# Postural Control Disturbances Induced by Virtual Reality in Stroke Patients
## 2 Methods  
Five age-matched SP (61.2 \(\pm\) 7.05 years old, range: 51-69 years, two women) and 15 age-matched HA (59.1 \(\pm\) 2.8 years old, range: 55-65 years, nine women) participated in the study conducted in the Physical Medicine and Rehabilitation department. The inclusion criteria were as follows: adults (>18 years old) with subacute paresis [14 days-6 months] having given informed agreement. The exclusion criteria were: (i) standing upright for less than 1 min without assistive device; (ii) cognitive impairment (Mini Mental State Examination < 24); (iii) inability to understand the instructions (Boston Diagnostic Aphasia Examination (BDAE) < 2); (iv) visual or auditory impairment; (v) motion sickness in VE by adverse events (e.g., vertigo); and (vi) neurological or orthopedic complications modifying quiet standing. All subjects gave their informed consent for inclusion before they participated in the study. The study was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by the Ethics Committee of the University Hospital Center of Poitiers (CHU86-RECH-R2021-02-01). A clinical assessment of fear of falling with the Falls Efficacy Scale-International (FES-I) and balance abilities with Postural Assessment Scale for Stroke Patients (PASS) was filled by the SP. Before starting the postural control assessment, each participant discovered neutral VE in a seated position and carried out a 30-s familiarization trial on a force platform. Then, postural control assessment was performed while participants were positioned with bare feet oriented at 30\({}^{\circ}\) from the sagittal midline in two control conditions: open eyes (OE) and closed eyes (CE); and in two types of VE (Figure 1A): a static virtual environment where the participant is in the middle of a room without any avatars (Figure 1B), and dynamic virtual environments, which represent a commercial center with moving avatars walking at spontaneous velocity following random displacements (1.1 m\(\cdot\)s\({}^{-1}\)) (Figure 1C-F). Dynamic virtual environments (DVEs) provided four visual conditions obtained by the modulation of virtual crowd density (sparse and dense) and avoidance space with avatars (wide: 0.8 m and narrow: 0.1 m). We thereby tested four DVE conditions: sparse and wide (SW); dense and wide (DW); sparse and narrow (SN); and dense and narrow (DN) (Figure 1C-F). Conditions were randomly assigned before the beginning of the session. After each condition, participants were asked to determine the difficulty in identifying feelings to maintain posture stability with a visual analogue scale (VAS) from 0 to 10 (0 no difficulty and 10 unable to perform). During assessment, safety devices (i.e., human security, metal frame) were put in place to prevent falls (Figure 1A). Postural data were collected over 30 s per trial with a 1-min rest between trials. A WIN-POSTURO force platform (Medicapteurs(r) Co. France) was used to measure the center of pressure (CoP) displacement (sampling rate: 40 Hz [17]), and an head-mounted display device (HTC(r) Vive, HTC Co., New Taipei City, China) with a large field of vision (110\({}^{\circ}\)) and a high resolution screen (1200 \(\times\) 1080 px each eye) to display VE (Virtualis(r) software, Co. France). CoP velocity was expressed in mm-s\({}^{-1}\) (corresponding to the cumulative distance over the sampling period) [18]. To investigate the effect of visual attention on postural control due to VE, we used the following equation: VRcost =(VE - OE)/OE\(\ast\)100) (in%). Raw signals were imported in MATLAB (Mathworks Inc., Natick, MA, USA) and low-pass filtered (fourth order low-pass zero-phase lag Butterworth filter, 7 Hz cut-off frequency) prior to computing the CoP velocity. The data are presented as median with data range. The Mann-Whitney U-test was performed to compare postural performance (i.e., CoP velocity and Vrcost) between the OA and HA groups. Statistical analysis was carried out with SPSS Statistics(r) 22 software (IBM Corp, Armonk, NY, USA) with a \(p\)-value \(<\) 0.05.