes) is as if not more important for preserving stability and preventing falls in older adults than whole-trial, averaged postural sway outcomes. As in previous studies [7; 11; 15], the indexes of the transient behavior of postural sway were generally not correlated with whole-trial outcomes, supporting the notion that additional information may be obtained by calculating the indexes of transient behavior. In this study, postural sway increased with task difficulty in both groups, and the increase was exaggerated in older adults compared to young adults, which is well in line with previous studies [22; 23]. In addition, older adults exhibited different transient behavior (larger changes throughout the trial) in postural sway as per CoP frequency. These results are difficult to interpret, as the differences between age groups were not consistent across tasks. This was likely observed because the older adults were relatively fit and their postural control was comparable to the young adults in the easier tasks (as also reflected in the whole-trial variables.)