Comparison of Movements in a Virtual Reality Mirror Box Therapy

for Treatment of Lower Limb Phantom Pain

Bartal Henriksen

, Ronni Nedergaard Nielsen

, Martin Kraus

and Bo Geng

School of Information and Communication Technology, Aalborg University, Rendsburggade 14, Aalborg, Denmark

Department of Architecture, Design and Media Technology, Aalborg University, Rendsburggade 14, Aalborg, Denmark

Department of Health Science and Technology, Aalborg University, Fredrik Bajers Vej 7, Aalborg, Denmark

Keywords: Virtual Reality, Phantom Limb Pain, Mirror Box Therapy.

Abstract: Many patients experience Phantom Limb Pain (PLP) after an amputation. Traditional Mirror Box Therapy

(MBT) has proven efficient for some patients, but the movements in MBT are physically limited, for lower

limb amputees. In this work, we investigated how anti-symmetrical mirroring compares to regular mirroring

in Virtual Reality (VR) MBT for lower limb amputees, as natural leg movements are anti-symmetrical, like

walking, running, and cycling. To motivate the patients, a game was developed that uses cycling and

swinging movements. We implemented the required movements into a goal-oriented game where the patient

must fly a gyrocopter through goal areas. The experiment was implemented as a within-subject design,

where the participants had to try three versions of the game and give preferential feedback. The findings

showed that the cycling version was more exhausting than the anti-symmetrical and symmetrical swinging

versions. Furthermore, we discovered that the required cycling motions were too difficult and tiresome to do

over a longer period of time. On the other hand, we found that it is possible to use anti-symmetrical

swinging of legs in VR MBT.

1 INTRODUCTION

When people have a limb amputated due to accident

or surgery, a condition called Phantom Limb Pain

(PLP) can occur. PLP is the pain experienced in the

amputated limb and occurs for up to 85% of all

amputees (Kooijman et al., 2000; Sherman, Sherman

and Parker, 1984). The PLP can relate to a certain

movement or position of the phantom limb, as well

as physical factors, such as: weather changes or

pressure on the residual limb (Flor, 2002). PLP is

not the same as physical pain and therefore the same

treatments, such as local anaesthesia and muscle

relaxant, are not effective for it (Flor, 2002).

Different hypotheses to why PLP occurs have been

explored, but the reason is still not known.

However, a method called Mirror Box Therapy

(MBT) has shown promising results in reducing PLP

for some patients (Ramachandran and Rogers-

Ramachandran, 1996). The MBT works by

providing the patients with an illusion that they

regain the missing limb. This is achieved by placing

a mirror perpendicular to the patient. The intact limb

is then mirrored onto the phantom limb. If

positioned correctly, this creates the illusion that the

amputated limb is still present. Why MBT is

successful is unclear, but one speculation is that

mirror illusion resolves the conflict between missing

visual feedback and movement intention, which

might cause PLP (Flor, 2002; Ramachandran and

Hirstein, 1998). The MBT is used both for lower

limb and upper limb amputees. In the MBT for

lower limb amputees, more requirements are needed

to establish the illusion. The patient needs to sit on a

bed or a couch and must look into a mirror from a

specific angle to establish the illusion. If the patient

moves or looks away, the illusion breaks down. The

patient further has limited movement, as the sitting

position requires them to bend forward to look into

the mirror, while putting their weight onto the leg.

1.1 Related Work

While the traditional MBT has produced promising

results for treatment of PLP, there are potential

advantages of developing a virtual MBT concept. In

a virtual environment (VE), there are less physical

restrictions to maintain the illusion of the virtual

Henriksen, B., Nielsen, R., Kraus, M. and Geng, B.

Comparison of Movements in a Virtual Reality Mirror Box Therapy for Treatment of Lower Limb Phantom Pain.

DOI: 10.5220/0006537801670174

In Proceedings of the 13th International Joint Conference on Computer Vision, Imaging and Computer Graphics Theory and Applications (VISIGRAPP 2018) - Volume 1: GRAPP, pages

167-174

ISBN: 978-989-758-287-5

167

limb in the mirror, as the patient does not need to sit

in a fixed position. Furthermore, virtual MBT also

enables the possibility to apply the MBT exercises

into more meaningful tasks and in more motivating

environments, such as games.

The concept of a virtual MBT has been explored

in the past decade, together with the advancing

technologies in motion sensors and the use of head-

mounted displays (HMDs) for depth perception in

virtual and/or augmented environments. Several

virtual MBT concept proposals have been developed

and a few systems have been tested with PLP

patients. These concepts use an HMD or custom

made display setups to visualize the system, and

while most concepts use a Virtual Reality (VR)

representation (Cole et al., 2009; Murray et al.,

2006; Osumi et al., 2017; Sato et al., 2010;

Zweighaft et al., 2012), a few have also developed

augmented reality systems that implement a virtual

limb to a representation of the stump of the patient

(Ortiz-Catalan et al., 2014; Trojan et al., 2013). To

track the movements of the patients, several systems

have been proposed, such as applying motion-

detection gloves, motion sensors, or pre-rendered

motions. The tested systems have shown promising

results, as the patients have experienced increased

control of the phantom limb by using some these

systems (Cole et al., 2009; Murray et al., 2006;

Osumi et al., 2017), and others have experienced a

short-term reduction in pain (Osumi et al., 2017;

Sato et al., 2010) as a result of the VR experiences.

In MBT, the exercise types used by the patients

vary depending on the pain and disability state of

individual patients (Brodie et al., 2003). For upper-

limb amputees the exercises include finger

movements, hand rotations and arm movements. The

patient usually starts with a small basic movement.

The movement is then repeated until the patient feels

comfortable and ready for a more challenging move.

A similar approach is used for lower-limb amputees.

The exercises consist of bending the knee, moving

the foot, rotating the ankle and toe movements

(Brodie, Whyte and Waller, 2003).

Even though the amount of lower-limb amputees

is a lot higher than upper-limb amputees (Brodie,

Whyte and Waller, 2003), most studies have focused

on developing different meaningful movements for

upper-limb amputees, such as: basic movement of

the upper limb (Murray et al., 2006; Ortiz-Catalan et

al., 2014; Osumi et al., 2017), punching an object

(Murray et al., 2006), grabbing and releasing objects

(Cole et al., 2009; Sato et al., 2010; Zweighaft et al.,

2012) and flexing the fingers (Trojan et al., 2013).

But there have only been a limited set of meaningful

movements explored for lower-limb amputees, such

as kicking an object or tapping a drum pedal (Cole et

al., 2009; Murray et al., 2006). Furthermore, while

the concepts proposed try to apply meaningful

movements to the MBT exercises, most focus on

applying standard mirrored movements. This makes

sense as mirrored movements often occur when

using both upper-limbs at the same time. The

movements used with the phantom limb in the MBT,

have a greater effect if they “feel” natural for the

patient for them to be convinced they are moving

their phantom limb (Henriksen et al., 2016). In this

study, three different mirrored applications were

developed: (1) grabbing and releasing boxes, (2)

pressing mirrored buttons and (3) a “bending game”

in which the patient had to bend a virtual rod with

both hands into different angles. While all three

versions were able to apply meaningful movements,

only the bending game was able to activate the

feeling of using the phantom upper-limb when tested

on an amputee (Henriksen et al., 2016). Based on

these results, it was hypothesized that by applying

mirrored movements, which required the use of both

lower-legs would help lower-limb patients feel they

used both their intact and phantom leg, when playing

the games (Nielsen et al., 2017). Nielsen et al.

developed two different games: (1) a shape game,

where the user needed to move both feet into

symmetrical square boxes in different angles, and

(2) a slingshot game, where the user needed to grab

the handle of a slingshot by doing a grabbing motion

with their toes, aim the handle by moving their feet

and release the handle by releasing their toes.

However, following an experiment with healthy

subjects and a test with a lower-leg amputee, it was

discovered that the mirroring movements were not

enough, as the movements were either too abstract

or too mundane (Nielsen et al., 2017). It should be

noted that this experiment also indicated the games

were constraining to do as they required the

participants to move their legs up in the air, in order

to do the required movements. Therefore, in order to

develop a VR MBT concept that encourages the use

of both lower-limbs, the concept must use motions

that feel natural to do with both legs. Furthermore,

the setup needs to be designed so it does not

constrain the legs of the user, as these constraints

would interfere with the overall experience and may

stop the user from using both legs.

While the long term goal is to design a VR

version of MBT to alleviate PLP, the goal for this

work is to encourage movement of both legs, and

engage users by having them play a game while

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168

using both legs. The focus of this work is on lower-

limb amputees and anti-symmetrical movements

2 METHOD

To compare symmetrical and anti-symmetrical

movements, a system using VR technology that

applied mirroring methods other than standard

mirroring for lower-limb MBT was developed. The

system uses a HTC Vive Head-Mounted Display

(HMD) to visualise the virtual environment. The two

HTC Vive controllers are attached to the left leg of

the participants in specific positions: one on the

thigh above the leg and one on top of the foot as

seen in Figure 1.

Figure 1: The setup of the experiment. The participant is

viewing the game through the HMD while controlling the

avatar through the two controllers attached to the left leg.

Through this setup, the user is able to control a

high detailed virtual avatar within the VE which is

positioned in the same position as the user. Two

avatar models were available; a male and a female.

The avatar is positioned so the user views the avatar

from the head position and is adjusted in size to fit

the user prior to game start. The virtual upper body

follows the HMD position while the pelvis is

stationary in a seated position. The controlled leg

follows the sensory input from the two controllers,

while the other leg moves based on one of three

movement: two anti-symmetrical movements and a

regular mirrored version for comparison: (1) Anti-

symmetrical swinging version: This version is based

on a simple anti-symmetrical movement, such as

walking, as the user needs to move the controlled leg

in a back and forward swinging motion. The

mirrored limb then follows this motion in an anti-

symmetrical manner. (2) Cycling version: This anti-

symmetrical version is based on applying cycling

motions. Here, the user needs to perform circular

motions with the controlled leg. The mirrored leg

then performs inverted movements, so the leg will

do anti-symmetrical movements both in the up/down

and forward/backwards directions. (3) Symmetrical

swinging version: In this version the user needs to

move the controlled leg in a back and forward

motion, similar to the anti-symmetrical version.

However, here the mirrored leg follows this motion.

The purpose of this version is to use as a control

version for comparison. The user perspective of the

avatar within the VE can be seen in Figure 2.

Figure 2: The avatar representation in the game. Left: the

field of view of the user; right: illustration of how the

avatar is seated in the gyrocopter.

When the games starts, the user applies one of

the movements to control the virtual avatar.

Whenever the user applies half a cycle of the given

movement, the altitude of the gyrocopter is

increased. If no movement is applied, the gyrocopter

slowly starts losing altitude. The gyrocopter moves

forward automatically in a steady pace. The user is

motivated to apply these movements correctly and in

a timely fashion in order to hit yellow transparent

checkpoints throughout the game in order to get as

menu checkpoints as possible before the finish line.

The three leg movements each apply the same

amount of altitude and the level is the same for each

iteration.

3 EVALUATION

This experiment was conducted with 18 participants

(aged 21 to 27, mean 25.9 years, 5 females and 13

males). The participants were mainly university

students from various educations. All participants

had at least some experience with using Virtual

Reality prior to this experiment. The experiment was

Comparison of Movements in a Virtual Reality Mirror Box Therapy for Treatment of Lower Limb Phantom Pain

169

conducted as a within-subject design as all

participants were instructed to play through all three

versions of the game. The order of the versions was

randomized in a counterbalanced order, to remove

any carry-over effects. Besides determining the

exertion rate of the three versions, another focus in

this experiment was to evaluate how much the users

applied movement to both legs, to find out if the

versions encouraged movement of both legs.

Furthermore, the anti-symmetrical movements,

which are not applicable in a standard MBT session,

were compared to the symmetrical version.

Therefore, this work focused on evaluation of the

two following aspects:

(1) The exertion rate of each movement was

measured, following the completion of the level for

each movement version.

(2) The amount of movement applied to both legs

during each version of the game were evaluated, in

order to compare the two anti-symmetrical versions

to the mirrored version.

3.1 Setup

The participant of the experiment sits on the edge of

a high table when playing through the three

iterations of the game in order to reduce the amount

of constrains in the legs. They are equipped with two

HTC Vive controllers on the left leg and attached the

HMD thereafter. It was decided to apply sensor

controls to only the left leg in order to avoid an

internal validity conflict as having sensors on both

legs could encourage the participants to apply

movement to both. Instead a camera setup was used

to record the leg movements for all participants in

each iteration. The footage was used to check if the

participants used one or both legs to apply the

movement cycles and to count how many of each

movement cycles were applied. It should be noted

that only a complete cycle using both legs counted

as an accepted applied movement cycle. Following

the completion of a single playthrough, the

participants answered questionnaires concerning the

exertion rate of the given version through a Borg

rating scale, and to rate the leg movements through

Likert-scale questions.

3.2 Procedure

The participants were initially instructed to sign a

consent form and to read a short description of the

test which included instructions on how to control

the gyrocopter using the three different movement

versions and an overview of the different features in

the panel of the gyrocopter. The description further

clarified the possibility to play the game using either

one or both legs, to reduce bias towards one

approach. The participants were hereafter equipped

with the controllers and HMD, and be seated in the

correct position. Following the introductory part, the

participants would complete the same game in each

of the three iterations, in a counterbalanced order.

Completion of the level took about 3.5 minutes.

Following each version, the participants were

instructed to complete a questionnaire, to get the

exertion results while they were still in their recent

memory and to get a short break from the game.

After all three game iterations were completed, the

participants completed the post-questionnaire.

4 RESULTS

The symmetrical and anti-symmetrical swinging

versions did not have a high exertion rate as their

respective mean value were 9.06 for the symmetrical

and 9.56 for the anti-symmetrical version, with 9

being relative to walking slowly, as seen in Figure 3.

The cycling version however received a mean

answer of 13.39, which is about “hard, but

manageable” and a median of 14. Following each

iteration, the participants were instructed to explain

if they experienced any discomfort during the level.

In the symmetrical and anti-symmetrical swinging

versions, two participants experienced discomfort,

while 16 did not. In the cycling version, ten

participants explained they felt a type of discomfort,

while eight did not. The main discomfort types

included: (1) strained muscles, (2) sweating under

the head-mounted display and (3) sweating in

general. No participants experienced nausea in any

of the three versions.

Figure 3: The exertion of each of the three versions, based

on a BORG scale rating.

Following the three iterations of the game the

participants were instructed to rank the three

versions in terms of exhaustion. They were asked to

rank the three versions from (1) least exhausting, (2)

second most exhausting and (3) most exhausting. As

seen in Figure 4, the symmetrical version was

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chosen as the least exhausting version with 11

choosing this version and 6 choosing the anti-

symmetrical swinging version. This corresponds

with the exertion rate data, as the symmetrical and

anti-symmetrical swinging versions are rated close

to each other in the Borg scale for the exertion rate,

but with the symmetrical version being less

exhausting. The cycling version was ranked as the

most exhausting version by 17 of 18 participants,

which also corresponds with the results from the

Borg scale.

Figure 4. The three versions ranked in terms of

exhaustion, ranked from (1) least exhausting, (2) second

most and (3) most.

4.1 Leg Movements

The leg movements of the participants were filmed

to evaluate the use of both legs during each of the

three versions of the game. In each version, the

number of completed cycles were counted, and split

into either “one leg” counters or “both leg” counters.

The results from the data set includes: (1)

Symmetrical swinging: In the symmetrical version

both legs were used overall 92.5% of the time, as 15

of the participants used both legs for all cycles, and

two used both legs during most of the cycles and one

used both legs in only 8.3% of the time, as seen in

Table 1, row (1). It was further noted that three

participants alternated between using the correct

symmetrical movements to anti-symmetrical

movements in some cases, either in the beginning of

the level or when having to apply a high pace. (2)

Anti-symmetrical swinging: Overall the participants

used both legs 88.4% of the time during the anti-

symmetrical movements, as seen in Table 1, row (2).

Here, 13 of the participants used both legs at all

times, while three used both more than half of the

time and two only used both in about ⅓ of the time.

One participant was noted applying symmetrical

movements in the beginning, but changed to the

correct anti-symmetrical movements after a short

while. (3) Cycling: Overall both legs were used

96% of the time as 15 participants used both legs all

the time and 3 used both legs more than half of the

time. One participant only used one leg in one cycle,

while using both legs in higher paced areas. Even

though many used both legs, how they used the legs

varied. Two participants applied small circular

motions, while two applied flat swinging motions

which looked close to the anti-symmetrical motions.

Finally, one participant applied motions that looked

similar to running motions.

Since the data is not normally distributed, a

Friedman’s test was used to compare the results. The

use of both legs for the participants was not

significantly different between the three versions,

2(2) = 0.89, p > .05 (p = 0.64). Following each

version, the participants were instructed to evaluate

the leg movements through five Likert-scale

questions ranked from 1: strongly disagree to 7:

strongly agree. The participants found the leg

movements required in the symmetrical and anti-

symmetrical swinging versions easy to perform as

the mean answers were 6.22 and 6.33 respectively

with 17 of 18 participants agreeing in both

questions, as seen in Figure 5. The cycling version

received a mean answer of 4.37, which indicate the

participants did not feel the cycling version was

particularly easy to perform.

Figure 5: Question (1): “I found the leg movements easy

to perform”.

When asked if the symmetrical and anti-

symmetrical swinging movements felt natural to

perform the mean answer was 5.89 for both answers,

as seen in Figure 6, which indicates the participants

felt these versions were natural to perform.

However, the cycling version received mixed

answers with a mean of 4.22 and a large variance of

3.71, which indicates not everyone found the cycling

movements natural.

Figure 6: Question (2): “The leg movements felt natural to

perform”.

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171

When asked if the leg movements felt consistent

with their real leg movements, the symmetrical

version received a mean answer of 6.11 and the anti-

symmetrical swinging version 6.00, as seen in

Figure 7. Furthermore, in both versions 17 of 18

participants agreed to some degree on this question.

The cycling version generally got positive results as

the mean answer was 5.11 and 13 agreeing to some

degree.

Figure 7: Question (3): “The leg movement felt consistent

with my real leg movement(s)”.

4.2 Game Data

Following the three iterations of the game, the

participants were instructed to rank the games as (1)

favorite version, (2) second favorite and (3) least

favorite version. As seen in Figure 8, the anti-

symmetrical swinging version was the favorite game

version, as 11 chose this as their favorite and six

chose it as their second favorite. The cycling was

their least favorite, with 14 choosing this version as

their least favorite game version.

Figure 8: The preferred versions, ranked by (1) favorite

version, (2) second favorite version and (3) least favorite

version.

5 DISCUSSION

The participants used both legs to a high degree in

all three versions and there was no significant

difference between the three versions in this regard.

This indicates that all versions do encourage the

use of both legs. However, as the experiment was

conducted using only healthy participants, the high

usage of both legs could be influenced by predefined

proprioceptive patterns within the participants, i.e.

they are used to applying these movements in real

life. With amputees the high degree of movement

may be lower as some amputees are not used to

apply movements with both limbs. However, as

many amputees are walking using prosthesis, or

doing swimming exercises and similar, these

movement patterns are not completely unfamiliar.

Furthermore, the high degree also shows that the

movements are intuitive to apply, which would help

encourage the use of both limbs for amputees and

thus help reduce phantom pain.

As the three versions controlled the gyrocopter

through the same system, the results indicate that

applying the cycling motions was a lot harder,

compared to the other two versions. One reason for

the high exertion rate for the cycle game could be

the increased amount of movements overall, as the

participants needed to lift their legs besides applying

the swinging motions. Furthermore, it was noted that

the participants used different approaches to apply

the cycling motions; some applied small circular

motions and did well in the game, while others

would apply very flat cycling motions which did not

help performing the cycle correctly. A reason for the

difficulties applying the cycling motion could be of

the lack of physical pedals. It should also be noted

that the large variability within how the participants

applied the motions could have had an effect on the

overall result, as the exertion rate differed greatly

between these approaches.

Regarding the symmetrical and anti-symmetrical

swinging versions, both received a low exertion rate

score on the Borg scale, with the symmetrical

version being marginally lower. However, the anti-

symmetrical version was favored by most

participants. Furthermore, it should be noted that

Table 1: How much both legs were used in full cycles, in percentage. First line (1) is the symmetrical version, (2) anti-

symmetrical and (3) is the cycling version.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

(1) 100 8.3 100 100 100 81.4 100 100 100 100 73 100 100 100 100 100 100 100

(2) 100 100 100 100 89.1 50.6 100 100 77.1 100 100 100 37.4 100 36.1 100 100 100

(3) 100 87.3 100 100 64.8 100 100 77.1 100 100 100 100 100 100 100 100 100 100

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some participants would change from symmetrical

to anti-symmetrical movements within the same

level. This could indicate that the anti-symmetrical

swinging motions indeed are natural movements,

though not significantly more than the symmetrical.

Overall these results show a promising result,

that it is possible to apply natural movements within

a virtual environment which are not possible through

the standard MBT. However, as the experiment was

conducted only with healthy participants the degree

of using both legs might have been influenced, as

their mobility of both legs cannot be compared to

amputees.

6 CONCLUSIONS

One of the goals of this work was to test if the three

chosen movements symmetrical swinging, anti-

symmetrical swinging, and cycling, encouraged the

use of both legs when playing the game. This was

evaluated by counting the percentages of how much

the participants used both legs in each game. The

evaluation showed that all three games did

encourage the use of both legs, as the symmetrical

version resulted in 92.5%, the anti-symmetrical

swinging resulted in 88.4% and the cycling resulted

in 96%.

Another focus of the project was to compare if

the anti-symmetrical movements were more natural

to perform, compared to the standard symmetrical

version. To this end, the use of both legs in the three

versions was compared. This did not show any

significant difference between the versions in using

both legs. Furthermore, the versions were compared

by having the participants rank the versions. Here,

the anti-symmetrical swinging version was the

favored version. This indicates that the anti-

symmetrical swinging movements are at least as

favorable to apply as the standard symmetrical. The

cycling, however, was not well received and the

participants did not find it easy to perform. The

symmetrical and the anti-symmetrical swinging

version did not seem to be too exhausting to

perform. On the other hand, the cycling version was

significantly more exhausting to perform. This

finding suggests that the cycling version cannot be

used for VR MBT, because the patients have to

perform these exercises over a longer period of time.

To address the issue of the cycling version, a

physical pedal could be used where patients could

feel some resistance while pushing the pedal.

Furthermore, the patients would be guided by the

pedal and would perform a correct sequence every

time.

Overall, it can be concluded that anti-

symmetrical swinging movements can be used in the

same manner as symmetrical swinging movements

in VR MBT for lower-limb amputees, which would

not be possible through the standard physical MBT.

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