# Postural Instability after Stepping on a Stair in Older Adults: A Pilot Study
## 1 Introduction  
Postural instability in older adults (OA) leads to impaired balance control when performing activities of daily living, potentially causing an increased risk and incidence of falls and a reduction of independence and quality of life [1]. Given that the biomechanical mechanisms of postural instability in OA have been well documented [2], the balance deficits of OA have been shown to manifest not only during static movements [3], but also during dynamic transitive movement in daily activities [4].  
A stair ascent task is a functionally relevant motor task that significantly challenges the locomotor and postural control system. Biomechanically, stair ascent requires significant momentum, which is necessary for conjoint upward and forward body propulsion [5]. Indeed, stabilizing one's body posture following stair ascent is needed to offset the propulsive momentum generated by the whole body. Accordingly, individuals with strength deficits may be more impaired at controlling stability after a stair ascent task, exhibiting alterations in their strength compensation strategy. Indeed, it has been reported that individuals with muscular and neurologic deficits (e.g., osteoarthritis and stroke) are at a greater risk of having a stepping-related fall due to biomechanical and environmental constraints [6, 7].  
When comparing OA and YA (young adults), OA are at higher risk of loss of balance and falls during stair ascent in comparison to YA, in part due to muscular deficits. Specifically, muscle weakness in the elderly causes abnormal gait patterns and changes their gait biomechanics, particularly affecting gait velocity, and with less strength in their lower extremities leading to their greater incidence of falls [8, 9]. Despite greater predicted possibilities that OA show balance problems and falls, studies have thus far focused less on motor control in dynamic activities, such as stepping up stairs, and even less on comparatively examining OA and YA.  
Additionally, stair ascent is a complex task that is cognitively demanding compared to other simple tasks (e.g., sit-to-stand) [10], and age differences in cognitive abilities have been clearly shown [11]. A study proposed that significantly more attentional resources are required during stair ascent in OA than YA, while greater attentional resources are not required during simple tasks, such as standing [12]. It is also highlighted that cognitive decline is a common problem observed in the elderly [13]. Thus, stair ascent task assessments provide important insights into biomechanical abilities that cannot be captured through simple tasks for OA, while being sensitive to different cognitive abilities by age.  
The purpose of the current study was to examine how stabilizing capabilities after stepping up stairs in OA faired in comparison to YA, hypothesizing that OA would take longer to stabilize their body posture and would have less ability to regain static plateau after the stepping performance. To find the exact time point at which one's postural sway is in plateau, we utilized the 'time to stabilization (TTS)' metric, which provides underlying information calculated based on overall information in a time series. It is useful and applicable when assessing dynamic balance capability.