## 2 Materials and Methods
### Balance Assessment  
Balance is frequently investigated to assess the recovery of patients who have suffered from trauma or disease [6]. Multiple approaches to assess gait, balance, and functional mobility have been developed including the Timed Up and Go (TUG) test, FES (Falls Efficacy Scale), ABC (Activities-specific Balance Confidence Scale), BESS (Balance Error Scoring System) [19]. To ensure static stability during the process of locomotion, one's center of mass must be vertically projected inside the polygon of support formed by the contact points between its limbs and the ground, or base of support [4]. Quiet (i.e., static) standing is an assessment of balance performance, representing an individuals' ability to constrain the center of mass (COM) or center of pressure (COP) within an established base of support (BoS) [2]. The COP trajectory is recorded using force platforms, which track the point of application of the ground reaction forces resultant under the feet. The COP represents the point of application of the ground reaction force vector and provides insight  
Figure 1: (**a**) VR training headsets and setup for (**b**) RABT with the NaviGAITor device. into postural stability by examining the displacement and variability of body sway. The resulting signal, called stabilogram, is frequently analyzed using either its one-dimensional variations in the mediolateral (ML) or anteroposterior (AP) direction, or its two-dimensional trajectory [3]. Here, to assess balance performance in older adults, pre- and post-training evaluations were conducted for both the VR and RABT interventions, balance performance was assessed using the Balance Error Scoring System (BESS) and center of pressure (COP) metrics were obtained from force plate data. BESS assessment was used as a clinical measure for balance which allows for variation in somatosensory cues (i.e., hard vs. foam support surface) as well as variation in base of support, BOS, (stance widths of wide, tandem, and single-leg) while the participant has no visual cues (eyes are closed), thereby allowing for measured differences in balance (in terms of BESS errors) with increases in task difficulty for six stance conditions [19]. Participants were instructed to stand upright with their eyes closed and hands on their hips for 20 s per condition. During each stance, observable deviations from the instructed posture, referred to as \"errors\", were recorded. Errors included actions such as extending the arms away from the hips, opening the eyes, stepping or stumbling, excessive hip flexion (>30\({}^{\circ}\)), or losing balance for more than five seconds. An evaluator scores the subject's postural stability using an objective list of errors. Each error committed is given 1 point, and the maximum allowable number of points for each position is 10. The subject's total BESS score is the sum of the individual stance position scores (maximum total BESS score is 60) [4]. Higher scores indicated greater postural instability, whereas lower scores reflected better balance control. The COP assessment was performed to examine postural control during balance tasks. Force plate data were collected using the Tekscan Strideway system (Tekscan Version 7.8, Norwood, MA) at a sampling frequency of 50 Hz during five 20 s trials per stance condition. Participants completed double-leg, single-leg, and tandem stances with eyes open and closed. In order to assess balance confidence, the ABC scale was used. The ABC scale is a self-reporting assessment designed to gauge an individual's confidence in their ability to carry out different daily living tasks without losing their balance. Balance confidence levels were reported by the participant for daily living scenarios in and out of one's home, using scores ranging from 0% (no confidence) to 100% (complete confidence). All ABC assessments were administered in person by trained research assistants.