Walking in virtual reality
Even though virtual reality is increasingly used in rehabilitation, the implementation of walking navigation in virtual reality still poses a technological challenge for current motion tracking systems. Different metaphors simulate locomotion without involving real gait kinematics, which can affect presence, orientation, spatial memory and cognition, and even performance. All these factors can dissuade their use in rehabilitation. To make walking in virtual reality a reality we designed a marker-based head tracking solution that estimates the location of the head in a room-size environment from the position of fiducial markers attached to the ceiling.
A feasibility study was conducted to determine the accuracy, the jitter, and the lag of the tracking system
and its elicited sickness and presence in comparison of a CAVE system.1
The accuracy and the jitter around the working area at three different heights and the lag of the head
tracking system were analyzed. In addition, forty-seven healthy subjects completed a search task that
involved navigation in the walking virtual reality system and in the CAVE system. Navigation was enabled by
natural locomotion in the walking virtual reality system and through a specific device in the CAVE system.
An HMD was used as display in the walking virtual reality system. After interacting with each system,
subjects rated their sickness and their presence. Better performance was registered at higher heights, where
accuracy was less than 0.6 cm and the jitter was about 6 mm. The lag of the system was 120 ms. Participants
reported that both systems caused similar low levels of sickness. However, ratings showed that the walking
virtual reality system elicited higher sense of presence than the CAVE system.
The marker-based solution provides accurate, robust, and fast head tracking to allow navigation in the virtual reality system by walking without causing relevant sickness and promoting higher sense of presence than CAVE systems, thus enabling natural walking in full-scale environments, which can enhance the ecological validity of virtual reality-based rehabilitation applications.
Motion tracking systems
Motion tracking systems are commonly used in virtual reality-based interventions to detect movements in the
real world and transfer them to the virtual environment. There are different tracking solutions based on
different physical principles, which mainly define their performance parameters. However, special
requirements have to be considered for rehabilitation purposes. We compared the accuracy and jitter of three
tracking solutions (optical, electromagnetic, and skeleton tracking) in a practical scenario and analyzed
the subjective perceptions of 19 healthy subjects, 22 stroke survivors, and 14 physical therapists.2
With regards to the objective parameters, a practical scenario was designed to consider the effect of distance in the tracking performance. A 6 × 6 grid with 25 cm × 25 cm squares was designed and printed on a plastic sheet that was fixed to the floor, covering an area of 1.5 m2. The right ankle joint (tibiotalar) of an experimenter was fixed in all the intersection points of the grid (49 in total) using a cast. The position of the joint was estimated by the different tracking systems during 5 s
With regards to the subjective parameters, three different configurations of a virtual reality-based system
with a stepping exercise3 (see
Balance) using the three different tracking systems were
installed in the physical therapy area of a neurorehabilitation unit in a large metropolitan hospital. The
experiences with the three tracking systems of healthy individuals, individuals with stroke, and therapists
were collected through two ad-hoc questionnaires.
The optical tracking system provided the best accuracy while the electromagnetic device provided the most
inaccurate results. However, this tracking solution provided the best jitter values, in contrast to the
skeleton tracking, which had the worst results. Healthy individuals and professionals preferred the skeleton
tracking solution rather than the optical and electromagnetic solution (in that order). Individuals with
stroke chose the optical solution over the other options. Our results show that subjective perceptions and
preferences are far from being constant among different populations, thus suggesting that these
considerations, together with the performance parameters, should be also taken into account when designing a
- Borrego A., Latorre J., Llorens R., Alcañiz M., Noé E. Feasibility of a walking virtual reality system for rehabilitation: objective and subjective parameters. Journal of NeuroEngineering and Rehabilitation, 2016. In press
- Llorens R., Noé E., Naranjo V., Borrego A., Latorre J., Alcañiz M. Tracking Systems for Virtual Rehabilitation: Objective Performance vs. Subjective Experience. A Practical Scenario. Sensors, 2015, 15(3), 6586-6606.
- Llorens R., Gil-Gómez J., Alcañiz M., Colomer C., Noé E. Improvement of balance using a virtual reality based stepping exercise: a randomized controlled trial involving individuals with chronic stroke. Clinical Rehabilitation, 2015. 29(3):261-8.