The aim of this study is to investigate the effect of manipulating the optic flow speed in the virtual environment on the gait pattern during virtual reality (VR) - enhanced treadmill walking in people post-stroke. Furthermore, the study will also investigate if the level of immersion has an effect on the rehabilitation outcomes by manipulating the optic flow speed in two different VR devices: the semi-immersive GRAIL system and the fully-immersive 'Oculus Rift S' HMD.

To properly understand the results of the people with a stroke, we will also investigate the effect of manipulating the optic flow speed and the level of immersion on the gait pattern in healthy people. That way, we can investigate whether virtual reality has a different influence on the gait pattern of people with a stroke than in healthy people.

STUDY DESIGN

This study is an experimental, 2-group, multicenter trial in which people post-stroke and healthy people will perform 2 different sessions of treadmill walking. The 2 sessions will be carried out on 2 separate days within 10 days. Both sessions will be identical, only the VR system used to manipulate the optic flow speed will differ. The order of the 2 sessions and the optic flow speed manipulation will be randomized.

MATERIALS

GRAIL system: The Gait Real-time Interactive Lab (GRAIL) is an integrative motion capture system consisting of 10 optical motion cameras (Vicon Inc., Oxford, UK), a dual belt treadmill with integrated force plates, a 180-degree cylindrical projection screen system, and D-Flow software (Motekforce Link, Amsterdam, Netherlands). The treadmill of the GRAIL system has two modes: fixed walking speed or self-paced. For this study, the treadmill will be self-paced, meaning that the patient is in control and can choose the walking speed. The treadmill will follow the walking speed of the patient, which allow the patient to start, stop and change speed at will. The projection screen of the GRAIL system will assure a semi-immersive virtual environment. The GRAIL system is located in the Smart Space lab (UZ Gent) and will be made available at the researchers of the VUB/UZ Brussel.

Head-mounted display (HMD): The HMD VR system 'Oculus Rift S' (Oculus, LLS, US) is a low-cost HMD that fully integrates the user into the virtual environment by blocking out perception of the real-world. The "Oculus Rift S" will assure a fully immersive virtual environment. The researchers of the VUB/UZ Brussel have the HMD 'Oculus Rift S' in their possession and will make it available for the study.

PROCEDURE

Both sessions will be carried out in the Smart Space lab at the University Hospital of Ghent (Corneel Heymanslaan 10, 9000 Gent) where the GRAIL system is located.

Prior to the start of both sessions, some preparations will have to be done with regard to the outcome measures:

• Patients will be asked to fill in a questionnaire (the Simulator Sickness Questionnaire-Pre).
• Surface electrodes will be placed bilateral on the M. rectus femoris, M. vastus lateralis, M. biceps femoris, M. tibialis anterior, M. gastrocnemius medialis, M. deltoideus anterior part, M. deltoideus posterior part, M. latissimus dorsi. Electrode placement will follow the SENIAM guidelines. The skin underlying the electrode will be shaved and cleaned with alcohol to improve electrode-skin contact and reduce impedance.
• Reflective markers will be placed on fixed points of the lower and upper limbs of the patient. Marker placement will follow the Plug-in Gait full body model (VICON). For this model, the investigator will have to measure the following things in advance: body length, body weight, leg length, knee width, ankle width, shoulder offset, elbow width, wrist width, hand thickness.

After these preparations, patients will start to walk on the self-paced treadmill of the GRAIL system. For safety only, patients will walk with the safety harness. Patients will first be habituated to walking on the self-paced treadmill for 8 minutes without any form of VR. Patients need this habituation trial in order to get used to the self-paced treadmill system. After these 8 minutes, the treadmill will be stopped and patients will be seated for a 5-minute rest period. During the rest period, patients will be asked to fill in two short questionnaires (the Simulator Sickness Questionnaire-1, the VAS scales).

GRAIL session: After the 5-minute rest period, patients will walk for another 3 times 8 minutes with the VR. Patients are now walking while looking at a virtual environment that is projected onto the projection screen of the GRAIL device (semi-immersive). The optic flow will be different during each 8-minute walk. Between each walk, patients will rest for 5 minutes and during each rest period, patients will be asked to fill in a short questionnaire (the Simulator Sickness Questionnaire-2,-3).

HMD session: After the 5-minute rest period, patients will walk for another 3 times 8 minutes with the VR. Patients are now walking while wearing the HMD 'Oculus Rift' and are being fully immersed in a virtual environment (fully-immersive). The optic flow will be different during each 8-minute walk. Between each walk, patients will rest for 5 minutes and during each rest period, patients will be asked to fill in the a short questionnaire (the Simulator Sickness Questionnaire-2,-3).

To end both sessions, participants will be asked to fill in 3 questionnaires (the Simulator Sickness Questionnaire-Post, the VAS scales and the Igroup Presence Questionnaire).

RANDOMIZATION

First, the type of session will be randomized with 2 possible options: GRAIL - HMD or HMD - GRAIL. Second, the optic flow speed manipulation (matched, slow, fast) within the session will be randomized. The order of the manipulation will be the same in both sessions. The first 8 minutes in each session will always be the one without VR. Thereafter, the optic flow speed will be manipulated. There are 6 possible options for the randomization of the optic flow speed manipulation: matched - slow - fast / matched - fast - slow / slow - fast - matched / slow - matched - fast / fast - slow - matched / fast - matched - slow. The randomizations will be done through block randomization in Microsoft Excel®.

STUDY ANALYSIS

This study will investigate (1) the effect of manipulating the optic flow speed and (2) the effect of the level of immersion (semi-immersive GRAIL system with full-immersive HMD).

The effect optic flow speed has on the spatiotemporal gait parameters, kinematics, kinetics and muscle activity will be compared with walking without VR. In a first stage, the data will be visualized using LO(W)ESS smoothing (locally weighted scatterplot smoothing) to explore the observed effects over time (per condition and outcome), allowing for flexibility using this quasi-nonparametric approach. Next, relevant values expressing onset, magnitude and duration of the effect will be extracted:

• Onset: time point(s) at which the minimal clinically important difference (MCID) is exceeded. In case MCID is unknown, a 10% threshold will be used
• Magnitude: magnitude of the maximum (or maxima) and time point(s) at which the maximum is reached
• Duration: time between the onset and the time point at which the MCID (or 10% threshold) is no longer exceeded

These values will be compared between conditions in a two-way repeated measures ANOVA (optic flow speed * device). If relevant (based on the exploratory analyses), additional in-depth statistics, such as functional data analysis, will be performed under the guidance of the Department of Statistics and Data Analysis of our university.

Inclusion Criteria stroke patients:

• diagnosed with stroke (as defined by the World Health Organization)
• stroke onset ≥ 3 months
• ambulatory with an impaired gait pattern (FAC-score 2, 3 or 4)
• ability to walk on a treadmill for 4x8 minutes without bodyweight support
• adult (≥ 18 years)
• ability to signal pain, fear and discomfort
• ability to give informed consent

Exclusion Criteria stroke patients:

• other neurological deficits than stroke leading to impaired gait (e.g. Parkinson's disease, multiple sclerosis)
• comorbidities (e.g. COPD, severe osteoporosis, cardiovascular instability)
• visual and/or vestibular disorders that can interfere with the VR
• uncontrolled spasticity significantly interfering with the movement of the lower extremities (Modified Ashworth Scale > 2)
• acute medical illness
• communicative/cognitive problems leading to the inability to understand and carry out instructions
• severe unilateral spatial neglect

Inclusion Criteria healthy people:

• ≥ 18 years of age
• normal or corrected-to-normal vision with glasses or contact lenses,
• no locomotion impairments

Exclusion Criteria healthy people:

• having a significant lower extremity injury during the last two years that might affect their gait
• having any type of vestibular/visual deficiency