Development of a serious game for gait intervention in Augmented reality: a prospective study on the impact of virtual feedback on walking speed.

Abstract
Background. Improving walking speed can be a time consuming and tiresome process for people with disabilities. Integrating augmented reality (AR) games into gait training programs could increase motivation and engagement in rehabilitation sessions.
Research question. Does an AR game with visual feedback help to achieve or exceed target walking speed without increasing variability in walking speed?
Methods. This prospective study presents the game framework used to develop five AR mini-game activities for gait training. These mini-games combine different feedback modalities. 15 he (Preprint)
Authors: Guinet Anne-Laure , Otmane Samir , Bouyer Guillaume , Desailly Eric 
Published: October 2021

BACKGROUND

Gait in patient with neurological disorders (ND) is characterized by a slower walking speed (WS), a shorter-stride length, a lower cadence, and more time spent in double support.1 Gait quality, especially WS, are often hard to improve for people with ND.2 Improved walking ability has a positive impact on achievement of daily activities and motivating social engagement.2
Current motor learning theories recommend task-specific, variable, and high intensity practice.3 New technologies, such as augmented reality (AR), appear to be a good way to combine repetition, practice specificity, motivation, feedback, and reward, both in a clinical context and at home.4 AR that delivers virtual elements superimposed on the person’s view of the real-world could be used for gait training without a treadmill. Using a mixed-reality system combining an AR head-mounted display (HMD) and a sensor-based motion capture system, a case study showed that a patient adapted his gait performance.5 However, AR promotes patients’ engagement in the therapy and potentiates therapeutic gains by increasing motivation.6,7
While the potential of AR feedback, particularly for enhancing motivation, has been demonstrated, few studies have delved into how different forms or presentations of AR feedback may impact gait parameters and performance.8 One concern to AR feedback for people with ND during gait training is the extent to which it may distract or increase cognitive load. Previous studies have shown that gait performance decreases for people with ND as dual task cognitive load increased.9 Moreover, several studies demonstrated that a high gait variability, including variability of WS, results in walking instability and a higher risk of falling.10
In this study, we investigate the acceptability of an AR HMD and the impact of different forms of visual AR feedback on gait in typically developing adults prior to future studies including people with ND.

OBJECTIVE

The primary research questions are: Can AR feedback be used to help individuals achieve or exceed a target WS? Does AR feedback result in increased variability in WS? Our hypothesis is that a game with AR feedback can help individuals to maintain or exceed a target WS without significantly affecting WS variability.

METHODS

Methods
AR game and feedback development framework
This work followed the development framework PROGame:11 the team (therapists, researchers, engineers) identified the need for the serious game and user categories (users, experts), they clarified the requirements of the game (operational objectives and restrictions). These identifications were based upon prior experience and literature review.2,12,13 The identified need was to improve motivation and feedback during gait training. Operational objectives included: to be safe and adaptable to the motor capabilities of diverse patient; to provide efficient gait training in line with motor learning theory; to provide high quality feedback on WS; to contain gamified content to motivate the patient to achieve the rehabilitation goals; to be fun. Additionally, the team identified crucial technical requirements: people must be free to walk without restriction of movement (e.g., wireless, low-weight/comfortable, secure) and they must be able to use their usual walking aids (crutches, posterior walker). It should be a “ready-to-use” application both for patient and clinicians to avoid wasting valuable clinical time.
The Hololens AR HMD was selected because its technical characteristics matched the identified requirements.14 It has also been demonstrated to be sufficiently accurate to evaluate the head pose and WS of the user without time drift in the global environment.15
To investigate our hypothesis, 5 mini games using different feedback were developed (Figure 1). Feedback are characterized by modalities that define their functioning, their role or their form of presentation:16
-	Focus of attention: knowledge of result (KR) focusing on the outcome of a movement or knowledge of performance (KP) addressing qualities of the movement itself,
-	Method of presentation: simple visual content or richer effect (animation),
-	Frame of reference: hologram can be world-locked (stationary frame of reference) or body-locked (attached frame of reference).

Subjects & Protocol
Participants were 15 adults (36.3+/-9.8y, 11 women/4 men) with typical or corrected-to-typical vision. The study was approved by the National Ethical Committee and all participants completed written consent. The protocol is detailed in Figure 2.
Evaluation Criteria
1.	WS was calculated with the AR HMD (100Hz). The three first and last meters (acceleration/deceleration) were excluded from the analysis.
2.	A questionnaire with a 5-point Likert scale from 1 to 5 (strongly disagree/agree) was completed to assess this experiment.
Statistical analysis
After checking the stochastic assumptions, within-subjects’ ANOVA for repeated measures and Dunnett test for multiple comparisons were performed for comparing experimental values (i.e., during each mini-game) and a single control value. Descriptive statistics were reported to describe participant feedback collected via questionnaires.

RESULTS

Results and Discussion
Walking speed
The mean WS during each mini-game and the control scene and target speed were not statistically different (F(5,76)=2.31,p=0.052) (Figure 3). These results are close to being significant, more participants are required to confirm the trend observed. Dunnett test revealed indeed that the mean WS during MG4 was significantly higher in comparison to the target WS. All mini-games helped the participant to exceed the target WS.
There was a significant difference in the standard deviation of WS during each mini-game and the control scene (F(5,76)=9.39,p=5.37.10-7) (Figure 3). Dunnett test revealed that MG4 and MG5 resulted in a high variability in WS compared to the control scene (respectively 0.26 m/s, 0.33 m/s, 0.13 m/s). Of note, the frame of reference of feedback in MG4 and MG5 was world-locked as opposed to being presented relative to the user’s head orientation. Due to the field of view (30°), some participants may have had to turn their head to follow holograms. This might have influenced speed variability. Other reason could be the aim of the mini-game. In MG4, some participants wanted to race with the butterfly, that may lead to speed variability as when some reached it, they slowed down. So, changing the spatial representation of MG4 from world-locked to body-locked could be a way to decrease this high variability.

User questionnaire
Each participant successfully understood the instructions during mini-games (median 5/5,IQR 0). None had previous experience with AR. Participants reported a high level of satisfaction about the game session (5/5,IQR 1). They rated the mini-games to be sufficiently efficient to reach the target speed (4/5,IQR 1). About the Hololens HMD, they did not report any limitations for walking (5/5,IQR 0), but they highlighted discomfort due to the weight (579g) bearing on the nose. Participants also reported some confusion about the MG5. Of note, certain participants hesitated to catch the token, particularly when it appeared in virtual trees. For participants, the ranking of the best mini-games was MG2, MG4 and MG3 in a tie, MG1 and last MG5.

CONCLUSIONS

This paper presented the collaborative design and evaluation of AR mini-games with visual feedback for gait training. This investigation demonstrated that gait parameters and user experience can vary depending on the type of AR feedback presented. Some feedback modalities increased WS whereas others had a larger impact on speed variability. Specific recommendations based on learnings in this study include using knowledge of results feedback to create a more challenging task that motivates participants to excel, body-locked holograms that are easier to track, and clarify game presentation. Currently, a new version of the game based on the MG4 but with body-locked characteristics, is being developed that will be tested with people with ND.

CLINICALTRIAL

This study is registered in the ClinicalTrials.gov database NCT04460833