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. 2021 May 26;16(5):e0251025. doi: 10.1371/journal.pone.0251025

Relationship between the skeletal muscle mass index and physical activity of Japanese children: A cross-sectional, observational study

Tadashi Ito 1,2,*, Hideshi Sugiura 2, Yuji Ito 3, Koji Noritake 4, Nobuhiko Ochi 3
Editor: Masaki Mogi5
PMCID: PMC8153420  PMID: 34038448

Abstract

Regular physical activity is an important component of physical health of children and has been associated with increasing skeletal muscle mass and muscle strength. Children with low levels of physical activity may experience health problems, such as loss of muscle mass, later in life. Thus, it may be valuable to identify declining physical function in children who do not perform the recommended amount of physical activity. Therefore, we aimed to evaluate the relationship between the amount of physical activity performed for ≥60 min per day for ≥5 days per week and the skeletal muscle mass index and physical function in young children. In total, 340 typically developing children aged 6–12 years (175 girls; average age, 9.5±1.9 years) were included in this cross-sectional study. We evaluated the proportion of children performing the recommended minimum of 60 min of daily moderate-to-vigorous physical activity at least 5 days per week. The skeletal muscle mass and Gait Deviation Index scores, gait speed, grip strength, Five Times Sit-to-Stand test results, Timed Up-and-Go test results, one-leg standing time, and gait efficiency were evaluated. Multiple logistic regression analyses were performed to assess the association of moderate-to-vigorous physical activity with the skeletal muscle mass index, percent body fat, and physical function, after controlling for confounding factors (age and sex). A logistic regression analysis revealed that the skeletal muscle mass index was independently associated with moderate-to-vigorous physical activity (odds ratio, 2.34; 95% confidence interval, 1.17–4.71; P = 0.017). Performance of moderate-to-vigorous physical activity for ≥5 days per week for ≥60 min per day was associated with the skeletal muscle mass index score of Japanese children. Our findings highlighted the importance of performing moderate-to-vigorous physical activity for the development of skeletal muscle mass in children.

Introduction

A physically active lifestyle improves muscle health or physical health and has a positive effect on body composition and physical function in childhood and adolescence [1]. Therefore, regular physical activity is important for the physical health of children [2,3]. International recommendations by the World Health Organization, 2018 United States guidelines, and United Kingdom Chief Medical Officers indicate that children and adolescents should perform moderate-to-vigorous physical activity (MVPA) for ≥60 min per day for >5 days per week [1,4,5]. Physical activity varies according to demographics and social factors, the most notable of which are ethnicity and sex [6]. In 2008, the global proportion of adolescents aged 13–15 years who did not perform the recommended MVPA was 80.3% [7]. In 2010, these rates among children and adolescents aged 11–17 years were 78.4% and 84.4% for boys and girls, respectively [8]. In 2016, a study on children aged 9–11 years from 12 countries revealed that 44.1% of them performed at the recommended MVPA levels [9]. In 2019, similar findings were reported in a study on healthy children aged 4–13 years in Australia (42.0%) [10]. Recently, a study in Japan showed that 74.7% of boys and 55.2% of girls from a sample of 657 children aged 9–15 years fulfilled the physical activity guidelines of the Japan Sports Association (≥7 h per week) [11]. However, these studies did not assess the relationship between the recommended levels of MVPA and physical function (e.g., muscle strength, balance function, and gait quality). Nonetheless, whether the physical function of children is prone to decline when they do not perform the recommended MVPA remains unclear.

Physical function refers to the ability to perform physical movements and includes grip strength, an indicator of total muscle strength; gait quality, speed, and efficiency, indicators of gait ability; the Five Times Sit-to-Stand Test (FTSST), an indicator of functional performance; the Timed Up-and-Go test (TUG), an indicator of mobility; and the one-leg standing time (OLST), an indicator of postural and balance control. Measuring grip strength may help track changes in children’s health [12,13]. Gait speed and quality are critical elements of physical function and have been reported to serve as fundamental indicators of gait development in children [14–16]. The physiological cost index of gait has been widely used to assessment gait efficiency in healthy children [17]. A previous study reported that the FTSST and TUG are feasible and reliable tools for both children exhibiting typical development and those with disabilities [18–22]. The OLST has been proposed as a fundamental indicator of balance function in children [23,24].

Colley et al. reported that walking ability is an environmental factor that may influence physical activity [25]. Additionally, physical activity has been reported to be important for increasing the skeletal muscle mass and muscle strength of children [26–29]. More recently, research has shown that children may develop sarcopenia and chronic diseases [30]. Children with low MVPA levels may experience health problems, such as sarcopenia, later in life. Therefore, it may be valuable to identify signs of declining physical function and skeletal muscle mass in children who do not perform the recommended levels of MVPA. However, not all studies have consistently confirmed the associations among the skeletal muscle mass, physical function, and physical activity of children. Furthermore, to the best of our knowledge, no study has investigated these relationships among school-age children in Asia. Therefore, in this study, we examined the associations among the skeletal muscle mass, physical function, and MVPA of Japanese school-age children.

Materials and methods

Study population

Of the 48 elementary schools in Okazaki City, two schools introduced to us by the Okazaki City Board of Education, from which we received informed consent for study participation, were included. All 1997 students attending these two elementary schools in Okazaki City were invited to Aichi Prefectural Mikawa Aoitori Medical and Rehabilitation Center for Developmental Disabilities to undergo a medical examination and physical function evaluation. In total, 400 school-age children (6–12 years) were recruited for this cross-sectional study between January 2018 and December 2019. The exclusion criteria were as follows: orthopedic, neurological, ophthalmologic, auditory, respiratory, or cardiovascular abnormalities that could affect the results of physical function tests; inability to complete the physical function tests; substandard scores for the Raven’s Colored Progressive Matrices and the Picture Vocabulary Test-Revised, which indicate intellectual disability [31,32]; and previously diagnosed autism spectrum or attention-deficit hyperactivity disorder. Of the 400 potentially eligible children, 60 were excluded, and finally, 340 participants were enrolled in this study (Fig 1). Of note, six of the excluded participants were being treated with medications that could influence body composition and body weight. None of the other participants were receiving such medications. The study was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by the Ethics Committee of the Aichi Prefectural Mikawa Aoitori Ethics Review Board (approval number, 29002). The manuscript was prepared according to the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines. The legal guardians of all participants provided written informed consent for the children’s participation in the study and for the publication of any identifying information. In addition, all the children assented to study participation.

Fig 1. Flowchart of the enrollment of study participants.

Fig 1

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Measurement of variables

Moderate-to-vigorous physical activity (MVPA) questionnaire

The Japanese version of the World Health Organization Health Behavior in School-age Children was used to evaluate the MVPA levels [33]. Participants provided self-responses to the physical activity questionnaire. The questionnaire was administered to individuals who performed ≥60 min MVPA per day for ≥5 days per week, which is the recommended MVPA levels.

Assessment of appendicular skeletal muscle mass

A multi-frequency bioelectrical impedance analyzer (MC-780; Tanita, Tokyo, Japan) was used to measure the appendicular skeletal muscle mass and percent body fat. Standard positioning was used to perform measurements, as described in the instruction manual. The participants stood with the soles of their feet in contact with the anterior and posterior foot electrodes and grabbed the hand electrodes to maintain contact with the palm and thumb of each hand. Arms were extended and hung down in a natural standing position, and skin-to-skin contact was avoided. Measurements were performed by well-trained physical therapists or research assistants and completed within 15 s. The bioelectrical impedance analyzer resistance was obtained at three electrical frequencies: 5, 50, and 250 kHz. The skeletal muscle mass index (SMI) score was calculated as the appendicular skeletal muscle mass divided by height squared. The percent body fat of participants was calculated using the bioelectrical impedance analyzer. The multi-frequency bioelectrical impedance analyzer is used to determine the relationship between the measured volume of an electrical resistance and the conductor. The analyzer uses multiple frequencies to differentiate intracellular fluid from extracellular fluid and, therefore, provides an estimation of total body water [34]. Fat and bone have correspondingly high impedance and low conductivity, and skeletal muscle is an electrolyte rich tissue with low resistance. The analyzer has been demonstrated to be non-invasive, convenient, and economic and has been used to evaluate muscle development and muscle cross-sectional areas of children [35,36]. Bioelectrical impedance analysis (BIA) was only performed 2 h after meals, as described in the instruction manual.

Gait analysis

Measurements were performed using an eight-camera motion analysis system with a sampling frequency of 100 Hz (type MX-T 20S; Vicon, Oxford, UK). The Plug-In-Gait lower body Ai marker set (Vicon) was positioned on the anterior and posterior superior iliac spines, lateral femoral, lateral part of the knee joint, lateral lower limb, lateral malleoli of the ankle joint, heads of the second metatarsal, and both sides of the calcaneus bones by a physiotherapist with 10 years of experience in clinical gait analysis. An instrumented three-dimensional gait analysis was performed using the Plug-In-Gait (Vicon) to determine the kinematics of the lower extremities at the pelvis, hips, knees, and both sides of the ankles. The participants were recorded as they walked barefoot at a self-controlled speed on 8-AMTI OPT force plates (Advanced Mechanical Technology, Inc., Watertown, MA, USA) during three trials [37,38]. The Gait Deviation Index (GDI) of the participants was used to detect the gait pattern and was calculated using the Vicon current pipeline, as described in the study by Schwartz and Rozumalski [16]. The mean GDI score was calculated using the results of three gait trials to analyze the right and left lower limbs [16]. The GDI is an important tool that represents the overall gait pattern using numerical values; it is determined based on a score derived from the kinematic data points of the three-dimensional gait analysis of the pelvis, hip, knee, and ankle in the sagittal plane; the pelvis and hip in the frontal plane; and the pelvis, hip, and foot progression in the horizontal plane [16]. The GDI has been demonstrated to have high validity and excellent intra-rater and inter-trial reliability [39,40]. The normal gait speed results were recorded in m/s [15]. The mean normal gait speed was calculated using the results of three gait trials analyzing the right and left lower limbs.

Grip strength

Grip strength was measured (in kg) as the participant’s mean grip strength of both hands using an adjustable Smedley-type handheld dynamometer (GRIP-D; Takei Ltd., Niigata, Japan). The dominant hand was measured first, followed by the non-dominant hand. Grip strength of both hands was measured once each with the participants in the sitting position with their shoulder abducted and neutrally rotated, the elbow at extension, and the forearm and wrist in the neutral position [41]. The participants were instructed to squeeze the handle of the dynamometer as hard as they could and sustain for 5 s [42]. Verbal encouragement (i.e., “Keep it up, keep it up”) was provided during the test [42]. Prior to grip strength assessment by an experienced physical therapist, the participants were allowed to perform one practice trial as a learning attempt. The reliability of grip strength measurement was reported to be excellent in a full elbow-extension position [43].

Five Times Sit-To-Stand test (FTSST)

The FTSST, which is a functional performance measurement test, was used to evaluate lower muscle strength. A bench chair, which was height-adjusted to 90° of knee flexion, was used for the test. The participants sat on a chair with their arms crossed over the chest. Next, they were asked to stand up and sit down five times as quickly as possible without using arm support [18,19,44]. The FTSST began with the word “go” and stopped when the participant stood up after the fifth repetition; the time duration was recorded using a stopwatch (s). The participants were allowed to perform one practice trial before the actual test [18,19,44].

Timed UP-and-Go test (TUG)

The TUG test was used to evaluate functional mobility and the dynamic balance function. To perform the TUG test, the participants were asked to sit on an adjusted chair with 90° knee flexion. This test involved rising from an armchair, walking 3 m at a usual speed, turning around, walking back to the chair, and sitting down [20–22,45,46]. These instructions were clearly communicated to participants. The TUG test score (in s) after one trial was used for the analysis. A demonstration was provided, and one practice trial was allowed for each participant.

One-leg standing time (OLST)

The OLST was used to determine how long a participant could remain standing on one leg with their eyes open, which indicates the static balance function [47–49]. The arms had to be held alongside the body. The participants were asked to lift one foot from the floor, making sure not to brace the lifted leg against the support leg. The test ended when the lifted leg touched the support leg, when the lifted leg touched the floor, or after 120 s of maintaining successful balance. The mean OLST was calculated using the mean OLST (in s) of the right and left lower limbs. OLST was reported to have a good test-retest reproducibility and inter-rater reliability [23,24,50].

Gait efficiency

During the 2-min walking distance test, the participants were instructed to walk at a comfortable pace and without further encouragement on a flat and hard surface for 2 min to determine how far they could walk during that time [51,52]. The 2-min walking distance test was conducted following a standardized protocol [53] along a 10-m straight corridor. The examiner walked behind the participant and provided clear instructions; the participants were allowed to stop and rest. Gait efficiency during the 2-min walking distance test was measured using the POLAR M 600 (Polar Electro, Espoo, Finland), a sport-optimized smartwatch designed to measure the heart rate. The smartwatch was held in place by a wristband on the right wrist of the participant. Before gait efficiency assessment, each participant was required to sit quietly for a 3-min rest period. Gait efficiency was calculated based on net efficiency as follows:

Physiological cost index = (gait heart rate − resting heart rate)/2-min walking distance average gait speed (beats/m/s) [54,55].

Sample size

The sample size was calculated using G*Power version 3.1.9.4 (Heinrich Heine University, Düsseldorf, Germany) [56,57]. Additionally, the number of children who performed MVPA ≥5 days per week for ≥60 min per day was used to calculate the sample size. Based on the assumption that 42.0% of healthy children performed the recommended physical activity levels [10], we calculated an odds ratio (OR) of 1.91. A power analysis was conducted for the logistic regression to determine the sufficient sample size using an alpha level of 0.05, power level of 0.8, OR of 1.91, and a two-tailed test. Based on the aforementioned assumption, the desired sample size was 308.

Statistical analysis

When appropriate, t-tests and chi-square tests were used to compare the physical function of children who performed MVPA ≥5 days per week at ≥60 min per day (defined as group 1) and that of children who performed MVPA <5 days (defined as group 2). A Bonferroni correction was used to compare the physical characteristics of groups 1 and 2, with an α threshold of 0.0125 (0.05/4). Moreover, a Bonferroni correction was used to compare the physical function of these groups, with an α threshold of 0.0056 (0.05/9). The effect sizes were calculated using r and the Cramer’s V test (sex), which were considered small, moderate, and large when r = 0.1 or -0.1, r = 0.3 or -0.3, and r = 0.5 or -0.5, respectively. We confirmed that the children showed no significant sex-based differences in their MVPA values (P = 0.109; effect size [r], -0.1) (S1 Table). Moreover, our results revealed that seasonal variations showed no significant associations with MVPA (P = 0.061; η2 = 0.02) (S2 Table). Participant data are expressed as means and standard deviations or median values (ranges), and were compared using the independent t-test or Mann–Whitney U-test. Spearman’s rank correlation analysis was performed to determine the relationship between the SMI score and percent body fat; it was confirmed that the correlation was not high (r = 0.454; P<0.0001) before the logistic regression analysis was performed. The logistic regression analysis was conducted to examine the associations among the SMI score, percent body fat, physical function, and MVPA. P<0.05 was considered statistically significant. A multivariate logistic regression model was used to determine the OR for the SMI score, percent body fat, GDI, gait speed, grip strength, FTSST, TUG test, OLST, gait efficiency, after controlling for confounding factors (age and sex) in group 1. Data management and statistical computations were performed using the IBM SPSS Statistics 24.0 software package (SPSS Inc., Armonk, NY, USA).

Results

The results indicated that 82 of 165 boys (49.7%) and 105 of 175 girls (60.0%) did not perform the recommended MVPA at the time of assessment. The percentages of children who performed the recommended MVPA levels stratified according to different age groups were as follows: age 6 years, 11.8%; age 7 years, 25.0%; age 8 years, 31.6%; age 9 years, 37.7%; age 10 years, 57.6%; age 11 years, 72.3%; and age 12 years, 53.0%. The participants’ characteristics and a comparison of children who performed and did not achieve the recommended MVPA levels are summarized in Table 1.

Table 1. Demographic characteristics of study participants.

Variables Children performing the recommended MVPA levels (n = 153) Children performing substandard MVPA levels (n = 187) P value* Effect size (r or Cramer’s V test)
Age (years)a 10.0 (6.0–12.0) 9.0 (6.0–12.0) 0.0001 -0.3
Height (cm)a 140.9 (110.8–164.2) 130.7 (106.5–163.6) 0.0001 -0.3
Weight (kg)a 31.4 (17.8–59.6) 27.2 (16.1–74.4) 0.0001 -0.2
Body mass index (kg/m2)a 16.01 (12.91–24.21) 15.68 (12.32–29.6) 0.069 -0.1
Sex: Female/Maleb 70/83 105/82 0.056 0.1
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aData are presented as medians (ranges).

bDifferences in the proportion of sexes were derived using the chi-square test.

*P values for age, height, weight, and body mass index were derived using the Mann–Whitney U test.

MVPA, moderate-to-vigorous physical activity.

Children who performed the recommended MVPA levels (group 1) were older (10.0 vs. 9.0 years; P<0.001), taller (140.9 vs. 130.7 cm; P<0.001), heavier (31.4 vs. 27.2 kg; P<0.0001) and had a higher SMI score (5.98 vs. 5.59; P<0.001), higher GDI score (95.76 vs. 93.38; P = 0.003), stronger grip strength (14.15 vs. 12.0; P<0.001), faster TUG time (7.29 vs. 7.62 s; P<0.001), and longer OLST (120.0 vs. 93.24 s; P<0.001) than those with substandard MVPA levels (group 2) (Table 2).

Table 2. Physical function outcomes of study participants.

Variables Children performing the recommended MVPA levels (n = 153) Children performing substandard MVPA levels (n = 187) P value* Effect size (r)
Skeletal muscle mass index (kg/m2) 5.98 (4.75−7.64)a 5.59 (4.55−8.48) 0.0001 -0.3
Percent body fat (%) 12.2 (5.4–34.8)a 13.2 (5.0–46.4) 0.241 -0.1
Gait Deviation Index (points) 95.76±7.40a 93.38±7.10 0.003 0.2
Gait speed (m/s) 1.20±0.17a 1.16±0.16 0.032 0.1
Grip strength (kg) 14.15 (6.15−26.4)a 12.0 (5.75−33.0) 0.0001 -0.3
Five Times Sit-To-Stand test (s) 5.94 (3.42−11.35)a 6.1 (3.28−10.6) 0.558 -0.03
Timed Up-and-Go test (s) 7.29 (4.74−10.21)a 7.62 (4.57−11.57) 0.001 -0.2
One-leg standing time (s) 120.0 (20.92−120.0)a 93.24 (2.64−120.0) 0.0001 -0.3
Physiological cost index (beat/m/s) 22.0 (0−57.61)a 23.46 (0−76.29) 0.446 -0.04
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aData are presented as means±standard deviations or medians (ranges).

*P values for the Gait Deviation Index score and gait speed were derived using an independent t-test. For all other variables, P values were derived using the Mann–Whitney U test.

MVPA, moderate-to-vigorous physical activity.

For the logistic regression analysis, 340 participants were selected after undergoing medical examination to evaluate physical function. There was a significant relationship between the MVPA and SMI score (OR, 2.34; 95% confidence interval [CI], 1.17–4.71; P = 0.017) (Table 3). Regarding the relationship between MVPA and percent body fat, group 1 had significantly lower OR values (OR, 0.93; 95% CI, 0.88–0.98) than group 2 (Table 3), indicating a significant relationship between MVPA and physical function. Children in group 1 were more likely to have higher physical function than those in group 2 (GDI: OR, 1.04 and 95% CI, 1.0–1.08; TUG test: OR, 0.68 and 95% CI, 0.53–0.87; OLST: OR, 1.02 and 95% CI, 1.01–1.03) (Table 3).

Table 3. Relationship between MVPA and physical function.

Children performing recommended (n = 153) and substandard MVPA levels (n = 187)
B SE Odds ratio (95% CI) P value
Age 0.18 0.12 1.2 (0.95−1.5) 0.13
Sex 0.25 0.31 1.28 (0.7−2.3) 0.428
Skeletal muscle mass indexa 0.85 0.36 2.34 (1.17−4.71)c 0.017
Percent body fata -0.08 0.03 0.93 (0.88−0.98)b 0.006
Gait Deviation Indexa 0.04 0.02 1.04 (1.0−1.08)b 0.032
Gait speed -1.12 0.92 0.33 (0.05−1.96) 0.22
Grip strength -0.02 0.05 0.98 (0.9−1.07) 0.678
Five Times Sit-to-Stand Test 0.03 0.1 1.03 (0.85−1.25) 0.743
Timed Up-and-Go testa -0.39 0.13 0.68 (0.53−0.87)b 0.002
One-leg standing timea 0.02 0.01 1.02 (1.01−1.03)b 0.0001
Physiological cost index -0.01 0.01 0.99 (0.97−1.01) 0.241
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aLogistic regression analyses revealed that skeletal muscle mass index, percent body fat, Gait Deviation Index score, Timed Up-and-Go test results, and one-leg standing time were independently associated with MVPA.

bVariables with a significant odds ratio.

cVariables with a significantly high odds ratio.

Hosmer–Lemeshow χ2 = 5.424, P = 0.711.

Nagelkerke R2 = 0.3.

B, partial regression coefficient; CI, confidence interval; MVPA, moderate-to-vigorous physical activity; SE, standard error.

Discussion

This study found a significant relationship between the skeletal muscle mass and substandard MVPA levels among school-aged children. Moreover, our study revealed the following important findings: 55% of children aged 6–12 years did not perform the recommended MVPA. Further, fulfillment of the recommended MVPA levels corresponded to favorable relationships with functional mobility and muscle strength (TUG), gait pattern (GDI), balance function (OLST), and percent body fat. Fulfillment of the physical activity recommendations was not associated with grip strength, FTSST, gait speed, or gait efficiency in 6–12-year-old Japanese children; the OR for skeletal muscle mass (SMI) was higher than that for gait pattern (GDI), functional mobility and balance (TUG), balance function (OLST), and percent body fat of children who performed the recommended MVPA levels. These findings highlighted the importance of performing MVPA levels to ensure the development of skeletal muscle mass in children.

Our findings may increase the awareness of the need to assess muscle mass of school-age children performing substandard MVPA levels. Although the loss of muscle mass has been associated with sarcopenia in older adults, recent evidence has shown that inactive children may also develop sarcopenia [58,59]. Sarcopenia is highly prevalent in children after liver transplantation and has been reported to be related to lower MVPA levels [28]. Moreover, a previous study revealed that disability significantly affected the muscle mass [60]. Furthermore, according to previous studies, children who are more physically active may have lower percent body fat values and higher muscle mass values [61,62]. The findings of a Chinese study group [63] and those of Babirekere-Iriso et al. [64] indicated that physical activity may have a vital role in the development of skeletal muscle mass in children. The findings of Doaei et al. [65,66] indicated that the FTO gene may play a vital role in the increase in skeletal muscle mass and body composition in male adolescents. Furthermore, lifestyle modifications may exert their effects on obesity through changes in the expression of the FTO and IRX3 genes [65]. Moreover, lifestyle modification appeared to have an impact on obesity through changes in the expression of the FTO and IRX3 genes [67]. Therefore, performing the recommended MVPA levels should be considered an important component of the skeletal muscle mass status of children with or without a disease. From a long-term perspective, it can be assumed that low SMI values are primarily caused by low physical activity levels of children. Therefore, maintaining an optimal SMI score during childhood could improve the maximum muscle mass [68]. Moreover, the effects of exercising the musculoskeletal system seemed to have a greater impact on muscle mass during childhood than during adulthood or old age [69]. A future longitudinal study is needed to confirm this hypothesis.

There was a significant difference in the GDI between children who performed the recommended MVPA levels and those who did not. We also found a significant relationship between MVPA levels and the GDI, suggesting that children with a good gait pattern engaged in more physical activity. Gait is an important activity of daily life [70]. However, logistic regression analysis showed a small OR, indicating that GDI was significantly related to MVPA. Therefore, gait pattern was less related to MVPA than the skeletal muscle mass. However, no differences were observed in the gait speed of children who did and did not perform the recommended MVPA levels. The physiological cost index, which is considered a simple evaluation tool, has been widely used to estimate energy cost during physical activity, including gait, in various clinical scenarios [17,71–73]. However, it was not associated with MVPA in this study, indicating that children with shorter physical activity times did not necessarily exhibit greater changes in their heart rate.

This study also showed that the grip strength of children who performed a substandard MVPA levels was lower than that of those who performed the recommended MVPA levels. However, there was no significant association between grip strength and MVPA in our study, which was in line with the findings of a previous study involving preschool children [74]. Therefore, the grip strength test was found to not be the most appropriate for this population. Furthermore, there was no clear relationship between the MVPA levels and FTSST results in this study. Nonetheless, a previous study data suggested that handgrip strength correlated well with the overall muscle strength [75]. These results are of great interest because muscle strength may be independent of MVPA.

We found a significant relationship among the MVPA, TUG test, and OLST results, suggesting that children with high functional mobility and balance function engage in more physical activity. As the TUG test is a practical tool used to assess functional mobility and balance function [46,76] and because the OLST could be a high-intensity task for children [77], these tests may help evaluate the physical activity of typically developing children. However, the logistic regression analysis showed a small OR, indicating that the TUG test and OLST were significantly related to MVPA. Therefore, the aforementioned tests were deemed to be less related to MVPA levels than the degree of skeletal muscle mass.

Our analysis showed that 54.8% of school-age (6–12 years) boys and girls did not fulfill the current recommendations for daily physical activity. This finding was in contrast to the rate (79.9%) observed among adolescents aged 9–15 years in Unnan City, Japan [78]. Furthermore, a survey of Thai children aged 6–17 years (n = 13,225) indicated that 76.6% of the respondents did not perform the recommended MVPA levels [79]. Another study reported that physical activity was also associated with positive mental health in a multi-ethnic Asian population [80]. We presumed that children in our study actively participated in school clubs, and the opportunities for participation in sports activities were numerous. Differences in the MVPA levels among the children in our study and those in the studies conducted in Unnan City and in Thailand may be attributable to regional characteristics or differences in the ages of participants. However, the categorization of MVPA relied on self-report questionnaire data that could be affected by recall bias. Subjective assessment was reported to overestimate MVPA levels compared with the objective assessment [81]. Moreover, as the items for measuring preference for MVPA were not investigated for reliability and validity, a future longitudinal study is needed to confirm this hypothesis.

This study had several limitations. First, it must be emphasized that clear and causal associations could not be established in this study. Second, although participants from Okazaki City participated in this study, the generalizability of our findings is limited because the research involved only a few schools located in the city. Further studies involving all schools in Okazaki City are required in the future. Third, the questionnaire used in this study was subjective; therefore, there is a possibility that the participants overestimated their physical activity [81]. Fourth, the validity of the BIA method depends on the participant’s hydration status. Limitations of BIA include assumptions involving a fixed hydration status [82]. Thus, BIA may not always be the best method to assess body composition. Finally, we could not account for the role of dietary intake, family income, and bone mineral density, which may be confounders of MVPA [26,83–85].

Conclusions

We found that MVPA ≥60 min per day for ≥5 days per week, defined as the recommended MVPA levels, was associated with the SMI of Japanese school-age children. Moreover, the GDI score, TUG test, OLST, and percent body fat were also associated with MVPA, although less significantly. These results indicated the importance of performing the recommended MVPA levels for the development of skeletal muscle mass in children.

Supporting information

S1 Table. Demographic characteristics and physical function outcomes of study participants.

(DOCX)

Click here for additional data file. (24.9KB, docx)
S2 Table. Demographic seasonal variations in physical activity of the participants.

(DOCX)

Click here for additional data file. (13.3KB, docx)
S1 Database

(XLSX)

Click here for additional data file. (63.9KB, xlsx)

Acknowledgments

We would like to thank the Okazaki City Board of Education, Okazaki City Medical Association, Aichi Pediatrics Medical Society, Yoshiki Nakanowatari, Ryohei Takasu, and Takayoshi Ono for their help recruiting the participants. We are also sincerely grateful to the staff at Naomichi Matsunaga, Yu Hamabe, Jun Mizusawa, Yoshiji Yamamoto, Yoshiki Fukaya, Yumi Aoki, Shuhei Takahashi, Daisuke Kawaguchi, Kento Iwamoto, Shota Sanada, Hiroto Takenaka, Arisa Omori, Yingzhi Gu, Yuya Shirai, Daiki Takahashi, and Aika Hishida for their assistance with data collection. We would like to thank Editage (www.editage.com) for English language editing.

Data Availability

All relevant data are within the paper and its Supporting Information files.

Funding Statement

The authors received no specific funding for this work.

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PONE-D-20-38059

Relationship between skeletal muscle mass index and physical activity of Japanese children: A cross-sectional, observational study

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The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

**********

4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

**********

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: Thank you for the hard-effort observational study. This study has tried to reveal the relationship between sufficient PA and skeletal muscle mass in growing children. And even more, authors tried to figure out the real physical performance by examining several time-consuming studies such as TUGT, one-leg standing time, sit to stand test results, grip strength, gait, etc.

Method

Please define when skeletal muscle mass assessment with BIA was done as the previous food, or liquid consumption could influence the results.

Would you please describe details about the grip strength? Did you have a practice trial or not? How many times did the participants evaluate?

What do you mean “The funder played no role in the design, conduct, or reporting of this study.” It is written that the manuscript has no funding resources.

Statistical analysis and results

Do you have any special reason for comparing median age, height, and weight? It seems a sufficient number to compare with t-test.

Are there any age-specific characteristics in the pattern of MVPA levels? I wonder if they had a different percentage of sufficient MVPA level according to the age group. With the supplementary data, I realized that a relatively small number of 6-aged students and they all are almost in suboptimal PA group, for example.

It would be better to demonstrate a general logistic regression table instead of a simple P-value and 95% CI.

Discussion

Recommend some of the points to discuss.

One study revealed that disability significantly affects muscle mass. (Pediatr Int. 2020 Aug;62(8):962-969. doi: 10.1111/ped.14248.)

Another study figured out that PA is also associated with positive mental health in a multi-ethnic Asian population. (Int J Environ Res Public Health. 2020 Nov 16;17(22):8489. doi: 10.3390/ijerph17228489.)

Reviewer #2: Overall Impressions: Well done study. Statistical analysis appears appropriate. Sample size appears appropriate. Some limitations in methodology need to be addressed.

Specific concerns:

a) Please report on the reliability and precision of muscle function and physical performance tests. Should provide some evidence of this to ensure tests were reliable.

b) Use of BIA to assess appendicular skeletal muscle mass is impacted by normative equations used by equipment. Authors need to address the limitations of BIA in the discussion and to report on precision and reliability data.

c) Use of subjective measures of PA (questionnaire) is limited and may have impacted overall study findings. It would be best to explore the extent to which bias could have impacted overall study findings in discussion. This should be expanded in more detail in the discussion as this would have limited overall study findings.

d) Making the assumption of physical activity outside of ‘school hours’ may not be appropriate and hence the authors should temper their overall conclusions on this point.

e) Any seasonal variations in physical activity noted? This would be important to include as school aged children PA patterns may vary substantively over the winter vs summer/spring months.

**********

6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

PLoS One. 2021 May 26;16(5):e0251025. doi: 10.1371/journal.pone.0251025.r002

Author response to Decision Letter 0

12 Feb 2021

Editor #1

Comments

Journal Requirements:

When submitting your revision, we need you to address these additional requirements.

1. Please ensure that your manuscript meets PLOS ONE's style requirements, including those for file naming. The PLOS ONE style templates can be found at

https://journals.plos.org/plosone/s/file?id=wjVg/PLOSOne_formatting_sample_main_body.pdf and

https://journals.plos.org/plosone/s/file?id=ba62/PLOSOne_formatting_sample_title_authors_affiliations.pdf

Response: The authors would like to thank the Editor for the constructive critique to improve the manuscript. We have made every effort to address the issues raised and to respond to all comments. The revisions are indicated in red font in the revised manuscript.

Comments

2. Thank you for stating the following financial disclosure:

'The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.'

At this time, please address the following queries:

Please clarify the sources of funding (financial or material support) for your study. List the grants or organizations that supported your study, including funding received from your institution.

State what role the funders took in the study. If the funders had no role in your study, please state: “The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.”

If any authors received a salary from any of your funders, please state which authors and which funders.

If you did not receive any funding for this study, please state: “The authors received no specific funding for this work.”

Please include your amended statements within your cover letter; we will change the online submission form on your behalf.

Response: We would like to thank the Editor for the comment. Please note that we have not received any specific funding for this work. This information has been included and clarified in the cover letter and manuscript files in accordance with the Editor’s comment.

Reviewer #1

The authors would like to thank the reviewer for his/her constructive critique to improve the manuscript. We have made every effort to address the issues raised and to respond to all comments. The revisions are indicated in red font in the revised manuscript. Please, find next a detailed, point-by-point response to the reviewer's comments.

Comment

Thank you for the hard-effort observational study. This study has tried to reveal the relationship between sufficient PA and skeletal muscle mass in growing children. And even more, authors tried to figure out the real physical performance by examining several time-consuming studies such as TUGT, one-leg standing time, sit to stand test results, grip strength, gait, etc.

Response: We would like to thank the reviewer for the positive evaluation of our work.

Comment

Method

Please define when skeletal muscle mass assessment with BIA was done as the previous food, or liquid consumption could influence the results.

Response: We would like to thank the reviewer for the constructive comment. Please note that we have added this information to the sentence describing the skeletal muscle mass assessment with BIA, as per the reviewer’s suggestion.

“Bioelectrical impedance analysis (BIA) was only performed at 2 h after the meals, as described in the instruction manual.” (Lines 145–147)

Comment

Would you please describe details about the grip strength? Did you have a practice trial or not? How many times did the participants evaluate?

Response: We would like to thank the reviewer for pointing this out. Grip strength was measured once each to the right and left limbs. Moreover, the participants were allowed to perform one trial, as a learning attempt. We have amended this information in the Methods section, as follows: “The dominant hand was measured first, followed by the non-dominant hand. The grip strength of both hands was measured once each with the participants in the sitting position with their shoulder abducted and neutrally rotated, the elbow at extension, and the forearm and wrist in the neutral position [28]. The participants were instructed to squeeze the handle of the dynamometer as hard as they could and sustain for 5 s [29]. Verbal encouragement (i.e., “Keep it up, keep it up”) was provided during the test [29]. Prior to the grip strength assessment by an experienced physical therapist, the participants were allowed to perform one practice trial as a learning attempt. The grip strength measurement reliability was reported to be excellent in a full elbow-extension position [30].” (Lines 173–181)

Comment

What do you mean “The funder played no role in the design, conduct, or reporting of this study.” It is written that the manuscript has no funding resources.

Response: We would like to thank the reviewer for the comment. Please note that we have received any specific funding for this work. This error has been corrected in accordance with the reviewer’s comment.

Comment

Statistical analysis and results

Do you have any special reason for comparing median age, height, and weight? It seems a sufficient number to compare with t-test.

Response: We would like to thank the reviewer for providing these insightful comments. The t-test cannot be performed because the age, height, and weight data were not normally distributed.

Comment

Are there any age-specific characteristics in the pattern of MVPA levels? I wonder if they had a different percentage of sufficient MVPA level according to the age group. With the supplementary data, I realized that a relatively small number of 6-aged students and they all are almost in suboptimal PA group, for example.

Response: We would like to thank the reviewer for pointing this out. The percentages of children that performed the recommended MVPA level according to their age groups were as follows: 6-year-old, 11.8%; 7-year-old, 25.0%; 8-year-old, 31.6%; 9-year-old, 37.7%; 10-year-old, 57.6%; 11-year-old, 72.3%; and 12-year-old, 53.0%. With respect to this study characteristic, we presumed that children aged 10–12 actively participated in school clubs, and the opportunities for participation in sports activities are numerous. We have provided this information in the Results section.

Comments

It would be better to demonstrate a general logistic regression table instead of a simple P-value and 95% CI.

Response: We would like to thank the reviewer for the comment. We agree with the reviewer’s suggestion. Therefore, we have included this information in Table 3.

Comment

Discussion

Recommend some of the points to discuss.

One study revealed that disability significantly affects muscle mass. (Pediatr Int. 2020 Aug;62(8):962-969. doi: 10.1111/ped.14248.)

Another study figured out that PA is also associated with positive mental health in a multi-ethnic Asian population. (Int J Environ Res Public Health. 2020 Nov 16;17(22):8489. doi: 10.3390/ijerph17228489.)

Response: We would like to thank the reviewer for the suggestion. We agree that the points mentioned need to be added to the Discussion section. Following the reviewer's insightful suggestion, we have revised these sentences in the Discussion section and cited the relevant literature, as follows:

“Moreover, one study revealed that disability significantly affected the muscle mass [47].” (Lines 313–314)

“Another study reported that PA was also associated with positive mental health in a multi-ethnic Asian population [65]. We presumed that children in our study actively participated in school clubs, and the opportunities for participation in sports activities are numerous.” (Lines 358–361)

Reviewer #2

We would like to thank the reviewer for providing insightful comments regarding our manuscript. We express our sincere gratitude for the time and effort the reviewer has spent to review our manuscript. The comments and suggestions have helped us immensely improve our work.

Comments

Overall Impressions: Well done study. Statistical analysis appears appropriate. Sample size appears appropriate. Some limitations in methodology need to be addressed.

Response: We would like to thank the reviewer for the positive comments regarding our study. The manuscript has been rechecked, and the necessary changes have been made in accordance with the reviewers’ suggestions. The responses to all comments have been prepared and given below.

Comments

a) Please report on the reliability and precision of muscle function and physical performance tests. Should provide some evidence of this to ensure tests were reliable.

Response: We agree with the reviewer’s comment. Therefore, we have provided this information in the Materials and Methods section (Assessment of appendicular skeletal muscle mass, Grip strength, Five times sit-to-stand test, Timed Up-and-Go test, One-leg standing time, and Gait efficiency).

Comments

b) Use of BIA to assess appendicular skeletal muscle mass is impacted by normative equations used by equipment. Authors need to address the limitations of BIA in the discussion and to report on precision and reliability data.

Response: Following the reviewer’s suggestion, we have added more information to the Discussion section as follows: “Fourth, the validity of the BIA method depends on the participant’s hydration status. Limitations of BIA include assumptions involving a fixed hydration status [67]. Thus, BIA may not always be the best method to assess body composition.” (Lines 375–377)

Comments

c) Use of subjective measures of PA (questionnaire) is limited and may have impacted overall study findings. It would be best to explore the extent to which bias could have impacted overall study findings in discussion. This should be expanded in more detail in the discussion as this would have limited overall study findings.

Response: We would like to thank the reviewer for pointing this out. We agree with the reviewer’s comment. Therefore, we have added the following part to the Discussion section:

" However, the categorization of MVPA relied on self-reported questionnaire data that could be affected by recall bias. Subjective assessment was reported to overestimate MVPA compared with objective assessment [66]. Moreover, as the items for measuring preference for MVPA were not investigated for reliability and validity, a future longitudinal study is needed to confirm this hypothesis.” (Lines 364–368)

Comments

d) Making the assumption of physical activity outside of ‘school hours’ may not be appropriate and hence the authors should temper their overall conclusions on this point.

Response: We agree that the assumption of physical activity outside of “school hours” needs improvement. Therefore, we have revised the corresponding part in the manuscript to avoid its misinterpretation as follows: “We presumed that children in our study actively participated in school clubs, and the opportunities for participation in sports activities are numerous.” (Lines 360–361)

Comments

e) Any seasonal variations in physical activity noted? This would be important to include as school aged children PA patterns may vary substantively over the winter vs summer/spring months.

Response: We would like to thank the reviewer for the suggestion. Differences between the four seasons and the physical activity time were examined using one-way analysis of the Kruskal–Wallis test. However, the physical activity levels were not significantly different because of the seasonal variations in this study (P=0.061; η2=0.02), indicating that these seasonal variations observed in the winter vs. summer/spring months did not necessarily exhibit significant changes in physical activity time. We agree that we should focus on the seasonal variations and, therefore, we have provided more information in S2 Table.

Attachment

Submitted filename: Reveiewers_Response_PLOSONE_20210212.docx

Click here for additional data file. (23.7KB, docx)

Decision Letter 1

Masaki Mogi

12 Mar 2021

PONE-D-20-38059R1

Relationship between skeletal muscle mass index and physical activity of Japanese children: A cross-sectional, observational study

PLOS ONE

Dear Dr. Ito,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

==============================

The manuscript is still necessary to be revised before acceptance.

See carefully the Reviewers' comments and respond them appropriately.

==============================

Please submit your revised manuscript by Apr 26 2021 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

  • A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.

  • A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.

  • An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

If applicable, we recommend that you deposit your laboratory protocols in protocols.io to enhance the reproducibility of your results. Protocols.io assigns your protocol its own identifier (DOI) so that it can be cited independently in the future. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols

We look forward to receiving your revised manuscript.

Kind regards,

Masaki Mogi

Academic Editor

PLOS ONE

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #3: All comments have been addressed

Reviewer #4: (No Response)

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #3: Yes

Reviewer #4: Partly

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #3: Yes

Reviewer #4: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #3: Yes

Reviewer #4: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #3: Yes

Reviewer #4: No

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #3: - Please define in the introduction "physical function" and which variables comprise it. Include what studies show evidence of it in this population.

- They should better justify why they have chosen these variables "gait speed", "Five times sit-to-stand test" and "Timed Up-and-Go test" in this population. They are validated tests and recommended mainly for older population. Since their study is not with children with cerebral palsy as the articles that they put as reference.

- LINE 342: As indicated, the test results do not correlate with handgrip strength. This is because the test is not the most appropriate for this population. Just a comment to be taken into account in future research.

If these considerations are included, the article is suitable for publication.

Reviewer #4: The subject is interesting and the study is well designed. However, the following corrections must be done before further considerations.

- The abstract should be revised in terms of scientific writing. For example, the first sentence is short and vague. Which confounders were adjusted in regression models?

-Introduction needs to be revised. It should be started with describing the health issue (body composition in adolescence) not the independent variable.

- Explain about school selection criteria.

- Some drugs may have effects on weight and body composition. Did you consider using them as the exclusion criteria? If not, suggested to mention it in your limitations.

- Add decryptions about you bio impedance analyzer.

- The role of dietary intake in determining body composition was ignored. Mention it as a limitation if you did not collect data on diet.

- Discuss about the possible effects of genetics and diet on body composition and also their effects on the association between physical activity and body muscle mass.

See these papers: https://doi.org/10.1016/j.clnu.2018.06.1124, https://dx.doi.org/10.1080%2F21623945.2019.1693745, https://dx.doi.org/10.1186%2Fs12967-019-1921-4.

- Conclusion must be drawn from your main results and should be revised.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #3: No

Reviewer #4: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

PLoS One. 2021 May 26;16(5):e0251025. doi: 10.1371/journal.pone.0251025.r004

Author response to Decision Letter 1

5 Apr 2021

Reviewer #3

The authors would like to thank the reviewer for his/her constructive critique to improve the manuscript. We have made every effort to address the issues raised and to respond to all the comments. The revisions are indicated in red font in the revised manuscript. Please, find our detailed, point-by-point responses to the reviewer's comments below.

Comment

Please define in the introduction "physical function" and which variables comprise it. Include what studies show evidence of it in this population.

Response: Thank you for pointing this out. We agree with the reviewer’s suggestion. We have added this information and cited the relevant literature in the Introduction as requested. (Lines 94–105)

Comment

They should better justify why they have chosen these variables "gait speed", "Five times sit-to-stand test" and "Timed Up-and-Go test" in this population. They are validated tests and recommended mainly for older population. Since their study is not with children with cerebral palsy as the articles that they put as reference.

Response: We agree it is very important to justify for the variables used in this study by citing previous work. The gait speed is a critical element of physical function and has been reported to serve as a fundamental indicator for gait development in children [14-16]. A previous study reported that the FTSST and TUG are feasible and reliable tools for both children with typical development and those with disabilities [18-22]. We have added this information in the introduction section. (Lines 96–105).

Comment

LINE 342: As indicated, the test results do not correlate with handgrip strength. This is because the test is not the most appropriate for this population. Just a comment to be taken into account in future research.

Response: We would like to thank the reviewer for the constructive comment. Please note that we have added this information to the sentence describing results of the grip strength, as per the reviewer’s suggestion.

“Therefore, the grip strength test was found to not be the most appropriate for this population. (Line 384)

Reviewer #4

We express our sincere gratitude for the time and effort the reviewer has spent reviewing our manuscript and for providing insightful comments and suggestions, which have helped us immensely to improve our work.

Comments

The subject is interesting and the study is well designed. However, the following corrections must be done before further considerations.

Response: Thank you for raising these issues. The manuscript has been rechecked carefully, and the necessary changes have been made in accordance with the reviewers’ suggestions. The responses to all comments have been prepared and are provided below.

Comments

The abstract should be revised in terms of scientific writing. For example, the first sentence is short and vague. Which confounders were adjusted in regression models?

Response: We agree with the reviewer’s comment. Following the reviewer’s suggestion, we have added the following additional information to the abstract section: “Regular physical activity is an important component of physical health of children and has been associated with increasing skeletal muscle mass and muscle strength. Children with low levels of physical activity may experience health problems, such as loss of muscle mass, later in life. Thus, it may be valuable to identify declining physical function in children who do not perform the recommended amount of physical activity. Therefore, we aimed to evaluate the relationship between the amount of physical activity performed for ≥60 min per day for ≥5 days per week and the skeletal muscle mass index and physical function in young children.” and “Multiple logistic regression analyses were performed to assess the association of moderate-to-vigorous physical activity with the skeletal muscle mass index, percent body fat, and physical function, after controlling for confounding factors (age and sex).” (Lines 50–57 and 62–65)

Comments

Introduction needs to be revised. It should be started with describing the health issue (body composition in adolescence) not the independent variable.

Response: We would like to thank the reviewer for pointing this out and agree with the reviewer’s comment. Thus, we have added the following details to the Introduction section: “A physically active lifestyle improves muscle health or physical health and has a positive effect on body composition and physical function in childhood and adolescence [1].” (Lines 74–75)

Comments

Explain about school selection criteria.

Response: We thank the reviewer for pointing this out and agree with the reviewer’s comment. Thus, we have added the following information to the Study population subsection: "Of the 48 elementary schools in Okazaki City, two schools introduced to us by the Okazaki City Board of Education, from which we received informed consent for study participation, were included." (Lines 120–122)

Comments

Some drugs may have effects on weight and body composition. Did you consider using them as the exclusion criteria? If not, suggested to mention it in your limitations.

Response: We agree with the reviewer’s comment. Therefore, we have added the following details to the Study population: "Of note, six of the excluded participants were being treated with medications that could influence body composition and body weight. None of the other participants were receiving such medications." (Lines 133–135).

Comments

Add decryptions about you bio impedance analyzer.

Response: Accordingly, we have added the following details to the “Assessment of appendicular skeletal muscle mass” section: "The multi-frequency bioelectrical impedance analyzer is used to determine the relationship between the measured volume of an electrical resistance and the conductor. The analyzer uses multiple frequencies to differentiate intracellular fluid from extracellular fluid and, therefore, provides an estimation of total body water [34]. Fat and bone have correspondingly high impedance and low conductivity, and skeletal muscle is an electrolyte rich tissue with low resistance." (Lines 165–170)

Comments

The role of dietary intake in determining body composition was ignored. Mention it as a limitation if you did not collect data on diet.

Response: Thank you for pointing this out. We had previously mentioned that we could not account for effects of dietary intake on the overall body composition in the study limitations in the original manuscript (Line 377), but in this revised version we have added the following to the limitation section to avoid any potential confusion: "we could not account for the role of dietary intake…" (Lines 420–421).

Comments

Discuss about the possible effects of genetics and diet on body composition and also their effects on the association between physical activity and body muscle mass.

See these papers: https://doi.org/10.1016/j.clnu.2018.06.1124, https://dx.doi.org/10.1080%2F21623945.2019.1693745, https://dx.doi.org/10.1186%2Fs12967-019-1921-4.

Response: Following the reviewer’s suggestion, we have discussed this issue in the revised manuscript, and we have cited the suggested papers. We have added the following to the Discussion: "The findings of Doaei et al. [65,66] indicated that the FTO gene may play a vital role in the increase in skeletal muscle mass and body composition in male adolescents. Furthermore, lifestyle modifications may exert their effects on obesity through changes in the expression of the FTO and IRX3 genes [65]. Moreover, lifestyle modification appeared to have an impact on obesity through changes in the expression of the FTO and IRX3 genes [67]." (Lines 354–359)

Comments

Conclusion must be drawn from your main results and should be revised.

Response: Thank you for pointing this out. This error has been corrected in accordance with your comment. (Lines 427–428)

Attachment

Submitted filename: Certificate_of_editing.pdf

Click here for additional data file. (315.1KB, pdf)

Decision Letter 2

Masaki Mogi

19 Apr 2021

Relationship between skeletal muscle mass index and physical activity of Japanese children: A cross-sectional, observational study

PONE-D-20-38059R2

Dear Dr. Ito,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

An invoice for payment will follow shortly after the formal acceptance. To ensure an efficient process, please log into Editorial Manager at http://www.editorialmanager.com/pone/, click the 'Update My Information' link at the top of the page, and double check that your user information is up-to-date. If you have any billing related questions, please contact our Author Billing department directly at authorbilling@plos.org.

If your institution or institutions have a press office, please notify them about your upcoming paper to help maximize its impact. If they’ll be preparing press materials, please inform our press team as soon as possible -- no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org.

Kind regards,

Masaki Mogi

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

No further comment.

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #3: All comments have been addressed

Reviewer #4: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #3: (No Response)

Reviewer #4: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #3: Yes

Reviewer #4: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

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Reviewer #3: Yes

Reviewer #4: Yes

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5. Is the manuscript presented in an intelligible fashion and written in standard English?

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Reviewer #3: Yes

Reviewer #4: Yes

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6. Review Comments to the Author

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Reviewer #3: Dear authors

The article is very interesting and the authors have made the requested changes and the article can be accepted for publication.

Congratulations

Best regards

Reviewer #4: This paper aimed to investigate the association between skeletal muscle mass index and physical activity of Japanese children. All comments have been addressed.

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Reviewer #3: No

Reviewer #4: No

Acceptance letter

Masaki Mogi

27 Apr 2021

PONE-D-20-38059R2

Relationship between the skeletal muscle mass index and physical activity of Japanese children: A cross-sectional, observational study

Dear Dr. Ito:

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Kind regards,

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on behalf of

Dr. Masaki Mogi

Academic Editor

PLOS ONE

Associated Data

    This section collects any data citations, data availability statements, or supplementary materials included in this article.

    Supplementary Materials

    S1 Table. Demographic characteristics and physical function outcomes of study participants.

    (DOCX)

    Click here for additional data file. (24.9KB, docx)
    S2 Table. Demographic seasonal variations in physical activity of the participants.

    (DOCX)

    Click here for additional data file. (13.3KB, docx)
    S1 Database

    (XLSX)

    Click here for additional data file. (63.9KB, xlsx)
    Attachment

    Submitted filename: Reveiewers_Response_PLOSONE_20210212.docx

    Click here for additional data file. (23.7KB, docx)
    Attachment

    Submitted filename: Certificate_of_editing.pdf

    Click here for additional data file. (315.1KB, pdf)

    Data Availability Statement

    All relevant data are within the paper and its Supporting Information files.

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