Clinical relevance and future research

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Methods

Subjects

Twelve volunteers participated in this study including 8 females and 4 males. All subjects were students or employees at University Medical Center Groningen (UMCG) and were between the ages of 20 and 30 years as in the previous study (Jeschke, 2017). The participants would be excluded, if they had any self-reported physical problems that could affect the walking pattern or were using any assistive walking devices or orthoses. Since the study was conducted in Dutch, it was required for the participants to be fluent in Dutch to receive the information given before and during the trial. Furthermore, to be eligible for this study, the subjects could not have impaired hearing or vision, due to the demands of the interventions and also needed to be unaware of the purpose of the study and the outcome measures.

Ethical considerations

All subjects participated voluntarily according to the Helsinki Declaration (World Medical Association, 2013) and were verbally informed about the study before signing the written informed consent form (Appendix 1) prior to the beginning of the study. Information about the participants was treated with confidentiality throughout the entire process including the final project and presentation. Since the study is not a medical research study, but a technological research study, it was considered not to expose the subjects to any health-related risks. The study was approved by the local Medical Ethics Committee (2017/188).

Study design

In this experimental study, all measurements were gathered during only one session with each subject. All subjects were assigned to an intervention group where they would function as their own control since no control group was formed. The subjects would be evaluated during each intervention in the trial and the study does therefore not fit perfectly in one of the standard study designs explained in the ”Study Design Classification Scale” (Hafner, 2008). This type of study is categorized as a repeated measure within subject design which resembles the “controlled before-and-after trial” (Hafner, 2008), with the difference being when the subjects are evaluated and the outcome measures assessed.

Experimental setting

The study took place at UMCG Beatrixoord in Haren, The Netherlands where all subjects were to walk on a self-paced dual-belt treadmill for a total of 18 minutes and 40 seconds, while a VR environment was projected on a 180° semi-cylindrical screen and onto the treadmill (GRAIL, Motek Forcelink BV, The Netherlands), showing a straight infinite path leading to a mill with a house next to it (Figure 3A). Before walking on the treadmill, the subjects’ balance was determined, and they were tested if they suffered from any color vision deficiencies through Romberg’s and Ishihara’s test respectively. Furthermore, the subjects’ preferred gait velocity was calculated by measuring the time it took for them to walk ten meters and they filled out the Physical Activity Readiness Questionnaire (PAR-Q) and a small questionnaire regarding their hobbies, sports, surgery and medication (Appendix 2). Afterwards, while walking on the treadmill, the subjects were to perform seven of eight different tasks called the interventions. During these interventions, subjects would perform a color-word Stroop task projected with words appearing on a white plane (40 x 140cm) with an interval of 1,5 seconds at the height of 165cm (Figure 3B) as in the previous study by Jeschke (2017). All subjects kept walking during their trial and all tasks were performed in Dutch. The eight different interventions are explained below under the section experimental conditions and interventions.

Experimental conditions and interventions

Ioriginal1 and Ioriginal2: This was an intervention similar to the original intervention described by Jeschke (2017). Here the subjects were to perform the color-word Stroop task without any feedback or other interferences. This intervention was given to all the participants as the first (Iorginal1) and last intervention (Ioriginal2) in the trial, to enable a within subject comparison to detect any effects that could be caused by tiredness, learning or a decline in concentration.
IzerolineA: This intervention consisted of the subjects performing a color-word Stroop task without being informed about the fact, that the acceleration-line was moved 15 cm to the back. The acceleration-line is the line on the treadmill that determines the area for acceleration and deceleration. If the subjects move in front of this line the treadmill will accelerate, and if they move behind this line the treadmill will decelerate. The aim of this intervention was to investigate if the increase in gait velocity when performing a color-word Stroop task while walking self-paced on a dual-belt treadmill in a VR environment (Jeschke, 2017) could be caused by the subjects trying to place themselves more in the forward direction on the treadmill, thereby making the increase in gait velocity a consequence of the subjects’ position. By moving the acceleration-line more to the back, it would be possible to do a within subject comparison between this intervention, IzerolineB and Ioriginal1, which has an acceleration-line at 0, to investigate how the subjects place themselves and with what self-selected gait velocity.
IzerolineB: This intervention consisted, like IzerolineA, of the subjects performing a color-word Stroop task without being informed about the fact that the acceleration-line was moved 15 cm to the front. The aim of this intervention was like with IzerolineA, to investigate if the found increase in gait velocity (Jeschke, 2017) was caused by the subjects placing themselves in a more forward direction on the treadmill making the increase in gait velocity a consequence of this more forward position. By moving the acceleration-line more to the front, it would like with IzerolineA, be possible to do a within subject comparison between this intervention, IzerolineA and Ioriginal1 to investigate the theory illustrated in figure 1.
Ineutralzone: In this intervention, the subjects performed a color-word Stroop task while walking self-paced on the treadmill, which was provided with a neutral zone ranging from 20 cm in front to 20 cm behind the original acceleration-line from Ioriginal1. The neutral zone consisted of an area in which the acceleration and deceleration was not active, resulting in the subjects having to cross the borders of the neutral zone to be able to change their gait velocity. The aim of this intervention was to investigate if the found increase in gait velocity (Jeschke, 2017) could be caused by the subjects positioning themselves in a more forward direction on the treadmill. By giving the subjects an area in which they would be able to place themselves without it affecting their gait velocity, it would be possible to do a within subject comparison between this intervention and Ioriginal1 to investigate the differences in gait velocity and position.
Ivisual: In this intervention, the subjects performed a color-word Stroop task while walking self-paced on the treadmill with the neutral zone and visual feedback active, thereby providing the subjects with real-time feedback on their whereabouts on the treadmill. The visual feedback consisted of the blue sky in the VR environment fading to red the closer the subjects got to the back edge of the treadmill, while fading to green the closer the subjects got to the front edge of the treadmill (Figure 3C and 3D). The fading of the visual color feedback began at the borders of the neutral zone at 20 cm in front and 20 cm behind the acceleration-line, as illustrated in figure 2. The purpose of this intervention was to explore, if the increase in gait velocity discovered by Jeschke (2017) could be caused by the subjects not knowing their position on the treadmill because of the lack of a reference point affecting the subjects to place themselves in a more forward direction on the treadmill thereby leading to the found increase in gait velocity. By providing the subjects with real-time visual feedback on their position on the treadmill, it  would be possible to investigate how having the opportunity of knowing your position on a treadmill affects gait velocity and position though a within subject comparison between this intervention and Ineutralzone. Since both the visual feedback and the color-word Stroop task required visual attention, the feedback intervention was also done having audio feedback instead, making the subjects use another sense to perceive the feedback given on their placement on the treadmill.
Iaudio: In this intervention, the subjects performed a color-word Stroop task while walking self-paced on the treadmill with the neutral zone and audio feedback active, thereby providing the subjects with real-time audio feedback on their position. The audio feedback consisted of a sequence of beeps increasing in frequency the closer the subjects got to the front or the back edges of the treadmill thereby functioning as the park assist in a car. The first beeps would begin at the borders of the neutral zone at 20 cm in the front and 20 cm behind the acceleration-line and hereafter increase in frequency the further away from the acceleration-line the subjects got. By providing the subjects with real-time audio feedback, it would like with Ivisual be possible to investigate how having the opportunity of knowing your position on a treadmill affects position and velocity and it if was possible to substitute the lack of a reference point through a within subject comparison between Ineutralzone and this intervention.
Ifalsevisual: In this intervention, the subjects performed a color-word Stroop task while walking self-paced on the treadmill with the neutral zone active and a false version of the visual feedback active. The false version of the visual feedback differed from the normal visual feedback intervention by changing the borders defining where the visual feedback would begin. The back border of the feedback would not be changed do to security reasons and remained therefore at the same distance from the acceleration-line at 20 cm behind this line. The front border of the false visual feedback was moved from 20 cm in front of the acceleration-line to exactly at the acceleration-line to let the feedback start sooner when the subjects would move forward thereby making the subjects believe that they were more anterior placed on the treadmill than they actually were, as illustrated in figure 2. The aim of this intervention was to investigate if the subjects would position themselves based on the information given through the feedback about their placement and if it was possible to place the subjects more in the back on the treadmill than they would otherwise do themselves because of the shorter distance to the back edge.
Ifalseaudio: In this intervention, the subjects performed a color-word Stroop task while walking self-paced on the treadmill with the neutral zone active and a false version of the audio feedback. The purpose of this intervention was, like with Ifalsevisual, to investigate if the subjects would position themselves based on the audio feedback given and if it is possible through providing them with false audio feedback on their position to make them place themselves more posterior on the treadmill than they otherwise would on their own. The false version of the audio feedback consisted of changing the borders from where the audio feedback would begin. These borders would be positioned at the same place as the borders in Ifalsevisual with the back border 20 cm behind the acceleration-line and the front border at the acceleration-line to enable the feedback to starter sooner, when the subjects would move more to the front on the treadmill. This way the false audio feedback would inform the subjects on being more anterior placed on the treadmill than their actual position, making it possible to do a within subject comparison between this intervention and Iaudio to investigate how this affects the subjects’ position on the treadmill.
All interventions lasted for 60 seconds and all subjects were exposed to all interventions except the intervention including false feedback. Here the participants would either receive the false audio feedback or the false visual feedback depending on which normal feedback intervention order they were randomized to. Since it could be expected that the subjects would gain too much information about the study’s intentions by performing the false feedback intervention, the false feedback intervention regardless which one they received, was in the end of the trial only followed by Ioriginal2.
Before each intervention the subjects were exposed to a control condition (CC) consisting of walking for 20 seconds at a fixed velocity of 1 m/s followed by walking self-paced for 60 seconds without any kind of Stroop task being displayed at any time. The fixed velocity was used to reset the subjects’ gait velocity after each intervention while the self-paced velocity was used to determine the subjects’ preferred gait velocity when they were not performing a color-word Stroop task and to examine, if there was an increase in velocity when performing a color-word Stroop task by comparing it with the subsequent intervention.
Furthermore, some of the control conditions included a learning period, where the subjects would walk self-paced for 60 seconds while being exposed to either the neutral zone, the audio feedback or the visual feedback after having walked 20 seconds at the fixed speed of 1 m/s. These control conditions including a learning period were called CCneutralzone, CCaudio, CCvisual, CCaudio2 and CCvisual2 and were used to let the subjects get familiar with the neutral zone and the feedback, before using them while performing the color-word Stroop task in the interventions. The difference between CCaudio/CCvisual and CCaudio2/CCvisual2 was that CCaudio and CCvisual included a pre-set voice in the beginning, informing the subjects how the feedback worked by saying either “you will now be provided with real-time visual feedback on your placement on the treadmill, the sky will fade to red the closer you get to the back edge of the treadmill, while it will fade to green the closer you get to the front edge” or “you will now be provided with real-time audio feedback on your position on the treadmill, a sequence of beeps will increase in frequency the closer you get to the front or back edge of the treadmill”. This information was not given during CCaudio2 and CCvisual2 since these control conditions would always be given as the last control condition only followed by a normal CC.
Below is a flowchart explaining one of the possible combinations for the trial when the interventions were randomized. The color of the different interventions explains the possible randomization for the trial.

Introduction
Background
Aim of study
Methods
Subjects
Ethical considerations
Study design
Experimental setting
Experimental conditions and interventions
Materials
Data analysis
Statistical analysis
Results
Velocity
Position
Discussion
Discussion of results
Ioriginal1, IzerolineA, IzerolineB
Ineutralzone, Ivisual, Iaudio, Ifalsevisual, Ifalseaudio
Discussion of methods
Subjects and generalizability
Overground walking
The neutral zone
Validity and reliability
Subjects-expectancy effect
Limitations
Clinical relevance and future research
Conclusion
Acknowledgments
References
Appendix
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