Association between age at onset of independent walking and objectively measured sedentary behavior is mediated by moderate-to-vigorous physical activity in primary school children
Association between age at onset of independent walking and objectively measured sedentary behavior is mediated by moderate-to-vigorous physical activity in primary school children
Shigeru Inoue 0 1 3
Chiaki Tanaka 0 1 3
Tomoko AoyamaID 0 1 2 3
Shigeho Tanaka 0 1 3
Maki Tanaka 0 1 3
Masayuki Okuda 0 1 3
0 a Current address: Japan Society for the Promotion of Science, Chiyoda-ku, Tokyo, Japan ¤b Current address: Department of Early Childhood Education, Kyoto Bunkyo Junior College , Uji-shi, Kyoto , Japan
1 Editor: William D. Phillips, University of Sydney , AUSTRALIA
2 Department of Nutritional Epidemiology and Shokuiku, National Institute of Health and Nutrition, National Institutes of Biomedical Innovation, Health and Nutrition, Shinjuku-ku, Tokyo, Japan, 2 Department of Nutrition and Metabolism, National Institute of Health and Nutrition, National Institutes of Biomedical Innovation, Health and Nutrition, Shinjuku-ku, Tokyo, Japan, 3 Department of Child Education, Kyoto Seibo College , Kyoto-shi, Kyoto , Japan , 4 Department of Environmental Medicine, Graduate School of Science and Engineering, Yamaguchi University , Ube-shi, Yamaguchi , Japan , 5 Department of Preventive Medicine and Public Health, Tokyo Medical University, Shinjuku-ku, Tokyo, Japan, 6 Division of Integrated Sciences, J. F. Oberlin University , Machida-shi, Tokyo , Japan
3 Citation: Aoyama T , Tanaka S, Tanaka M, Okuda M, Inoue S , Tanaka C (2018) Association between age at onset of independent walking and objectively measured sedentary behavior is mediated by moderate-to-vigorous physical activity in primary school children. PLoS ONE 13(9): e0204030
Data Availability Statement: All relevant data are
within the paper and its Supporting Information
Funding: This work was supported by JSPS
KAKENHI Grant Number JP24500832 to CT
(https://kaken.nii.ac.jp/en/grant/KAKENHIPROJECT-24500832/) and JP13J07359 to TA
(https://kaken.nii.ac.jp/en/grant/KAKENHIPROJECT-13J07359/). The funders had no role in
study design, data collection and analysis, decision
Age at onset of walking has been shown as an early predictor of physical activity in infants
and children. However, little is known about whether age at onset of walking may predict
sedentary behavior (SB). The aim of the present study was to examine the association
between the timing of onset of walking and objectively measured SB, and whether this
association is mediated by moderate-to-vigorous physical activity (MVPA) in children.
The subjects were 388 elementary school children aged 6±12 years. Current weight and
height data were collected. Birth weight and the age in months the child first walked
independently were reported based on the parents' recall. Children's SB and physical activity were
objectively measured using a triaxial accelerometer (Active style Pro HJA-350IT, OMRON).
The following summary outcome variables were derived from accelerometer data: Time
(min/day) spent in SB ( 1.5 metabolic equivalents [METs]) and MVPA ( 3.0 METs).
The mean ± SD time (min/day) spent in sedentary was 376 ± 62 and MVPA was 67.6 ± 20.8.
Multiple linear regression analyses revealed that a later age at independent walking was
to publish, or preparation of the manuscript. There
was no additional external funding received for this
Competing interests: Dr. Shigeho Tanaka received
consigned research funds from Omron Healthcare
Co., Ltd. This does not alter our adherence to PLOS
ONE policies on sharing data and materials. The
remaining authors declare no competing interests.
associated with increased time spent in SB (β = 0.15, P < 0.001) and decreased time spent
in MVPA (β = -0. 18, P < 0.001) after adjusting for gender, birth weight, current age, body
weight, schools, and time spent wearing the accelerometer. When MVPA was introduced as
a covariate in the model predicting SB, the association between the age at independent
walking and time spent in SB was completely attenuated (β = 0.04, P = 0.215), while MVPA
was significantly associated with SB (β = -0.61, P < 0.001).
Our results indicate that infants who walked at a later age spent more time in SB in
childhood, and this association is mediated by MVPA. Appropriate interventions which focus on
increasing MVPA and thereby reducing SB may be beneficial in infants who demonstrate a
later age at onset of independent walking.
Numerous epidemiological studies have identified consistent associations between physical
inactivity, defined as ªnot performing sufficient amounts of moderate-to-vigorous physical
], and all-cause and cardiovascular disease mortality [
Sedentary behavior (SB), defined as ªany waking activity characterized by an energy
expenditure 1.5 metabolic equivalents and a sitting or reclining postureº [
], has also increasingly
been recognized as an important risk factor associated with health outcomes [
studies have shown that having a high level of SB negatively impacts health after considering the
benefits of MVPA [
]. SB is an independent risk factor for diseases and is therefore worth
considering in addition to MVPA. Sedentary lifestyles have become major health problem
worldwide, and the identification of factors which determine SB is of great importance to public
Previous reviews have shown that SBs [
] and physical activities [
] track at moderate levels
from childhood, suggesting that SB and physical activity (PA) during childhood may have a
particularly important role over an individual's life course. Thus, a better understanding of the
determinants of children's SB and PA is needed in order to effectively decrease SB and increase
PA across the lifespan. Systematic reviews have mainly focused on environmental [
psychological, social, and behavioral [
] factors as correlates of SB and PA during childhood and
adolescence. Recent evidence also suggests that early-life factors make a significant
contribution to SB  and PA [
] in young people, and a few studies have shown that indicators of
motor development were an early predictor of PA in children [14±16].
A birth cohort study by Ridgway et al. (2009) reported that an earlier age at standing
unaided and walking supported in infancy predicts higher levels of PA as indicated by an
increased frequency of sports participation during adolescence [
]. Mattocks et al. (2008)
reported motor coordination at 6 months, which was based on combined score from 12
questions, was positively associated with objectively measured PA (cpm) using accelerometry in
children aged 11±12 years [
]. Associations between age at onset of walking and objectively
measured PA patterns already exist in the first two years [
]. Hnatiuk et al. (2013) reported
that 19-month-old toddlers who walked earlier had a higher total time spent in
light-to-vigorous-intensity PA than those who walked later, regardless of how long the toddler had been
walking . A study by Prioreschi et al. (2017) reported diurnal distributions of mean vector
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magnitude by developmental stage and showed that in most cases walkers were more active
during the day than crawlers, even after adjusting for age [
]. Therefore, these associations
between age at onset of walking and PA during the first two years may carry over to
We also recently reported that a later age at onset of independent walking predicts lower
levels of MVPA (min/day) measured by an accelerometer in 6- to 12-year-old children [
However, no studies have shown significant associations between indicators of motor
development and SB. It's important that increased SB and decreased MVPA often occur in
combination, although these factors are mutually independent to some degree [
]. Therefore, it is
plausible that a later age at onset of walking may also predict increased time spent in SB.
However, interventions designed to increase PA or MVPA generally resulted in reductions in
sedentary time [
]. These studies suggest that the differences in sedentary time may be partly
caused by the amount of MVPA in an individual. Therefore, we hypothesized that MVPA
might mediate a potential association between age at independent walking and SB.
In order to develop successful strategies to prevent prolonged sedentary time during
childhood, it is essential to elucidate the substantial role of the timing of onset of walking in the
determining levels of SB in later life, taking MVPA into consideration. The aim of the present
study was to examine the association between the age at onset of independent walking and
objectively measured SB, and whether this association is mediated by MVPA in children.
Materials and methods
The subjects were Japanese primary school children, who were recruited from 14 primary
schools in urban areas of Tokyo, Kanagawa, and Kyoto prefectures. The anthropometry,
accelerometry, and questionnaire data were collected from June 2012 to January 2015 during the
A total of 569 individuals participated in this study. A questionnaire filled out by parents
was used to evaluate the children's medical history, bedtime and wake-up time, age at onset of
independent walking, and birth weight. Those who rescinded their consent (n = 8), had no
questionnaire data (n = 16), or had a history of conditions affecting PA such as respiratory
disease or heart disease (n = 28) were excluded. Additional subjects were excluded if the
accelerometer data did not conform with the study criteria (see below) (n = 91). A final data set for
388 children was used for subsequent analyses after the exclusion of children with incomplete
data on age at onset of independent walking (n = 24) including an outlier ( 25 months) [
those born with a very low birth weight (<1.5 kg) (n = 2), and those who were multiples
(n = 12).
The research project was approved by the Ethical Committee of Oberlin University (receipt
number: 12023). The study procedures were explained in writing to all children and parents,
and written informed consent was obtained from each participant and his/her parents.
Age at independent walking and birth weight
Information on independent walking and birth weight in children was retrospectively reported
on a questionnaire according to the parents' recall. The parents were asked to provide their
children's birth weight and the age in months when their children were first able to walk
without assistance [
]. Birth weight was investigated as a factor potentially related to SB 
and PA [
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Sedentary behavior and physical activity
Daily SB and PA were objectively measured using a triaxial accelerometer (Active style Pro
HJA-350IT, Omron Healthcare, Kyoto, Japan; dimensions 74 × 46 × 34 mm and weight 60 g
including batteries) for seven consecutive days. The device is described in detail elsewhere
]. The subjects wore the accelerometer on the left side of the waist and were requested to
wear the device at all times except under special circumstances such as dressing, bathing, and
swimming. An epoch of 10 seconds was used, and the data were converted using following
conversion equations for primary school children based on the results of Hikihara et al.
(2014), because the metabolic equivalent (MET) values recorded by the accelerometer are
overestimated in primary school children [
Ambulatory activities : 0:6237
MET value of Active style Pro 0:2411
ambulatory activities : 0:6145
MET value of Active style Pro 0:5573
Ambulatory activities (e.g., walking and running) and non-locomotive activities (e.g.,
playing games, cleaning, playing with blocks, tossing a ball, and aerobic dance) were discriminated
based on the ratio of unfiltered synthetic acceleration to filtered synthetic acceleration [
Filtered synthetic acceleration was defined as the integrated acceleration ((X2+ Y2+ Z2)0.5) after
the gravitational acceleration was removed from each dimensional acceleration (X, Y, Z) by
passing it through a second-order Butterworth high-pass filter with a cut-off frequency of 0.7
We analyzed data collected between 7:00 and 21:00 to exclude sleep. The average (± SD)
bedtime and wake-up time of children assessed by a questionnaire were 21:27 ± 2:00 and
6:49 ± 0:27, respectively, in this study. Therefore, we determined the time window for the
analyses as above (between 7:00 and 21:00). Awake data were partly excluded but data
accumulated during sleep were not included in the analyses in many cases. It was difficult to
discriminate the bedtime and wake-up time of each child day by day. If sleep periods were
included in the time window (7:00 to 21:00), misclassifications of sleep into SB would
increase, while excluding awake periods between 21:00 and 7:00 from the analyses would
hardly affect duration of MVPA because little MVPA is observed during the time
immediately after wake-up and before bedtime. Subjects with data obtained from wearing the
accelerometer >10 hours on at least two weekdays and at least one weekend day [
included in the analysis. Periods with >60 min of consecutive zero counts (no signal) were
classified as ªno wearing timeº.
Three variables were analyzed in this study: time (min/day) spent in SB ( 1.5 METs), light
PA (LPA, 1.6±2.9 METs), and MVPA ( 3.0 METs). The mean weekly values were then
calculated. The mean values were calculated by weighting for five weekdays and two weekend days
(weighted data = ([mean for weekdays × 5] + [mean for weekend days × 2]) / 7).
Body height and weight were measured without shoes, but with clothing to the nearest 0.1 cm
and 0.1 kg, respectively. We used scales that were typically used in primary schools; otherwise,
we brought a ªKarada Scan HBF-370º (Omron Healthcare, Kyoto, Japan) scale into the schools
and used it. Net body weight was calculated as the measured body weight minus the weight of
the clothing. We used 0.5 kg as the weight of clothing in all children except for children who
underwent measurements in physical exercise uniforms; their light clothing was regarded as
weighing 0.35 kg. The weight used for clothing was determined by weighing typical children's
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The measured and calculated values are presented as means ± standard deviations (SD).
Student's t-test was used to investigate potential differences between boys and girls. Partial
correlation analysis was used to test the relationships between study variables controlled for gender
and months of age as covariates. Multiple linear regression analyses were performed to assess
the associations between age at onset of independent walking and SB or PA. We first entered
the age at independent walking (Model 1) and then added the birth weight and current weight
as independent variables to examine how the association is altered by body weights (Model 2).
In order to investigate whether MVPA acts as a mediator in the association between motor
development and SB, we finally introduced MVPA as a covariate in the model for predicting
SB according to the methods suggested by Baron and Kenny [
For illustrative purposes, the age at onset of walking was categorized into three groups
according to tertiles: early (10.4 ± 0.7 months), middle (12.4 ± 0.5 months), and late (15.4 ± 2.0
months). Linear trends of SB, LPA, and MVPA were evaluated using an ordinal variable for
the three groups [
]. All models were adjusted for gender, birth weight, current weight,
months of age, interaction term (gender × months of age), schools, and accelerometer wearing
The statistical analyses were performed using IBM SPSS statistics 22.0 for Windows (IBM
Japan Ltd., Tokyo, Japan). The statistical significance level was set at P < 0.05.
The characteristics of the subjects are shown in Table 1. The average age of the subjects was
111.8 ± 19.4 months for boys and 112.0 ± 19.0 months for girls. No significant differences
between boys and girls were observed in current age, height, weight, or the age at which they
walked independently in infancy. Birth weight was slightly higher in boys than girls (95% CI:
7.5, 161.2). Boys showed higher MVPA values compared with girls (95% CI: 13.3, 20.9),
whereas SB was significantly lower in boys than in girls (95% CI: -26.3, -1.6).
Table 2 shows the partial correlation matrix between early life factors (birth weight and age
at onset of independent walking) and current measures (height, weight, SB, LPA, and MVPA)
controlled for gender and months of age as covariates. Birth weight was positively and weakly
correlated with current height (P < 0.01) and weight (P < 0.05). The age at independent
walking was weakly correlated positively with current weight (P < 0.05) and SB (P < 0.01) and
inversely with MVPA (P < 0.001), but these associations were weak. Current height (P < 0.05)
and weight (P < 0.01) were also weakly associated with SB and LPA. SB was strongly associated
with LPA and moderately associated with MVPA (P < 0.001). LPA was weakly associated with
MVPA (P < 0.001).
Table 3 presents the results of the multiple linear regression analyses used to examine the
associations of age at independent walking with SB and PA. As shown in model 1, the age at
independent walking was positively associated with SB (β = 0.13, 95% CI: 4.40, 17.37) and
inversely associated with MVPA (β = -0.17, 95% CI: -6.72, -2.35) after adjusting for gender,
months of age, interaction term (gender × months of age), schools, and accelerometer wear
time. A further adjustment for birth weight and current weight made little difference in the
relationships between age at independent walking and SB (β = 0.15, 95% CI: 5.43, 18.30) and
MVPA (β = -0.18, 95% CI: -6.93, -2.56) (model 2 and Fig 1).
When MVPA was introduced as a covariate in the model predicting SB (Table 4), the
association with the age at independent walking was completely attenuated and ceased to be
significant (β = 0.04, 95% CI: -1.92, 8.49) and MVPA was significantly associated with SB (β = 0.61,
95% CI: -2.05, -1.57).
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Data are means ± SD (range) or proportions.
SB, sedentary behavior; LPA, light physical activity; MVPA, moderate-to-vigorous physical activity.
² P values were calculated for gender difference by t-test.
We also examined the associations between age at independent walking and SB, LPA, and
MVPA in boys and girls separately (S1±S3 Tables). As a result, similar results were obtained in
both genders, except that the association between the age at independent walking and LPA
was not significant in boys (β = -0.07, P = 0.335 for model 1; β = -0.09, P = 0.199 for model 2),
while the association was still significant in girls (β = -0.12, P = 0.029 for model 1; β = -0.13,
P = 0.024 for model 2) (S2 Table).
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Model 1: Adjusted for gender, months of age, interaction (gender × months of age), schools, and accelerometer wear time.
Model 2: As Model 1 plus birth weight and current weight.
B, unstandardized regression coefficient; β, standardized regression coefficient.
SB, sedentary behavior; LPA, light physical activity; MVPA, moderate-to-vigorous physical activity.
This study was performed to examine the association between age at onset of independent
walking and SB in children and to test whether this association is mediated by MVPA. The
main finding of this study was that later age at independent walking was significantly (albeit
weakly) associated with increased time spent in SB. To our knowledge, only one study by
Wijtzes et al. (2013) has investigated the association between motor development and SB, and
the investigators found no association between having a delayed gross motor development at
one year and sedentary time in two-year-old toddlers [
]. As such, we believe that the present
study is the first to show the link between an indicator of infant motor development and
objectively measured SB. The difference in accelerometry may be a potential reason for the
discrepancy between our findings and those of Wijtzes et al. (2013); They used a uniaxial
accelerometer (ActiGraph) during one weekday and one weekend day in their study [
while we used a triaxial accelerometer during seven days. The mean wearing days of
accelerometer in our study (6.3 ± 1.0 days) was considerably greater than the minimum criteria (at
least three days) which is required for reliable PA monitoring in young children [
results indicate that on average, each increase of one month in a child's age at independent
walking was associated with almost 12 min/day more of SB (Table 3, model 2). This may
suggest important clinical implications, given that the age at which children first walk
independently is distributed over more than a nine-month range .
However, an important possibility to be considered was that MVPA may play a mediator
role in the observed association between age at independent walking and SB in model 2. To
elucidate the substantial association between the timing of onset of independent walking and
SB, we tested whether this association is mediated by MVPA. As a result, it was found that the
association between the age at independent walking and the time spent in SB was completely
mediated by MVPA, and MVPA was significantly associated with SB (Table 4). This means
that the association between age at independent walking and SB in model 2 is caused by
indirect effect through MVPA. Thus, it is plausible that the timing of onset of walking in infancy
may influence the amount of MVPA in childhood, and then MVPA may cause the differences
in sedentary time.
Independent walking, or upright and bipedal walking, a unique distinction of the human
species, is the major motor developmental task during the first two years of life [
]. Age at
onset of walking has often been used as an indicator of the progress of motor development in
early life [
] and been shown to be associated with health risks in later life, such as bone
strength  and blood pressure [
]. The age at which a child first walks independently varies
widely from person to person and typically ranges from 8 months to 17±18 months, while
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Fig 1. Estimated means of SB and PA among tertiles of the age at independent walking. (Dark gray) SB. (Light gray) LPA. (Black) MVPA. Adjusted
for gender, months of age, birth weight, current weight, interaction (gender × months of age), schools, and accelerometer wear time. SB, sedentary
behavior; LPA, light physical activity; MVPA, moderate-to-vigorous physical activity.
Age at independent walking (mos.)
Adjusted for gender, months of age, birth weight, current weight, interaction (gender × months of age), schools, and
accelerometer wear time.
B, unstandardized regression coefficient; β, standardized regression coefficient.
SB, sedentary behavior; MVPA, moderate-to-vigorous physical activity.
PLOS ONE | https://doi.org/10.1371/journal.pone.0204030
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2.7% of children were not be able to walk independently at 24 months in a healthy sample [
Muscle strength and balance-control are thought to be rate-limiting factors for the onset of
]; however, the timing of the onset of walking is modifiable. Exercise intervention
such as stimulations to facilitate walking accelerate the development of walking [
et al. (1972) showed that a few minutes of daily stepping practice with an upright posture over
several weeks result in the onset of walking occurring 1.3±2.2 months earlier compared with
control groups [
]. A more recent study has also shown that nutritional intervention
improves gross motor development in small infants [
]. It is currently unclear whether these
interventions to accelerate the gross motor development may have long-term benefits.
In the present study, the earlier age at onset of walking was associated with spending more
time in MVPA in children (Table 3). This result is in line with previous studies showing that
walking at an earlier age is associated with higher PA levels in infants and toddlers [
Although the detailed mechanisms by which the timing of onset of walking has an association
with the amount of MVPA are not fully understood, some factors associated with child-rearing
practices (e.g., encouragement to move by parents or caregivers) may carry over to the
childhood years and therefore may increase MVPA. Another possibility is that potential motor
proficiency may be associated with both the timing of the development of walking and the
amount of MVPA. The link between higher motor proficiency and increased PA is widely
]. A few studies have shown a relation between earlier infant motor development
and higher motor proficiency in later life [
]. Ridgway et al. (2009) reported that the
age at which infants stand unaided and walking supported were associated with muscle
strength, muscle endurance, and cardiorespiratory fitness at the age of 31 years . Kuh et al.
(2006) found that the age at which a child walked was a predictor of standing balance, chair
], and muscle performance [
] at the age of 53 years. Prospective studies that include
the assessment of factors associated with child-rearing practices and motor fitness would be
beneficial to elucidate a causal association between the development of walking and PA
patterns in later life.
In the case of analyzing by gender, the association between motor development and LPA
was no longer significant in boys, while girls still showed a significant association. It is not
clear why LPA was not predictive in boys, however, fewer sample size for boys would be partly
related to these results and otherwise the patterns of LPA may be more strongly associated
with other determinants such as psychological, social, and behavioral factors in boys.
The present study has several limitations. First, we relied on the parents' recall for
information on birth weight and the age at which the children walked independently. Registered birth
weight has been shown to be in good agreement with birth weight recalled by mothers of
school children age 8±11 years (interclass coefficient = 0.95, mean differences = 1.2 g) [
The reliability of age at walking recalled by parents of children six years of age and older is
]. As this variable is also based on parental reports rather than on an objective
assessment, reporting bias may exist. In addition, this parameter is also accompanied by a
problem of definition: independent walking has been variously defined as walking two or
three steps without support [
] or walking at least five steps independently , or it was not
specifically defined [
]. Therefore, recall bias, reporting bias, and the definition problem
might potentially influence the data. Nevertheless, the 10th, 50th, and 90th percentile values
for age at the onset of independent walking in this study (10.0, 12.0, and 15.0 mos.,
respectively) were almost the same as in the WHO Motor Development Study (10.0, 12.0, and 14.4
mos., respectively) [
] in that motor development was objectively assessed. Second, as this
was a retrospective study, long term follow-up studies are needed to confirm the influence of
development timing of onset of walking on PA and SB in later age. While this remains to be
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investigated, this study provides valuable information among infants with a later age at onset
of independent walking.
In summary, the present study showed that a later age at onset of walking in infancy was
associated with prolonged sedentary time in childhood. However, when MVPA was introduced as
a covariate, this association was completely mediated by MVPA. MVPA was significantly
associated with SB. These results indicate that a later age at onset of independent walking may have
a negative influence on MVPA, with an associated increase in sedentary time in children. This
suggests that the timing at which a child walked for the first time in early life may have
longterm implications for subsequent activity patterns. Our findings also suggest that appropriate
interventions which focus on increasing MVPA and thereby reducing SB may be beneficial in
infants who demonstrate a later age at onset of independent walking.
S1 Table. Partial correlation matrix controlled for months of age in boys and girls
separately. P < 0.05; P < 0.01; P < 0.001. SB, sedentary behavior; LPA, light physical
activity; MVPA, moderate-to-vigorous physical activity.
S2 Table. Multiple linear regression analyses with SB or PA as the dependent variable in
boys and girls separately. Model 1: Adjusted for months of age, schools, and accelerometer
wear time. Model 2: As Model 1 plus birth weight and current weight. B, unstandardized
regression coefficient; β, standardized regression coefficient. SB, sedentary behavior; LPA,
light physical activity; MVPA, moderate-to-vigorous physical activity.
S3 Table. Multiple linear regression analysis with SB as the dependent variable in boys and
girls separately. Adjusted for months of age, birth weight, current weight, schools, and
accelerometer wear time. B, unstandardized regression coefficient; β, standardized regression
coefficient. SB, sedentary behavior; MVPA, moderate-to-vigorous physical activity.
The authors would like to thank the participants for their cooperation in the study. We also
wish to thank the staff of the National Institute of Health and Nutrition for their help with the
Conceptualization: Tomoko Aoyama, Maki Tanaka, Masayuki Okuda, Shigeru Inoue, Chiaki
Data curation: Tomoko Aoyama, Maki Tanaka.
Formal analysis: Tomoko Aoyama.
Funding acquisition: Tomoko Aoyama, Chiaki Tanaka.
Investigation: Tomoko Aoyama, Maki Tanaka, Chiaki Tanaka.
Methodology: Shigeho Tanaka, Masayuki Okuda, Shigeru Inoue, Chiaki Tanaka.
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Project administration: Shigeho Tanaka, Masayuki Okuda, Shigeru Inoue, Chiaki Tanaka.
Resources: Shigeho Tanaka, Chiaki Tanaka.
Software: Shigeho Tanaka.
Validation: Shigeho Tanaka.
Writing ± original draft: Tomoko Aoyama.
Supervision: Shigeho Tanaka, Masayuki Okuda, Shigeru Inoue, Chiaki Tanaka.
Writing ± review & editing: Tomoko Aoyama, Maki Tanaka, Masayuki Okuda, Shigeru
Inoue, Chiaki Tanaka.
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