J. Phys. Ther. Sci. 36: 372–377, 2024

The Journal of Physical Therapy Science
Original Article

Analyzing muscle thickness changes in lateralabdominal muscles while exercising using
virtual reality
Masashi Kitano1, 4), Masami Nakamoto2, 3)*, Kengo Kawanishi2, 3), Masahiko Hara2, 4),
Shintarou Kudo1–3, 5)1) Graduate School of Health Science, Morinomiya University of Medical Science, Japan

2) Inclusive Medical Science Research Institute, Morinomiya University of Medical Sciences:

1-26-16 Nankokita, Suminoe-ku, Osaka-shi, Osaka 559-0034, Japan

3) Department of Physical Therapy, Morinomiya University of Medical Sciences, Japan4) mediVR, Inc., Japan

5) AR-Ex Medical Research Center, Japan

Abstract. [Purpose] Virtual reality (VR) rehabilitation has become popular in the medical field. VR-guided exercises (VR-ge) have demonstrated positive effects on gait and trunk control. Trunk muscle activation, particularlythat of the transversus abdominis (TrA), is responsible for these improvements. However, the difference in muscle
activation between VR and real space remains unclear. Therefore, this study aimed to clarify the differences intrunk muscle activation during exercise therapy performed in VR and real space. [Participants and Methods] A
total of 22 healthy male volunteers were divided into two equal groups: VR-ge and Control exercise (C-e) groups.
Both groups performed reaching exercises in a seated position. Ultrasound imaging was used to measure the thicknesses of the right external oblique, internal oblique, and TrA muscles, both at rest and during the reaching exercises
performed in six different directions. [Results] No significant differences were observed in TrA muscle thickness
changes between the groups before the intervention. However, after the intervention, the VR-ge group showed significantly greater TrA muscle thickness changes during reaching compared to that of the C-e group. [Conclusion]
VR-ge increased TrA activation during reaching compared to exercising in real space.
Key words: Exercise, Biomechanical phenomena, Virtual reality(This article was submitted Feb. 5, 2024, and was accepted Apr. 4, 2024)

INTRODUCTION
Rehabilitation using virtual reality (VR) has recently attracted attention in the medical field1). The mediVR KAGURA(mediVR Inc., Osaka, Japan) is an exercise therapy in which patients perform arm-reaching exercises in a seated position
against a target in VR space2, 3). Its effects include the extension of continuous walking distance in older patients with disusemuscle weakness after a 2-week intervention without an increase in lower limb muscle strength2). Additionally, studies haveshown that it improves abnormal pelvic and trunk movements during walking after hallux valgus surgery4). Thus, VR-guided
exercise improves gait function, despite being performed in a sitting position2, 4).
Electromyography (EMG) has shown that reaching exercises performed in a seated position activate the erector spinae andabdominal muscles5). Furthermore, stability during walking relies on the coordinated activation of the trunk muscles6), andthere is a positive correlation between trunk muscle thickness and walking speed7, 8). The transversus abdominis (TrA) is alateral abdominal muscle that plays a vital role in trunk control9). These findings suggest that activation of the trunk muscles,*Corresponding author. Masami Nakamoto (E-mail: nakamoto@morinomiya-u.ac.jp)©2024 The Society of Physical Therapy Science. Published by IPEC Inc.
This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives (by-nc-nd) License. (CC-BY-NC-ND 4.0: https://creativecommons.org/licenses/by-nc-nd/4.0/)372

especially the TrA, is involved in improving walking function through the VR-guided exercise2). However, the differencesin muscle activation between reaching movements in real space and those performed in VR conditions have not yet beenclarified.
Surface EMG is not suitable for evaluating the trunk muscles, which are composed of multiple layers. Instead, magnetic
resonance imaging10), computed tomography11), and ultrasound (US) imaging systems12) are used. Among these, the simplicity of US is useful during motion13). Muscle thickness changes at rest and during contraction, as assessed using US, canreflect muscle activity14, 15). Therefore, observing muscle thickness changes in the lateral abdominal muscles during reaching
movements in VR and in real space may help to clarify the mechanism of treatment efficacy of VR-guided exercise.
This study aimed to compare the changes in the thickness of the lateral abdominal muscles before and after reaching
exercises performed in VR and those performed in real space.

PARTICIPANTS AND METHODS
Twenty-two healthy male volunteers were included in this study. The inclusion criteria were no history of back pain,
injury, or surgery. The exclusion criteria were presence of internal diseases and difficulty in imaging the lateral abdominal
muscles owing to obesity. This cross-sectional study was approved by the Research Ethics Review Subcommittee of the
Academic Committee of Morinomiya University School of Medicine on July 4, 2022 (2021-137). This study was registeredin the University Hospital Medical Information Network Clinical Trials Registry (UMIN-CTR ID: UMIN000048720). All
study participants were informed of the experimental details and provided written informed consent to participate.
The participants were randomly and equally assigned to the VR-ge and C-e groups using the block substitution method(permutation block). Blinding was not possible because an interventionist administered the outcome measures. In the VR-ge
group, mediVR KAGURA (mediVR, Inc., Osaka, Japan) was used as the intervention method. After the participants wereplaced in the end-sitting position, they wore a head-mounted display and grasped the controller using both hands. The reaching directions were set to six directions (forward, 45° diagonally forward, and lateral) with the left and right upper limbs, andthe reaching distance was set at 90% of the maximum reaching distance. The reaching exercise comprised 150 “horizontal
games” (25 times in each direction) for a duration of 20 min (speed, 1 cm/s; outer edge of the circle, 20 cm radius from thecenter; inner edge of the circle, 10 cm radius from the center; controller’s sensing range, 2.0 cm radius). The therapist set thedirection of the target emergence to avoid movements requiring a 180° neck rotation, such as lateral to contralateral rotation.
Figure 1 depicts the VR image and reaching scene. In the C-e group, the participants were placed in an end-sitting position
and grasped the controller with both hands, but did not wear the head-mounted display. Similar to the VR condition, thereaching direction was set to six directions, with a maximum reaching distance of 90%. The reaching motion was performed
against the targets described previously (circle: outer edge of the circle [circular ring], 20 cm radius from the center; inneredge of the circle, 10 cm radius from the center).
The outcome measures included measurements of the right external oblique (EO), internal oblique (IO), and TrA muscles,
taken using an US imaging system. The transducer was positioned on the anterolateral aspect of the abdomen, midway
between the iliac crest and lower edge of the thorax, with short-axis scanning. The medial end of the transducer was placed
approximately 10 cm away from the midline (Fig. 2)16). The seven measurement conditions were a resting sitting position
and a posture in which the patient reached the target point in six directions with the right and left upper extremities (resting,ipsilateral forward, ipsilateral diagonally forward, ipsilateral lateral, contralateral forward, contralateral diagonally forward,
contralateral lateral [Fig. 3]). The outcome was defined as the difference in muscle thickness changes in the EO, IO, and TrA
during reaching from rest to each of the six directions.

Fig. 1. Virtual reality image and actual exercise scene during the implementation of the exercise using mediVR KAGURA.

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A preliminary study was conducted beforehand17), and G*power software (version 3.1.9.7; Franz Faul, University of Kiel,
Kiel, Germany) was used to calculate the sample size. This calculation estimated that 11 individuals were required in eachgroup to identify significant differences between the groups. Repeated-measures two-way analysis of variance was used toassess the influence of the two factors, group (VR-ge and C-e groups) and timing (pre- and post-intervention), on the amountof muscle thickness change at each reach. Significant interactions were assessed, and if a main effect was found, a paired
t-test was performed for the time period, and an uncorrelated t-test was performed for the group. The significance level was
set at 5%. Jeffrey’s Amazing Statistics Program version 0.17.2.1 (University of Amsterdam, Amsterdam, The Netherlands)
was used for statistical analyses.

RESULTS
Ultimately, 22 participants were enrolled in this study and randomized to either the VR-ge (n=11) or C-e (n=11) group(Fig. 4). No significant differences in the physical characteristics were observed between the two groups. The participant
information is shown in Table 1.
There was no significant difference in the change in muscle thickness of the ipsilateral TrA muscle between the groups
before the intervention (forward [VR-ge, 0.3 ± 1.1 mm; C-e, 0.2 ± 1.0 mm], diagonally forward [VR-ge, 0.3 ± 1.3 mm; C-e,
0.7 ± 1.2 mm], lateral [VR-ge, 0.0 ± 1.3 mm; C-e, 0.4 ± 1.2 mm]). After the intervention, significant differences in the muscle
thickness change were observed in the VR-ge group (forward [VR-ge, 1.6 ± 2.0 mm; C-e, 0.0 ± 0.7 mm]; diagonally forward

Fig. 2. Ultrasound image of the lateral abdominal muscles and the site of measurement.
TrA: transverse abdominal muscle; IO: internal oblique; EO: external oblique.

Fig. 3. Each reaching direction in the intervention.
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Fig. 4. Disposition of participants in the study.

Table 1. Baseline demographics of the study participantsAge (years)
Height (cm)Body weight (kg)

VR-ge

C-e

20.9 ± 1.8
170.7 ± 4.5
57.5 ± 5.4

21.4 ± 2.8
169.6 ± 4.8
62.0 ± 5.8*Significant difference p<0.05 by unpaired t-test.
VR-ge: virtual reality (VR)-guided exercise group; C-e: control exercise group.

[VR-ge, 1.9 ± 1.6 mm; C-e, 0.2 ± 0.6 mm]; lateral [VR-ge, 1.6 ± 1.5 mm; C-e, 0.2 ± 0.7 mm]). Nonetheless, there was only asignificant group-by-time interaction in the TrA during the ipsilateral reach. The results of the two-way analysis of varianceare shown in Table 2. The changes in muscle thickness of the IO and EO did not differ significantly between the groups either
before or after the intervention.

DISCUSSION
In this study, we hypothesized that the change in the thickness of the TrA muscle would be greater during reaching
movements performed under VR conditions. As expected, the results of this study showed significant differences in TrA
muscle thickness changes during ipsilateral anterior, oblique, and lateral reaching movements in the VR space compared withreaching movements performed in real space. This is the first study to demonstrate that the muscle activity during posturalcontrol in VR environments may differ from that in real space.
A previous study showed the abdominal muscles, as well as the erector spinae muscles, were activated during seatedreaching5). ShahAli et al.18) have measured muscle thickness using an US system and EMG activity in the TrA, IO, and EO
during bracing and hollowing, and reported a positive correlation between the rate of change in muscle thickness and EMG
activity. Dieterich et al.14, 15) also reported that changes in muscle thickness reflected trunk muscle activity during resting andisotonic contractions using an US device. This study found that the TrA was more active in the transversus abdominis muscle
during seated reaching under VR conditions compared to real space conditions.

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Table 2. Changes in muscle thickness of the lateral abdominal muscles before and after the start of the intervention
VR-ge
Ipsilateral (mm)Forward

Diagonally forward

LateralContralateral (mm)Forward

Diagonally forward

Lateral

C-e

Pre

Post

Pre

Post

EO
IO
TrA
EO
IO
TrA
EO
IO
TrA

0.9 ± 3.4
1.2 ± 3.9
0.3 ± 1.1
1.6 ± 3.0
1.8 ± 5.3
0.3 ± 1.3
1.2 ± 3.3
1.9 ± 5.7
0.0 ± 1.3

−0.2 ± 2.5
1.7 ± 2.8
1.6 ± 1.5
0.8 ± 3.6
4.2 ± 5.3
1.9 ± 1.6
0.2 ± 3.1
5.0 ± 5.0
1.6 ± 1.5

0.5 ± 1.0
2.4 ± 3.7
0.2 ± 1.0
0.7 ± 1.6
2.6 ± 3.6
0.7 ± 1.2
1.5 ± 3.3
3.3 ± 5.5
0.4 ± 1.2

0.2 ± 2.8
1.8 ± 1.1
0.0 ± 0.7
−0.4 ± 0.9
2.8 ± 1.9
0.2 ± 0.6
−0.7 ± 2.4
2.8 ± 2.6
0.2 ± 0.7

EO
IO
TrA
EO
IO
TrA
EO
IO
TrA

0.5 ± 1.9
1.8 ± 1.7
0.4 ± 1.4
0.1 ± 2.5
−0.5 ± 2.8
0.1 ± 1.6
0.2 ± 1.8
−0.7 ± 2.4−0.1 ± 1.0

1.0 ± 2.6
1.4 ± 4.3
1.7 ± 2.0
−0.3 ± 2.8
0.4 ± 4.2
1.0 ± 2.4
0.2 ± 1.9
0.1 ± 3.8
0.4 ± 1.6

0.8 ± 2.6
2.9 ± 2.5
0.2 ± 0.8
0.0 ± 1.6
0.4 ± 2.1
−0.6 ± 1.0
0.3 ± 2.0
1.0 ± 3.1
0.2 ± 0.8

1.2 ± 3.7
3.2 ± 2.5
0.5 ± 0.7
−0.1 ± 0.8
1.8 ± 1.8
−0.1 ± 0.9
0.7 ± 1.7
2.4 ± 3.4
0.1 ± 0.7

*#†

†
#
#†

#

#

Values are represented as mean ± SD. *#†indicates test results by repeated-measures 2-way ANOVA, *Group:
Significant difference p<0.05, #Timing: Significant difference p<0.05, †Interactions: Significant difference p<0.05.
VR-ge: virtual reality (VR)-guided exercise group; C-e: control exercise group; TrA: transverse abdominal muscle;
IO: internal oblique; EO: external oblique; SD: standard deviation.

Previous studies have reported that VR rehabilitation exercise results in a significantly lower center of pressure comparedto that resulting from conventional balance exercise in healthy participants19), and that VR is more effective than vestibular
rehabilitation alone in improving vestibular function in vestibular dysfunction20). However, no study has yet measured theactivity of the lateral abdominal muscles during VR exercise. Therefore, this study presents novel results that VR-guided
reaching in a seated position increased TrA activation.
The TrA is one of the deep abdominal muscles involved in trunk control18) and is thought to play a role in the segmental
control of the lumbosacral spine during exercise. It also plays an important role in anticipatory postural adjustment (APA)21).
The feed-forward muscle activity patterns associated with APA are planned and coordinated by the central nervous system,
resulting in involuntary TrA contractions that contribute to trunk stability during early physical activity22). The TrA is also
involved in the control of the lumbosacral spine. During normal reaching exercises, target points and body positional information are provided through visual and proprioceptive feedback23). However, during reaching exercise performed in VR
space, it is not possible to visually perceive one’s own body, which may have resulted in the training of postural control basedon prediction.
This study has several limitations. First, the study participants were all healthy adults, and the results may differ for
older participants or those with central nervous system diseases. Further, the present study only confirmed the effects on the
thickness of the TrA muscle during treatment and did not investigate the effects on the performance of actual postural control
tasks; thus, further studies are required. Second, kinematic changes during the VR exercise therapy were not investigated,
including the muscle activity output of the TrA, which varies with the anterior-posterior pelvic tilt24). It is possible that thekinematic changes in the trunk and pelvis are higher in the VR group compared with those in the normal reaching group,and these kinematic changes need to be clarified in future studies. Finally, because the present study required imaging of
the lateral abdomen during reaching, the therapist and evaluator could not be blinded to the treatment groups, which might
have induced bias. However, because there was no change in the muscle thicknesses of the IO and EO, the inability to fully
implement blinding had little effect on the results.
The results of this study showed significant differences in TrA muscle thickness changes during ipsilateral anterior,oblique, and lateral reaching movements performed in the VR space compared to those performed in real space. This suggeststhat VR-guided exercise may activate the TrA during reaching movements.

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376

Funding

This work was funded by JSPS KAKENHI Grant Number JP21K17503.

Conflicts of interest

Masahiko Hara is a director of mediVR Inc., in which he holds private equity. Masashi Kitano is full-time employee of
mediVR Inc.

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