## 1 Introduction  
Postural stability is paramount for older adults in order to be able to perform the activities of daily living [1] and minimize the risk of falling [2, 3]. While various methods are being used to assess postural stability [3], the analysis of center of pressure (CoP) movement remains the gold standard [4, 5]. Several variables extracted from CoP movement during quiet stance have been shown to be linked to a risk of falling in older adults [3]. Using longer trials for the collection of CoP data improves the reliability of the outcomes [6]. However, averaging the data across a longer trial (e.g., 30 s or 60 s) may mask important information regarding the transient behavior of postural sway [7].  
Postural stability is maintained based on sensory information from the vestibular, visual and somatosensory systems [8, 9]. The relative contribution of each system is dependent on environmental conditions and the postural task [10]. When a different sensory condition is suddenly introduced (e.g., when closing the eyes), the central nervous system must change the relative contribution of each sensory system (i.e., the sensory reweighing) [9, 10]. This results in a transient period of increased postural sway [7, 11]. While this phenomenon has been known for a long time [12, 13], [PERSON] et al. [7] were the first to suggest that analyzing the transient behavior of postural sway after changes in postural conditionscould offer additional information to traditional outcome variables, such as whole-trial estimates. In the context of this paper, whole-trial estimates refer to the traditional postural sway's outcome variables, where the CoP movement (e.g., CoP movement velocity) is averaged over an entire trial (e.g., 30 s). The indexes of transient behavior represent the difference in CoP movement between specific time intervals within the trial (e.g., between the first 10 s and the last 10 s).  
Researchers have focused on transient behavior after a transition from the parallel stance to the single-leg stance [11; 14; 15]. Specifically, it has been shown that the transient behavior of postural sway in the single-leg stance differs between ballet dancers and the general population [11] and between preferred and non-preferred legs [15], but it is similar in athletes with and without a history of ankle sprain injury [14]. In their study, [PERSON] et al. [7] reported that the transient behavior of postural sway in the parallel stance after closing the eyes distinguished younger adults from older adults, supporting its potential clinical relevance. In addition, they reported that transient characteristics and whole-trial estimates exhibited negligible correlations [7], which means that the analysis of transient behavior could offer additional information to traditional whole-trial variables. This finding was also supported by studies using single-leg stance tasks [11; 15]. Apart from the study by [PERSON] et al. [7], the results by [PERSON] et al. [16] and [PERSON] et al. [17] indicated that time interval choice within the single-leg postural sway trial importantly affects age-related differences, but they did not calculate any indexes of the transient behavior of postural sway.  
The purpose of this study was to compare young and older adults in terms of whole-trial estimates as well as the transient behavior of postural sway in four different postural tasks. We hypothesized that postural sway (measured by whole-trial estimates) would increase with task difficulty and that the increase would be more pronounced in older adults than in younger adults. We also hypothesized that the two age groups would exhibit different transient behaviors, with the relative decrease in postural sway being larger in the older group.