Relative age effect on the physical activity and sedentary behavior in children and adolescents aged 10 to 18 years old: A cross-sectional study in Japan

Relative age effect on the physical activity and sedentary behavior in children and adolescents aged 10 to 18 years old: A cross-sectional study in Japan | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Relative age effect on the physical activity and sedentary behavior in children and adolescents aged 10 to 18 years old: A cross-sectional study in Japan Takaaki Mori, Takumi Aoki, Kan Oishi, Tetsuo Harada, Chiaki Tanaka, and 21 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4937775/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 25 Nov, 2024 Read the published version in BMC Public Health → Version 1 posted 11 You are reading this latest preprint version Abstract Background A few studies have shown that relatively younger children and adolescents (those born later in the same school year) were less likely to engage in physical activity in a phenomenon termed the relative age effect. Although these studies mainly targeted elementary and middle school students, limited studies have reported on the relative age effect on physical activity in ordinary high school students. Moreover, the relative age effect on sedentary behavior might show an opposite association with physical activity. Therefore, we aimed to clarify the relative age effects on physical activity and sedentary behavior across different school stages in Japanese children and adolescents. Methods A cross-sectional questionnaire survey was conducted with 21,491 children and adolescents (elementary, middle, and high school students aged 10–18 years) in various Japanese regions from January 2018 to July 2019. Overall, 18,281 children and adolescents (10,299 boys and 7,982 girls) were finally included in our analysis. Data on the birth month, as well as the frequency and duration of their physical activity (vigorous, moderate, and moderate-to-vigorous intensity) and sedentary behavior (weekday and weekend total sedentary time, television viewing, playing video games, and Internet use) were obtained. We utilized two-part model regression analyses Statistically significant association with birth month indicated that a relative age effect was observed. Results The relatively younger individuals were less likely to engage in physical activity (especially vigorous physical activity); this association was observed in middle and high school students. Conversely, relatively younger boys spent more time on the weekend with sedentary behaviors and Internet use. Because of the school stage, the relative age effect on physical activity was observed in middle and high school students, whereas the relative age effect on sedentary behavior was observed on the weekend with sedentary behaviors, playing video games and Internet use for male high school students and on television viewing for female middle school students. Conclusion The relative age effect on physical activity can persist after middle school, and relatively younger individuals are more likely to replace physical activity with sedentary behavior. physical activity sedentary behavior relative age effect school adolescence Figures Figure 1 1. Background The physical activity and sedentary behaviors of children and adolescents are related to various lifestyles and health conditions. Previous studies targeting children and adolescents have shown that more physical activity enhances cardiorespiratory fitness endurance capacity, obesity, and cardiovascular disease [ 1 , 2 ]. Conversely, children and adolescents who engage in prolonged sedentary behavior have a lack of cardiorespiratory fitness and mental health conditions [ 3 – 5 ]. Based on this evidence [ 1 – 5 ], the World Health Organization (WHO) recommends that children and adolescents aged 5–17 should engage in at least an average of 60 min per day of moderate-to-vigorous intensity physical activity (MVPA) across the week and reduce sedentary behavior, especially recreational screen time [ 6 ]. Besides this, the Japan Sports Agency reported that some Japanese children and adolescents (particularly approximately 25% of female middle school students) hardly spent any time on exercise, including active play (less than 60 min per week) [ 7 ]. Therefore, Health Japan 21 (the third term), which is a national health measure in Japan, set a goal to reduce the number of children and adolescents who spend less than 60 min of physical activity per week [ 8 ]. The relative age effect is defined as the difference caused by chronological age within the same age cohort [ 9 ]. The school year in Japan begins in April and ends in March of the following year. Children and adolescents born in April were approximately 11 months older than those born in March. Therefore, “the relatively younger individuals” (those born in the latter half of the school year) are disadvantaged in terms of physical, mental, and social development. For example, several studies have reported that relatively younger individuals are shorter and have lower levels of physical fitness (e.g., grip strength, standing long jumping, and 50 m sprint) [ 10 , 11 ]. Additionally, systematic reviews and meta-analyses on the relative age effect on mental and social development clarified that relatively younger individuals have fewer friends [ 12 ], poor prosocial emotions (e.g., self-esteem and competence), and a higher risk of depression and problematic behavior (e.g., smoking as a minor and drug abuse) [ 13 , 14 ]. A few studies have examined the relative age effects on physical and sports activities. Yamaguchi et al. reported that relatively younger children and adolescents aged 10–15 years spend more time on sports and art activities than relatively older children and adolescents [ 15 ]. A previous study by our team has already shown that relatively younger individuals aged 10–15 years are less likely to engage in MVPA and to participate in organized sports activity [ 16 ]. However, it remains unknown whether the relative age effects are apparently observed in higher-intensity physical activities. As a possible hypothesis, the relative age effects may be observed in higher-intensity physical activities rather than moderate-intensity activities. Moreover, previous studies have targeted elementary and middle school students [ 15 , 16 ]; however, no study has examined the relative age effect on physical activity among high school students. The relative age effect on physical activity might persist in high school students because Telama et al. have reported that childhood physical activity tracks through adolescence and adulthood [ 17 ]. In contrast to physical activity, evidence of the relative age effects on sedentary behavior is limited. Yamaguchi et al. reported that relatively younger people within the same age cohort spend more time reading books and attending cram school (private lessons to catch up with school studies or to pass the entrance examination) [ 15 ]. Conversely, the relative age effect on recreational sedentary behavior remains unclear. Educational sedentary behavior has been shown to be beneficial for physical fitness and academic performance [ 18 ]. Recreational sedentary behavior (for example, viewing television, playing video games, and Internet use) is broadly different from educational sedentary behavior (e.g., reading books and learning). As mentioned previously, a reduction in recreational sedentary behavior is recommended, while educational sedentary behaviors have been shown to be beneficial in mental health and academic performance [ 6 ]. Particularly, among the Organization for Economic Co-operation and Development member countries, Japanese children and adolescents spent the least amount of time learning Information and Communications Technology and rather spent more time on video games and online games [ 19 ]. Therefore, we targeted Japanese children and adolescents aged 10–18 years and aimed to clarify whether relative age effects of physical activity and sedentary behavior exist and at what stage of school these relative age effects are likely to be observed. We hypothesized that the relative age effect on physical activity could be observed among high school students and that the relative age effect on sedentary behavior could be shown as opposed to that on physical activity. 2. Methods 2.1. Sampling and participants Japanese schools are categorized into 6 years of elementary school (1st − 6th grades), 3 years of middle school (7th − 9th grades), and 3 years of high school (10th − 12th grades). Our group comprises 26 members belonging to 21 academic organizations. A questionnaire survey was conducted in 76 schools (36 elementary, 23 middle, and 17 high schools) from January 2018 to July 2019 [ 16 , 20 , 21 ]. The survey areas covered all eight regions of Japan (Hokkaido, Tohoku, Kanto, Chubu, Kinki, Chugoku, Shikoku, and Kyushu). Responses from 21,491 participants were obtained. Next, 3,210 participants were excluded because of missing data for analysis as follows: sex: n = 502, height: n = 794, weight: n = 1,927, physical activity: n = 383, total sedentary behavior: n = 518, screen time: n = 746 with duplication. Ultimately, 18,281 children and adolescents (10,299 boys and 7,982 girls) were targeted for analysis. Figure 1 shows the procedure for selecting participants for analysis. Figure 1 . Protocol of research participants for analysis Ethics approval was obtained from the Ethics Committee of Doshisha University (approval number: 17095). The purpose, methods, benefits, and risks of our study were explained to the principals of the schools. We also obtained informed consent from participants and their guardians by explaining that we would protect their personal information and that their answers were never related to their school records. 2.2. Basic information and the birth month of the participant In the questionnaire survey, participants were asked about their sex, birth year, birth month, height, and weight. Body mass index (BMI) was calculated based on the participant’s height and weight. The birth months of the participants were divided into 12 months and reordered depending on the Japanese school year (April as the first and March as the last). 2.3. Physical activity and sedentary behavior of the participant The International Physical Activity Questionnaire for Japanese Early Adolescents (IPAQ-JEA), a previous Japanese study that improved the IPAQ to ensure that adolescents can easily understand the question’s meanings, was used [ 22 ]. Participants were asked about their vigorous physical activity (VPA) and moderate physical activity (MPA) in the last 1 week, “How many days per week did you engage in physical activity?” and “ How long per day did you engage in physical activity”? Participants included practice time for organized sports club activities in the VPA. MVPA per day was calculated from the accumulated MVPA time for 7 days using the following formula [ 16 , 20 ]: MVPA time = {(MPA frequency/week × MPA time/day) + (VPA frequency/week × VPA time/day)} / (7 days) We determined the cut-off value for MVPA time per week and utilized dichotomous variables. Regarding physical inactivity, participants with MVPA time less than 60 min per day were defined as “physically inactive,” while those with 60 or more min per day were classified as “physically active,” based on WHO physical activity and sedentary behavior guidelines [ 6 ]. Regarding the lack of information on exercise habits, we categorized participants with 60 or more min per week (approximately 8.6 min per day) as an “exerciser” and less than 60 min per week as an “unexerciser” based on the goal of Health Japan 21 [ 8 ]. Similar to physical activity, the total sedentary time on weekdays and weekends was obtained from the IPAQ-JEA. Additionally, participants were asked about their usual screen time per day associated with television viewing, playing video games, and Internet use, referring to the Health Status Surveillance of School Children Project Report [ 23 ]. Television viewing points included watching television programs and movies on television, digital versatile discs, and Blu-rays. Playing video games excluded online games. Internet use included video-sharing websites, online games, web searches, e-books, communication with social network services, and e-mail. Subsequently, we summed the three screen times to obtain the total screen time. 2.4. Statistical analysis For continuous dependent variables (e.g., physical activity frequency and duration, sedentary time, and screen time), we utilized a two-part regression model using the “twopartm” R package [ 24 ] because some children and adolescents spent no physical activity or sedentary behavior. For example, 7075 boys and girls (38.7%) did not engage in VPA, and 7616 boys and girls (41.7%) did not engage in playing video games. Therefore, we had to consider a zero-inflated model. The two-part model comprises the first part (logistic regression analysis to examine whether dependent variables equal zero or not) and the second part (linear regression analysis without those who did not engage in physical activity or sedentary behavior) and partial regression coefficient of birth month were estimated. Subsequently, we predicted the average margin effect (AME) of birth month. AME presents a partial regression coefficient of birth month for the entire population, including those who engaged in no physical activity or sedentary behavior. For dichotomous dependent variables (physical inactivity and lack of exercise habits), logistic regression analysis was executed. The odds ratio (OR) was calculated using maximum likelihood methods, while a 95% confidence interval (95% CI) was estimated using the Wald method. Previous studies have shown that boys engage in more high-intensity physical activity than girls [ 25 , 26 ]. Additionally, the details of the sedentary behavior differ between boys and girls. Boys spend more total sedentary time playing video games, whereas girls spend more time on the Internet [ 27 ]. Therefore, all analyses were conducted on both boys and girls separately. First, we analyzed children and adolescents from 5th -grader in elementary school to high school and then analyzed them based on school stage to examine the school stage where the relative age effect is apparently observed. However, the relative age effect might be confounded by aging because previous studies have reported that children and adolescents showed decreased physical activity but increased sedentary behavior as they age [ 28 , 29 ]. Furthermore, younger individuals may engage in less physical activity and more sedentary behavior simply because of disadvantageous physiques, such as lower weight and higher obesity risk, as previous studies have shown [ 30 ]. Hence, we adjusted for the adolescent grade and BMI. Statistical analyses were conducted using IBM SPSS Statistics for Windows, version 29 (IBM Corp., Armonk, N.Y., USA) and R version 4.4.1 and RStudio. Statistical significance was set at p < 0.05. 3. Results Table 1 shows the characteristics of the participants and the distribution of their birth months. The average age was similar between boys and girls, and birth months showed no biased distribution. Boys spent more time engaging in both VPA, MPA, and MVPA than girls. Regarding total sedentary time, girls spent more weekday sedentary time than boys, whereas boys spent more weekend sedentary time and had a higher total screen time than girls. Most of the total screen time was for Internet use for both boys and girls. Overall, 48.2% and 65.5% of boys and girls, respectively, did not engage in the minimum MVPA time of 60 min per day, as recommended by WHO. Besides this, 15.5% and 25.0% of boys and girls, respectively, did not engage in exercise for 60 min per week MVPA. VPA frequency and time in middle school students and MVPA time for girls were higher than those at any other school stage. Furthermore, as the school stage increased, MPA frequency, MPA time, and boys’ MVPA time were likely to increase. Regarding sedentary behavior, a higher school stage correlated with more weekday total sedentary time, weekend total sedentary time, and Internet use but less television viewing and playing video games. Table 1. Characteristics of participants and distribution of the birth months of the participants Table 2 shows the relative age effect on continuous and dichotomous physical activities. Among boys, a later birth month was negatively associated with VPA frequency (AME=-0.03, p < 0.01), VPA time (AME=-0.73, p < 0.01), and MVPA time (AME=-0.93, p < 0.01). Upon analyzing based on school stage, the relative age effect was observed on the VPA frequency (middle school students: AME=-0.04, p < 0.01; and high school students: AME=-0.03, p < 0.01), VPA time (middle school students: AME=-0.74, p < 0.01; and high school students: AME=-0.69, p < 0.01), and MVPA time (middle school students: AME=-0.88, p < 0.01; and high school students: AME=-0.96, p < 0.01) of the boys. Among girls, a later birth month showed a negative association with VPA frequency (AME=-0.02, p < 0.01) and VPA time (AME=-0.37, p < 0.01). Upon analyzing based on the school stage, a later birth month was associated with VPA frequency (high school students: AME=-0.02, p < 0.05), VPA time (high school students: b=-0.53, p < 0.05), and MPA time (middle school students: AME=-0.02, p < 0.05) of the girls. Conversely, the opposite of the relative age effect was observed in MPA time for female elementary school students (AME = 0.46, p < 0.05), who reported that a later birth month was associated with more MPA time. The results of the two-part model (the first and second parts) are shown in Additional file 1. Regarding dichotomous physical activity variables, a later birth month correlated to more physically inactive and unexercised boys (physical inactivity: OR = 1.02, p < 0.01; lack of exercise habits: OR = 1.03, p < 0.01). However, a later birth month was associated with more unexercised girls (OR = 1.02, p < 0.05), whereas the birth month was not associated with physical inactivity. As for results based on the school stage, a relative age effect was observed on physical inactivity only in male middle school students (OR = 1.03, p < 0.01), and the birth months showed a positive association with a lack of exercise habits for male elementary and middle school students (elementary school students: OR = 1.06, p < 0.01; and middle school students: OR = 1.03, p < 0.05). However, no relative age effect was found on physical inactivity or the lack of exercise habits for girls at any school stage. Table 3 shows the effect of relative age on sedentary behavior. Later birth months were positively associated with the weekend total sedentary time and Internet use in boys, whereas no association was found between birth month and sedentary behavior among girls. As for the analysis based on school stage, the relative age effect was observed for weekend total sedentary time (b = 2.38, p < 0.01), total screen time (AME = 2.04, p < 0.01), and Internet use (b = 1.35, p < 0.01) only for male high school students. Regarding girls, later birth month was negatively associated with weekday total sedentary time in elementary school students (AME=-3.85, p < 0.05) but positively associated with total screen time (AME = 2.44, p < 0.05) and television viewing in middle school students (AME = 1.12, p < 0.05). The results of the two-part model (the first and second part) are shown in Additional file 2. 4. Discussion 4.1. Relative age effect on physical activity in children and adolescents Our study showed that the relatively younger individuals were likely to have low levels of VPA frequency and time and less likely to have exercise habits for both boys and girls. Additionally, our study showed that relatively younger individuals, particularly among boys, spent less MVPA time and engaged in insufficient physical activity. These results indicated that the relative age effect on physical activity was more likely to be observed at higher intensities of physical activity. Many studies have reported that the relative age effect was more apparent in boys than in girls, thereby supporting our results [ 31 , 32 ]. Girls usually undergo secondary sexual characteristics and body shape changes that are disadvantageous to exercise, such as short legs and rounded hips [ 33 ]. Therefore, relatively older girls do not always have physical advantages, and the relative age effect can weaken. Regarding the school stage, the relative age effect on physical activity was observed in middle and high school students for boys but not in elementary (only VPA frequency) and high school students. Particularly, childhood experiences in succeeding in sports activities and selection may influence the relative age effect on VPA among middle and high school students. A previous study showed that relatively younger children are less likely to have disadvantages in physique [ 30 ] and physical fitness [ 10 , 11 ] and have a low level of competence for exercise [ 34 ]. Additionally, the sports club activities for Japanese students are significantly different between elementary school and later years. Half of all Japanese elementary school students participate in private or regional sports clubs [ 7 ]. However, most Japanese middle and high schools accept school club activity, called “bukatsudo,” with 70–90% of students belonging to school sports clubs [ 7 ]. Therefore, it is suggested that relatively younger individuals are likely to select sports activities and have lower levels of physical activity frequency and time owing to a lack of success in sports activities. Interestingly, the relative age effect on partial physical activity has already emerged among elementary school students. Relatively younger boys were less likely to exercise among elementary school students; however, the relative age effect on physical inactivity was not significant. These results indicate that the relatively younger boys in elementary school hardly engage in MVPA, although both the relatively younger and older boys did not meet MVPA levels recommended by the WHO. Less than half of elementary school boys engaged in more than 60 min/day MVPA (45.8%), while over half of middle and high school boys engaged in more than 60 min/day MVPA (53.9% and 52.3%, respectively). Therefore, it is possible that the relative age effect on physical inactivity was not significantly observed because elementary school students entirely spent less MVPA time. Additionally, our study showed that relatively younger girls in elementary school somewhat engaged in more MPA time, which was not inconsistent with previous studies [ 15 , 16 ]. Although we could not perfectly explain because of the lack of evidence of the relative age effect on MPA, we offered one possible reason. In our study, MPA time was referred to the unorganized playing or exercise on their own as we included practice time for organized sports club activities in VPA. These results indicate that relatively younger girls in elementary school may less frequently participate in organized sports activities but may compensate for physical activity by unorganized playing or exercise in a place that is different from that of the relatively older girls. However, since this cause could not exceed our speculation, future studies should examine what kinds of MPA relatively younger girls are more likely to spend. 4.2. Relative age effect on the sedentary behavior in children and adolescents In contrast to physical activity, we clarified that the relatively younger boys spent more weekend total sedentary time, although no association was observed between birth month and weekday total sedentary behavior. These results indicated that the relative age effect may be observed on weekends when they have more free time to spend on screen time than on weekdays. Adolescents are less likely to have leisure time on weekdays because of school classes, after-school activities, and private lessons. Our study could not accurately report whether the relative age effect on sedentary behavior was caused by leisure or educational sedentary time because we could not ask about adolescents’ leisure or educational sedentary time respectively by weekdays and weekends. Nonetheless, our study actually showed that the weekend total sedentary time was longer than the weekday sedentary time, which may be caused by extracurricular activities. Hence, the observed relative age effect on weekend total sedentary time may be attributed to adolescents having more leisure time that they can use as they prefer. Considering the relative age effect on playing video games and Internet use, it is suggested that on weekends, relatively younger people choose to play video games and Internet use rather than participate in sports. In the analyses based on the school stage, the relatively younger boys were likely to have more weekend total sedentary time, total screen time, playing video games, and Internet use among high school students. Based on the results of physical activity, these findings indicate that sedentary behavior may cause a relative age effect later than physical activity and that relatively younger boys may replace physical activity with sedentary behavior, especially playing video games and Internet use. Previous studies have demonstrated that relatively younger individuals have fewer friends [ 12 ], less time to play out [ 15 ], and a higher risk of dropping out (quitting sports activities) [ 35 ]. Atkin et al. reported that good friendship qualities were associated with higher MVPA and lower sedentary time [ 36 ]. The relatively younger individuals spend more screen time because they may have fewer friends than the relatively older ones. Additionally, middle school students are more likely to participate in sports club activities than at any other school stage [ 37 ]. High school students are less likely to select sports clubs than middle school students [ 37 ]. We could not determine whether high school students dropped out of sports activities because we did not ask them about their participation in sports. Nonetheless, we suggest that the relatively younger adolescents in high school did not engage in physical activity and spent more screen time, possibly because they could not be as successful in sports as in childhood. However, contrary to our hypotheses, the relatively younger girls were likely to spend less weekday sedentary time in elementary school. We do not know the clear reason why we obtained the opposite result for relatively younger girls when compared with boys. Nevertheless, judging from the lack of association between the birth months and total screen time in girls, relatively older adolescents might spend more sedentary time on education. For example, some Japanese elementary school students study out of school (for example, at home and in cram schools) to gain admission to national or private schools. Previous studies have reported a relative age effect on academic scores, where older individuals have higher academic scores [ 13 , 38 ]. Therefore, relatively older girls might spend more weekday sedentary time because they spend more time learning outside of school. However, we could not investigate educational sedentary behavior; therefore, further studies are needed to establish evidence for the relative age effect on recreational and educational sedentary behavior. Regarding gender differences in screen time, we interestingly observed a relative age effect on boy’s playing video games and Internet use (overall and high school students). According to a scoping review that summarizes adolescents’ screen time change since 2000, boys spend more time playing video games than girls, and girls spend more time using cell phones than boys [ 27 ]. As a possible reason, the purpose of Internet use in Japan may have influences on our results. Japanese children and adolescents aged 10–17 years old use the Internet primarily for watching videos (92.9%), following web searches (84.5%), and playing online games (83.0%) [ 39 ]. Therefore, streaming and online games may result in increased Internet use of the relatively younger boys. However, a relative age effect was observed on girl’s television viewing (overall and middle school students), which is inconsistent with those of this review. An Australian longitudinal study for children aged 10 years, followed by 4 years, showed that television viewing in girls significantly increased between 12 and 14 years of age [ 40 ]. Although our study could not completely explain why the relative age effect on television viewing was observed only among female middle school students, relatively younger girls might have had a greater increase in television viewing than relatively older girls, which might reflect our results. 4.3. Educational and political suggestion based on the relative age effect on children and adolescents Physical activity is an important factor in mitigating the relative age effect. Watanabe and Tamura report that relatively young individuals can achieve a high level of physical fitness by engaging in physical activity [ 41 ]. However, it is difficult for un-exercisers to increase their physical activity independently. As most previous studies have indicated, teachers and coaches who educate children and adolescents on sports should comprehend that birth month is associated with not only the physical fitness of children and adolescents but also future sports activity [ 42 ]. Additionally, based on our results, which show that the relative age effect on physical activity emerges among elementary school students, it is necessary to provide opportunities to freely participate and succeed in sports activities. The Japan Association Athletics Federation emphasized that elementary and middle school students should experience various kinds of sports without limiting them to only one sport and that high school students can start to specialize in a few kinds of sports without sticking to their sports performance [ 43 , 44 ]. Furthermore, teachers and coaches should delay the determination of player selection and specialization. Seasonal sports systems may be effective in avoiding relative age effects. In seasonal sports systems, children and adolescents can participate in different sports depending on the season, and seasonal sports systems can prevent them from concentrating on only one sport [ 44 , 45 ]. Seasonal sports systems are accepted in countries, such as New Zealand [ 45 ] and the U.S. [ 46 ]. However, Japan has not introduced a seasonal sports system, and most Japanese adolescents participate in only one sports activity [ 45 ]. Jayanthi et al. concluded that training specializing in a single sport should be delayed until late adolescence based on evidence that training specializing in a single sport is associated with injury, psychological stress, and dropout [ 47 ]. Moreover, Brenner recommends that athletes take a break from their specialized sports for 1 month, 6 months, or 2 months annually [ 48 ]. Introducing a seasonal sports system is expected to minimize the inequity of player selection between relatively younger and older adolescents, as well as to prevent both adolescents from injury and dropping out. Regarding sedentary behavior, it may be necessary to pay attention to the potential effects of birth month on the sedentary behavior of elementary and middle school students. We found that the relative age of sedentary behavior may emerge later than the relative age of physical activity. However, an umbrella review of interventions to reduce sedentary behavior reported that five of the seven reviews supported significant improvement in children’s sedentary behavior but that only one of the four reviews for adolescents assisted significant improvement in reducing sedentary behavior [ 49 ]. Previous studies have clarified that childhood sedentary behavior continues until adolescence and adulthood [ 50 ]. The sedentary behavior of adolescents increases with aging [ 29 ]. Therefore, it is desirable to prevent extreme sedentary behavior, especially during recreational screen time, at an earlier school stage. Furthermore, it is difficult to demand that middle and high school students who do not have exercise habits increase their physical activity and reduce sedentary behavior. Recently, “green time (outdoor time to attach nature environments such as greenspace, park, and forest)” has been remarked as the opposite of screen time. Better access to greenspaces and more outdoor activity in nature reduce mental health risks such as depression, anxiety, and stress [ 51 ]. Of course, it remains unknown whether an increase in green time can mitigate the relative age effect on screen time because a few studies have simultaneously examined both screen time and green time [ 51 ]. Nevertheless, an increase in green time could naturally reduce the relative age effect on sedentary behavior, although increasing VPA time is challenging. Therefore, we expect that the school and educational field will inform children and adolescents of green time benefits in health education and that policymakers will encourage the provision of a neighborhood environment where anyone, including the relatively younger, can freely and easily interact with nature. 4.4. Strength and limitations We examined the relative age effect on physical activity and sedentary behavior in children and adolescents after adjusting for grade, physique, and school differences. Various aspects of physical activity, such as intensity, frequency, time, and cut-off value, were considered, and evidence of the relative age effect among high school students was added. Furthermore, we analyzed the relative age of sedentary behavior based on weekday and weekend differences and according to the type of screen time, which has not been examined in previous studies. Our study clarified that the relatively younger individuals showed insufficient physical activity (especially higher intensity) and more sedentary behavior (especially weekend total sedentary behavior and Internet use), regardless of age and physique. Additionally, we analyzed the effect of the school stage on the relative age effect, which was apparently observed. Consequently, we found that the relative age effect differed from the school stage in terms of the intensity and type of screen time. Moreover, we extend the target school stage to high school. Particularly, it is unique in suggesting that high school students are at the school stage where the relative age effect on physical activity remains, and the relative age effect on sedentary behavior emerges. However, our study had some limitations. First, we could not clarify causal relationships because our study was cross-sectional. For example, although sedentary behavior showed an opposite relative age effect compared to physical activity, we could not perfectly clarify whether relatively younger children and adolescents dropped out of physical activity and replaced them with sedentary behavior. Future longitudinal studies should follow the same person for a longer time to ensure the relative age effect on physical activity and sedentary behavior change. Second, we cannot perfectly deny the overestimation of physical activity and underestimation of sedentary behavior because of the characteristics of the questionnaire survey. Nonetheless, we made our maximum efforts to ensure that participants could comprehend the questionnaire’s intention and easily remember it (physical activity in the last 1 week and usual sedentary behavior on average). Third, we could not investigate the places where children and adolescents engaged in physical activity or sedentary behavior and the details of physical activity or sedentary behavior. Therefore, we could not distinguish between organized physical activity and unorganized recreational behavior. Additionally, due to the lack of information on exercise habits, we assumed that participants with 60 or more min per week are “exercisers.” Regarding screen time, we could not specify the detailed purpose (for example, playing games, web search, and communication). The relative age effect emerges not only in behavioral development but also in social development (e.g., number of friends and prosocial emotions) [ 12 – 14 ]. Hence, future studies should focus on the locations and purposes of physical activity and sedentary behavior. Moreover, future studies should clarify how the relative age effect influences the interaction between behavioral and social development. 5. Conclusions We observed the relative effects of age on physical activity and sedentary behaviors among children and adolescents living in various areas of Japan. Additionally, the relative age effect on physical activity had already emerged in elementary school students and was observed in middle and high school students. Particularly, relatively younger individuals engaged in less high-intensity physical activity than relatively older individuals. Conversely, relatively younger male high school students spent more sedentary time, especially on weekends, playing video games and Internet use. Meanwhile, we could show the relative age effect on television viewing for female middle school students. Hence, it is suggested that the relative age effect on physical activity remains after middle school and that relatively younger individuals are more likely to replace physical activity with sedentary behavior. Abbreviations MVPA: moderate-to-vigorous physical activity, VPA: vigorous physical activity, MPA: moderate physical activity, WHO: World Health Organization, IPAQ-JEA: International Physical Activity Questionnaire for Japanese Early Adolescents, BMI: body mass index, AME: average margin effect, Declarations Ethics approval and consent to participate Ethics approval was obtained from the Ethics Committee of Doshisha University (approval number: 17095). The purpose, methods, benefits, and risks of our study were explained to the principals of the schools. We also obtained participants and their guardians by explaining that we would protect their personal information and that their answers were never related to their school records. Consent for publication Not applicable. Availability of data and materials The datasets are publicly unavailable because the publication of datasets, including schools and personal information (e.g., birth month), is prohibited in Japan. Nevertheless, the datasets are available from the corresponding author upon reasonable request. Competing interests The authors do not have any competing interests. Funding This work was supported by JSPS KAKENHI Grant Number JP 18H01000, JSPS KAKENHI Grant Number JP 19K21797, and JSPS KAKENHI Grant Number JP 21H00848. Authors’ Contributions TM and KI conceptualized and designed this study. TA (Aoki) and KI acquired funding. TA (Aoki), TH (Harada), CT, ST, HT, KF, YK, NT, KK, SK, NM, KS, MW, RK, TH (Hara), RM, TA (Abe), KY, DK, HA, NY, TT and KI conducted the survey. TA, KO, and KI sorted out the data. TT and TN provided technical information about statistical analysis. TM performed statistical analysis and drafted the manuscript. TM and KI reviewed and edited the manuscript. All authors read and approved the final manuscript. Acknowledgments We appreciate all participants and their teachers for their cooperation with the questionnaire survey. We would also like to thank Editage (http://www.editage.jp/) for English language editing. References Poitras VJ, Gray CE, Borghese MM, Carson V, Chaput JP, Janssen I, et al. Systematic review of the relationships between objectively measured physical activity and health indicators in school-aged children and youth. Appl Physiol Nutr Metab. 2016;41;Suppl 3:S197-239. 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Sato M, Ishii K, Shibata A, Mano Y, Oka K. Gender and grade differences in school recess physical activity among Japanese elementary school children. Jpn J Hum Growth Dev Res. 2010;54:11-7. Thomas G, Bennie JA, De Cocker K, Castro O, Biddle SJH. A descriptive epidemiology of screen-based devices by children and adolescents: a scoping review of 130 surveillance studies since 2000. Child Indic Res. 2020;13:935-50. Kallio J, Hakonen H, Syväoja H, Kulmala J, Kankaanpää A, Ekelund U, et al. Changes in physical activity and sedentary time during adolescence: gender differences during weekdays and weekend days. Scand J Med Sci Sports. 2020;30:1265-75. Kontostoli E, Jones AP, Pearson N, Foley L, Biddle SJH, Atkin AJ. Age-related change in sedentary behavior during childhood and adolescence: a systematic review and meta-analysis. Obes Rev. 2021;22:e13263. Hemati Z, Keikha M, Riahi R, Daniali SS, Goudarzi M, Kelishadi R. A systematic review on the association of month and season of birth with future anthropometric measures. Pediatr Res. 2021;89:31-45. Cobley S, Baker J, Wattie N, McKenna J. Annual age-grouping and athlete development: a meta-analytical review of relative age effects in sport. Sports Med. 2009;39:235-56. Nakata H, Sakamoto K. Sex differences in relative age effects among Japanese athletes. Percept Mot Skills. 2012;115:179-86. Vincent J, Glamser FD. Gender differences in the relative age effect among US Olympic Development Program youth soccer players. J Sports Sci. 2006;24:405-13. Kawata Y, Kamimura A, Oki K, Yamada K, Hirosawa M. Relative age effect on psychological factors related to sports participation among Japanese elementary school children. Advances in human factors in sports and outdoor recreation. Florida: Walt disney World®; 2017. p. 199-211. Crane J, Temple V. A systematic review of dropout from organized sport among children and youth. Eur Phys Educ Rev. 2015;21:114-31. Atkin AJ, Corder K, Goodyer I, Bamber D, Ekelund U, Brage S, et al. Perceived family functioning and friendship quality: cross-sectional associations with physical activity and sedentary behaviours. Int J Behav Nutr Phys Act. 2015;12:23. Sasagawa Sports Foundation. The 2021 SSF national sports-life survey of children and young people. Tokyo: Sasagawa Sports Foundation; 2021. p. 91-103. Urruticoechea A, Oliveri A, Vernazza E, Giménez-Dasí M, Martínez-Arias R, Martín-Babarro J. The relative age effects in educational development: a systematic review. Int J Environ Res Public Health. 2021;18:8966. Cabinet Office of Japan. Internet use environment survey of young people; 2022 (in Japanese). https://warp.ndl.go.jp/info:ndljp/pid/12772297/www8.cao.go.jp/youth/kankyou/internet_torikumi/tyousa/r04/net-jittai/pdf-index.html. Accessed 8 Mar 2024. Thomas G, Bennie JA, De Cocker K, Ireland MJ, Biddle SJH. Screen-based behaviors in Australian adolescents: longitudinal trends from a 4-year follow-up study. Prev Med. 2020;141:106258. Watanabe M, Tamura M. Characteristics of children born later in the school year, showing better physical fitness. Jpn J Hum Growth Dev Res. 2017;2017:1-8. Webdale K, Baker J, Schorer J, Wattie N. Solving sport’s ‘relative age’ problem: a systematic review of proposed solutions. Int Rev Sports Exerc Psychol. 2020;13:187-204. Japan association athletics federations. JAAF athlete development model (in Japanese). https://www.jaaf.or.jp/development/model/. Accessed 8 Mar 2024. Mori T, Ishii K. Relative age effect among Japanese athletes who participated in the World Athletics Championships and Olympic Games. Jpn J Hum Growth Dev Res. 2023;2023:18-24. Nishio T. The plan for the reform of Japanese school club sports with reference to the New Zealand context. J Jpn Soc Sports Ind. 2017;27:69-72. Nakazawa A. Post World War Ⅱ and the present of extracurricular sports activity -Why sports are connected with school education?- Tokyo: Seikyusha; 2014. p. 46-51. Jayanthi N, Pinkham C, Dugas L, Patrick B, Labella C. Sports specialization in young athletes: evidence-based recommendations. Sports Health. 2013;5:251-7. Brenner JS, Council on Sports Medicine and Fitness. Sports specialization and intensive training in young athletes. Pediatrics. 2016;138:e20162148. Nguyen P, Le LK, Nguyen D, Gao L, Dunstan DW, Moodie M. The effectiveness of sedentary behaviour interventions on sitting time and screen time in children and adults: an umbrella review of systematic reviews. Int J Behav Nutr Phys Act. 2020;17:117. Biddle SJH, Pearson N, Ross GM, Braithwaite R. Tracking of sedentary behaviours of young people: a systematic review. Prev Med. 2010;51:345-51. Oswald TK, Rumbold AR, Kedzior SGE, Moore VM. Psychological impacts of “screen time” and “green time” for children and adolescents: a systematic scoping review. PLoS One. 2020;15:e0237725. Tables Tables are available in the Supplementary Files section. Additional Declarations No competing interests reported. Supplementary Files Tables.docx Additionalfile1.docx File name: Additional file 1 File format: doc Title of data: Results of two-part model (physical activity) Additionalfile2.docx File name: Additional file 2 File format: doc Title of data: Results of two-part model (sedentary behavior) Cite Share Download PDF Status: Published Journal Publication published 25 Nov, 2024 Read the published version in BMC Public Health → Version 1 posted Editorial decision: Revision requested 24 Sep, 2024 Reviews received at journal 22 Sep, 2024 Reviewers agreed at journal 16 Sep, 2024 Reviews received at journal 15 Sep, 2024 Reviewers agreed at journal 15 Sep, 2024 Reviewers agreed at journal 14 Sep, 2024 Reviewers invited by journal 14 Sep, 2024 Editor invited by journal 21 Aug, 2024 Editor assigned by journal 20 Aug, 2024 Submission checks completed at journal 20 Aug, 2024 First submitted to journal 19 Aug, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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reported.","formattedTitle":"Relative age effect on the physical activity and sedentary behavior in children and adolescents aged 10 to 18 years old: A cross-sectional study in Japan","fulltext":[{"header":"1. Background","content":"\u003cp\u003eThe physical activity and sedentary behaviors of children and adolescents are related to various lifestyles and health conditions. Previous studies targeting children and adolescents have shown that more physical activity enhances cardiorespiratory fitness endurance capacity, obesity, and cardiovascular disease [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Conversely, children and adolescents who engage in prolonged sedentary behavior have a lack of cardiorespiratory fitness and mental health conditions [\u003cspan additionalcitationids=\"CR4\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Based on this evidence [\u003cspan additionalcitationids=\"CR2 CR3 CR4\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], the World Health Organization (WHO) recommends that children and adolescents aged 5\u0026ndash;17 should engage in at least an average of 60 min per day of moderate-to-vigorous intensity physical activity (MVPA) across the week and reduce sedentary behavior, especially recreational screen time [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Besides this, the Japan Sports Agency reported that some Japanese children and adolescents (particularly approximately 25% of female middle school students) hardly spent any time on exercise, including active play (less than 60 min per week) [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Therefore, Health Japan 21 (the third term), which is a national health measure in Japan, set a goal to reduce the number of children and adolescents who spend less than 60 min of physical activity per week [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe relative age effect is defined as the difference caused by chronological age within the same age cohort [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. The school year in Japan begins in April and ends in March of the following year. Children and adolescents born in April were approximately 11 months older than those born in March. Therefore, \u0026ldquo;the relatively younger individuals\u0026rdquo; (those born in the latter half of the school year) are disadvantaged in terms of physical, mental, and social development. For example, several studies have reported that relatively younger individuals are shorter and have lower levels of physical fitness (e.g., grip strength, standing long jumping, and 50 m sprint) [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Additionally, systematic reviews and meta-analyses on the relative age effect on mental and social development clarified that relatively younger individuals have fewer friends [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e], poor prosocial emotions (e.g., self-esteem and competence), and a higher risk of depression and problematic behavior (e.g., smoking as a minor and drug abuse) [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eA few studies have examined the relative age effects on physical and sports activities. Yamaguchi et al. reported that relatively younger children and adolescents aged 10\u0026ndash;15 years spend more time on sports and art activities than relatively older children and adolescents [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. A previous study by our team has already shown that relatively younger individuals aged 10\u0026ndash;15 years are less likely to engage in MVPA and to participate in organized sports activity [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. However, it remains unknown whether the relative age effects are apparently observed in higher-intensity physical activities. As a possible hypothesis, the relative age effects may be observed in higher-intensity physical activities rather than moderate-intensity activities. Moreover, previous studies have targeted elementary and middle school students [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]; however, no study has examined the relative age effect on physical activity among high school students. The relative age effect on physical activity might persist in high school students because Telama et al. have reported that childhood physical activity tracks through adolescence and adulthood [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn contrast to physical activity, evidence of the relative age effects on sedentary behavior is limited. Yamaguchi et al. reported that relatively younger people within the same age cohort spend more time reading books and attending cram school (private lessons to catch up with school studies or to pass the entrance examination) [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Conversely, the relative age effect on recreational sedentary behavior remains unclear. Educational sedentary behavior has been shown to be beneficial for physical fitness and academic performance [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Recreational sedentary behavior (for example, viewing television, playing video games, and Internet use) is broadly different from educational sedentary behavior (e.g., reading books and learning). As mentioned previously, a reduction in recreational sedentary behavior is recommended, while educational sedentary behaviors have been shown to be beneficial in mental health and academic performance [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Particularly, among the Organization for Economic Co-operation and Development member countries, Japanese children and adolescents spent the least amount of time learning Information and Communications Technology and rather spent more time on video games and online games [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Therefore, we targeted Japanese children and adolescents aged 10\u0026ndash;18 years and aimed to clarify whether relative age effects of physical activity and sedentary behavior exist and at what stage of school these relative age effects are likely to be observed. We hypothesized that the relative age effect on physical activity could be observed among high school students and that the relative age effect on sedentary behavior could be shown as opposed to that on physical activity.\u003c/p\u003e"},{"header":"2. Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Sampling and participants\u003c/h2\u003e \u003cp\u003eJapanese schools are categorized into 6 years of elementary school (1st \u0026minus;\u0026thinsp;6th grades), 3 years of middle school (7th \u0026minus;\u0026thinsp;9th grades), and 3 years of high school (10th \u0026minus;\u0026thinsp;12th grades). Our group comprises 26 members belonging to 21 academic organizations. A questionnaire survey was conducted in 76 schools (36 elementary, 23 middle, and 17 high schools) from January 2018 to July 2019 [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. The survey areas covered all eight regions of Japan (Hokkaido, Tohoku, Kanto, Chubu, Kinki, Chugoku, Shikoku, and Kyushu). Responses from 21,491 participants were obtained. Next, 3,210 participants were excluded because of missing data for analysis as follows: sex: n\u0026thinsp;=\u0026thinsp;502, height: n\u0026thinsp;=\u0026thinsp;794, weight: n\u0026thinsp;=\u0026thinsp;1,927, physical activity: n\u0026thinsp;=\u0026thinsp;383, total sedentary behavior: n\u0026thinsp;=\u0026thinsp;518, screen time: n\u0026thinsp;=\u0026thinsp;746 with duplication. Ultimately, 18,281 children and adolescents (10,299 boys and 7,982 girls) were targeted for analysis. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the procedure for selecting participants for analysis.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Protocol of research participants for analysis\u003c/p\u003e \u003cp\u003e \u003cstrong\u003eEthics approval\u003c/strong\u003e \u003cp\u003ewas obtained from the Ethics Committee of Doshisha University (approval number: 17095). The purpose, methods, benefits, and risks of our study were explained to the principals of the schools. We also obtained informed consent from participants and their guardians by explaining that we would protect their personal information and that their answers were never related to their school records.\u003c/p\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Basic information and the birth month of the participant\u003c/h2\u003e \u003cp\u003eIn the questionnaire survey, participants were asked about their sex, birth year, birth month, height, and weight. Body mass index (BMI) was calculated based on the participant\u0026rsquo;s height and weight. The birth months of the participants were divided into 12 months and reordered depending on the Japanese school year (April as the first and March as the last).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Physical activity and sedentary behavior of the participant\u003c/h2\u003e \u003cp\u003eThe International Physical Activity Questionnaire for Japanese Early Adolescents (IPAQ-JEA), a previous Japanese study that improved the IPAQ to ensure that adolescents can easily understand the question\u0026rsquo;s meanings, was used [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Participants were asked about their vigorous physical activity (VPA) and moderate physical activity (MPA) in the last 1 week, \u0026ldquo;How many days per week did you engage in physical activity?\u0026rdquo; and \u0026ldquo; How long per day did you engage in physical activity\u0026rdquo;? Participants included practice time for organized sports club activities in the VPA. MVPA per day was calculated from the accumulated MVPA time for 7 days using the following formula [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]:\u003c/p\u003e \u003cp\u003eMVPA time = {(MPA frequency/week \u0026times; MPA time/day) + (VPA frequency/week \u0026times; VPA time/day)} / (7 days)\u003c/p\u003e \u003cp\u003eWe determined the cut-off value for MVPA time per week and utilized dichotomous variables. Regarding physical inactivity, participants with MVPA time less than 60 min per day were defined as \u0026ldquo;physically inactive,\u0026rdquo; while those with 60 or more min per day were classified as \u0026ldquo;physically active,\u0026rdquo; based on WHO physical activity and sedentary behavior guidelines [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Regarding the lack of information on exercise habits, we categorized participants with 60 or more min per week (approximately 8.6 min per day) as an \u0026ldquo;exerciser\u0026rdquo; and less than 60 min per week as an \u0026ldquo;unexerciser\u0026rdquo; based on the goal of Health Japan 21 [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eSimilar to physical activity, the total sedentary time on weekdays and weekends was obtained from the IPAQ-JEA. Additionally, participants were asked about their usual screen time per day associated with television viewing, playing video games, and Internet use, referring to the Health Status Surveillance of School Children Project Report [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Television viewing points included watching television programs and movies on television, digital versatile discs, and Blu-rays. Playing video games excluded online games. Internet use included video-sharing websites, online games, web searches, e-books, communication with social network services, and e-mail. Subsequently, we summed the three screen times to obtain the total screen time.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4. Statistical analysis\u003c/h2\u003e \u003cp\u003eFor continuous dependent variables (e.g., physical activity frequency and duration, sedentary time, and screen time), we utilized a two-part regression model using the \u0026ldquo;twopartm\u0026rdquo; R package [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e] because some children and adolescents spent no physical activity or sedentary behavior. For example, 7075 boys and girls (38.7%) did not engage in VPA, and 7616 boys and girls (41.7%) did not engage in playing video games. Therefore, we had to consider a zero-inflated model. The two-part model comprises the first part (logistic regression analysis to examine whether dependent variables equal zero or not) and the second part (linear regression analysis without those who did not engage in physical activity or sedentary behavior) and partial regression coefficient of birth month were estimated. Subsequently, we predicted the average margin effect (AME) of birth month. AME presents a partial regression coefficient of birth month for the entire population, including those who engaged in no physical activity or sedentary behavior. For dichotomous dependent variables (physical inactivity and lack of exercise habits), logistic regression analysis was executed. The odds ratio (OR) was calculated using maximum likelihood methods, while a 95% confidence interval (95% CI) was estimated using the Wald method.\u003c/p\u003e \u003cp\u003ePrevious studies have shown that boys engage in more high-intensity physical activity than girls [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Additionally, the details of the sedentary behavior differ between boys and girls. Boys spend more total sedentary time playing video games, whereas girls spend more time on the Internet [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. Therefore, all analyses were conducted on both boys and girls separately. First, we analyzed children and adolescents from 5th -grader in elementary school to high school and then analyzed them based on school stage to examine the school stage where the relative age effect is apparently observed. However, the relative age effect might be confounded by aging because previous studies have reported that children and adolescents showed decreased physical activity but increased sedentary behavior as they age [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Furthermore, younger individuals may engage in less physical activity and more sedentary behavior simply because of disadvantageous physiques, such as lower weight and higher obesity risk, as previous studies have shown [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Hence, we adjusted for the adolescent grade and BMI. Statistical analyses were conducted using IBM SPSS Statistics for Windows, version 29 (IBM Corp., Armonk, N.Y., USA) and R version 4.4.1 and RStudio. Statistical significance was set at p\u0026thinsp;\u0026lt;\u0026thinsp;0.05.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results","content":"\u003cp\u003eTable\u0026nbsp;1 shows the characteristics of the participants and the distribution of their birth months. The average age was similar between boys and girls, and birth months showed no biased distribution. Boys spent more time engaging in both VPA, MPA, and MVPA than girls. Regarding total sedentary time, girls spent more weekday sedentary time than boys, whereas boys spent more weekend sedentary time and had a higher total screen time than girls. Most of the total screen time was for Internet use for both boys and girls. Overall, 48.2% and 65.5% of boys and girls, respectively, did not engage in the minimum MVPA time of 60 min per day, as recommended by WHO. Besides this, 15.5% and 25.0% of boys and girls, respectively, did not engage in exercise for 60 min per week MVPA.\u003c/p\u003e\n\u003cp\u003eVPA frequency and time in middle school students and MVPA time for girls were higher than those at any other school stage. Furthermore, as the school stage increased, MPA frequency, MPA time, and boys\u0026rsquo; MVPA time were likely to increase. Regarding sedentary behavior, a higher school stage correlated with more weekday total sedentary time, weekend total sedentary time, and Internet use but less television viewing and playing video games.\u003c/p\u003e\n\u003cp\u003eTable\u0026nbsp;1. Characteristics of participants and distribution of the birth months of the participants\u003c/p\u003e\n\u003cp\u003eTable \u003cspan\u003e2\u003c/span\u003e shows the relative age effect on continuous and dichotomous physical activities. Among boys, a later birth month was negatively associated with VPA frequency (AME=-0.03, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01), VPA time (AME=-0.73, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01), and MVPA time (AME=-0.93, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01). Upon analyzing based on school stage, the relative age effect was observed on the VPA frequency (middle school students: AME=-0.04, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01; and high school students: AME=-0.03, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01), VPA time (middle school students: AME=-0.74, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01; and high school students: AME=-0.69, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01), and MVPA time (middle school students: AME=-0.88, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01; and high school students: AME=-0.96, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01) of the boys. Among girls, a later birth month showed a negative association with VPA frequency (AME=-0.02, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01) and VPA time (AME=-0.37, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01). Upon analyzing based on the school stage, a later birth month was associated with VPA frequency (high school students: AME=-0.02, p\u0026thinsp;\u0026lt;\u0026thinsp;0.05), VPA time (high school students: b=-0.53, p\u0026thinsp;\u0026lt;\u0026thinsp;0.05), and MPA time (middle school students: AME=-0.02, p\u0026thinsp;\u0026lt;\u0026thinsp;0.05) of the girls. Conversely, the opposite of the relative age effect was observed in MPA time for female elementary school students (AME\u0026thinsp;=\u0026thinsp;0.46, p\u0026thinsp;\u0026lt;\u0026thinsp;0.05), who reported that a later birth month was associated with more MPA time. The results of the two-part model (the first and second parts) are shown in Additional file 1.\u003c/p\u003e\n\u003cp\u003eRegarding dichotomous physical activity variables, a later birth month correlated to more physically inactive and unexercised boys (physical inactivity: OR\u0026thinsp;=\u0026thinsp;1.02, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01; lack of exercise habits: OR\u0026thinsp;=\u0026thinsp;1.03, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01). However, a later birth month was associated with more unexercised girls (OR\u0026thinsp;=\u0026thinsp;1.02, p\u0026thinsp;\u0026lt;\u0026thinsp;0.05), whereas the birth month was not associated with physical inactivity. As for results based on the school stage, a relative age effect was observed on physical inactivity only in male middle school students (OR\u0026thinsp;=\u0026thinsp;1.03, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01), and the birth months showed a positive association with a lack of exercise habits for male elementary and middle school students (elementary school students: OR\u0026thinsp;=\u0026thinsp;1.06, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01; and middle school students: OR\u0026thinsp;=\u0026thinsp;1.03, p\u0026thinsp;\u0026lt;\u0026thinsp;0.05). However, no relative age effect was found on physical inactivity or the lack of exercise habits for girls at any school stage.\u003c/p\u003e\n\u003cp\u003eTable\u0026nbsp;3 shows the effect of relative age on sedentary behavior. Later birth months were positively associated with the weekend total sedentary time and Internet use in boys, whereas no association was found between birth month and sedentary behavior among girls. As for the analysis based on school stage, the relative age effect was observed for weekend total sedentary time (b\u0026thinsp;=\u0026thinsp;2.38, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01), total screen time (AME\u0026thinsp;=\u0026thinsp;2.04, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01), and Internet use (b\u0026thinsp;=\u0026thinsp;1.35, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01) only for male high school students. Regarding girls, later birth month was negatively associated with weekday total sedentary time in elementary school students (AME=-3.85, p\u0026thinsp;\u0026lt;\u0026thinsp;0.05) but positively associated with total screen time (AME\u0026thinsp;=\u0026thinsp;2.44, p\u0026thinsp;\u0026lt;\u0026thinsp;0.05) and television viewing in middle school students (AME\u0026thinsp;=\u0026thinsp;1.12, p\u0026thinsp;\u0026lt;\u0026thinsp;0.05). The results of the two-part model (the first and second part) are shown in Additional file 2.\u003c/p\u003e\n"},{"header":"4. Discussion","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e4.1. Relative age effect on physical activity in children and adolescents\u003c/h2\u003e \u003cp\u003eOur study showed that the relatively younger individuals were likely to have low levels of VPA frequency and time and less likely to have exercise habits for both boys and girls. Additionally, our study showed that relatively younger individuals, particularly among boys, spent less MVPA time and engaged in insufficient physical activity. These results indicated that the relative age effect on physical activity was more likely to be observed at higher intensities of physical activity. Many studies have reported that the relative age effect was more apparent in boys than in girls, thereby supporting our results [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. Girls usually undergo secondary sexual characteristics and body shape changes that are disadvantageous to exercise, such as short legs and rounded hips [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. Therefore, relatively older girls do not always have physical advantages, and the relative age effect can weaken.\u003c/p\u003e \u003cp\u003eRegarding the school stage, the relative age effect on physical activity was observed in middle and high school students for boys but not in elementary (only VPA frequency) and high school students. Particularly, childhood experiences in succeeding in sports activities and selection may influence the relative age effect on VPA among middle and high school students. A previous study showed that relatively younger children are less likely to have disadvantages in physique [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e] and physical fitness [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] and have a low level of competence for exercise [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. Additionally, the sports club activities for Japanese students are significantly different between elementary school and later years. Half of all Japanese elementary school students participate in private or regional sports clubs [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. However, most Japanese middle and high schools accept school club activity, called \u0026ldquo;bukatsudo,\u0026rdquo; with 70\u0026ndash;90% of students belonging to school sports clubs [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Therefore, it is suggested that relatively younger individuals are likely to select sports activities and have lower levels of physical activity frequency and time owing to a lack of success in sports activities.\u003c/p\u003e \u003cp\u003eInterestingly, the relative age effect on partial physical activity has already emerged among elementary school students. Relatively younger boys were less likely to exercise among elementary school students; however, the relative age effect on physical inactivity was not significant. These results indicate that the relatively younger boys in elementary school hardly engage in MVPA, although both the relatively younger and older boys did not meet MVPA levels recommended by the WHO. Less than half of elementary school boys engaged in more than 60 min/day MVPA (45.8%), while over half of middle and high school boys engaged in more than 60 min/day MVPA (53.9% and 52.3%, respectively). Therefore, it is possible that the relative age effect on physical inactivity was not significantly observed because elementary school students entirely spent less MVPA time. Additionally, our study showed that relatively younger girls in elementary school somewhat engaged in more MPA time, which was not inconsistent with previous studies [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Although we could not perfectly explain because of the lack of evidence of the relative age effect on MPA, we offered one possible reason. In our study, MPA time was referred to the unorganized playing or exercise on their own as we included practice time for organized sports club activities in VPA. These results indicate that relatively younger girls in elementary school may less frequently participate in organized sports activities but may compensate for physical activity by unorganized playing or exercise in a place that is different from that of the relatively older girls. However, since this cause could not exceed our speculation, future studies should examine what kinds of MPA relatively younger girls are more likely to spend.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e4.2. Relative age effect on the sedentary behavior in children and adolescents\u003c/h2\u003e \u003cp\u003eIn contrast to physical activity, we clarified that the relatively younger boys spent more weekend total sedentary time, although no association was observed between birth month and weekday total sedentary behavior. These results indicated that the relative age effect may be observed on weekends when they have more free time to spend on screen time than on weekdays. Adolescents are less likely to have leisure time on weekdays because of school classes, after-school activities, and private lessons. Our study could not accurately report whether the relative age effect on sedentary behavior was caused by leisure or educational sedentary time because we could not ask about adolescents\u0026rsquo; leisure or educational sedentary time respectively by weekdays and weekends. Nonetheless, our study actually showed that the weekend total sedentary time was longer than the weekday sedentary time, which may be caused by extracurricular activities. Hence, the observed relative age effect on weekend total sedentary time may be attributed to adolescents having more leisure time that they can use as they prefer. Considering the relative age effect on playing video games and Internet use, it is suggested that on weekends, relatively younger people choose to play video games and Internet use rather than participate in sports.\u003c/p\u003e \u003cp\u003eIn the analyses based on the school stage, the relatively younger boys were likely to have more weekend total sedentary time, total screen time, playing video games, and Internet use among high school students. Based on the results of physical activity, these findings indicate that sedentary behavior may cause a relative age effect later than physical activity and that relatively younger boys may replace physical activity with sedentary behavior, especially playing video games and Internet use. Previous studies have demonstrated that relatively younger individuals have fewer friends [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e], less time to play out [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e], and a higher risk of dropping out (quitting sports activities) [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. Atkin et al. reported that good friendship qualities were associated with higher MVPA and lower sedentary time [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. The relatively younger individuals spend more screen time because they may have fewer friends than the relatively older ones. Additionally, middle school students are more likely to participate in sports club activities than at any other school stage [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. High school students are less likely to select sports clubs than middle school students [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. We could not determine whether high school students dropped out of sports activities because we did not ask them about their participation in sports. Nonetheless, we suggest that the relatively younger adolescents in high school did not engage in physical activity and spent more screen time, possibly because they could not be as successful in sports as in childhood. However, contrary to our hypotheses, the relatively younger girls were likely to spend less weekday sedentary time in elementary school. We do not know the clear reason why we obtained the opposite result for relatively younger girls when compared with boys. Nevertheless, judging from the lack of association between the birth months and total screen time in girls, relatively older adolescents might spend more sedentary time on education. For example, some Japanese elementary school students study out of school (for example, at home and in cram schools) to gain admission to national or private schools. Previous studies have reported a relative age effect on academic scores, where older individuals have higher academic scores [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. Therefore, relatively older girls might spend more weekday sedentary time because they spend more time learning outside of school. However, we could not investigate educational sedentary behavior; therefore, further studies are needed to establish evidence for the relative age effect on recreational and educational sedentary behavior.\u003c/p\u003e \u003cp\u003eRegarding gender differences in screen time, we interestingly observed a relative age effect on boy\u0026rsquo;s playing video games and Internet use (overall and high school students). According to a scoping review that summarizes adolescents\u0026rsquo; screen time change since 2000, boys spend more time playing video games than girls, and girls spend more time using cell phones than boys [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. As a possible reason, the purpose of Internet use in Japan may have influences on our results. Japanese children and adolescents aged 10\u0026ndash;17 years old use the Internet primarily for watching videos (92.9%), following web searches (84.5%), and playing online games (83.0%) [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. Therefore, streaming and online games may result in increased Internet use of the relatively younger boys. However, a relative age effect was observed on girl\u0026rsquo;s television viewing (overall and middle school students), which is inconsistent with those of this review. An Australian longitudinal study for children aged 10 years, followed by 4 years, showed that television viewing in girls significantly increased between 12 and 14 years of age [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. Although our study could not completely explain why the relative age effect on television viewing was observed only among female middle school students, relatively younger girls might have had a greater increase in television viewing than relatively older girls, which might reflect our results.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e4.3. Educational and political suggestion based on the relative age effect on children and adolescents\u003c/h2\u003e \u003cp\u003ePhysical activity is an important factor in mitigating the relative age effect. Watanabe and Tamura report that relatively young individuals can achieve a high level of physical fitness by engaging in physical activity [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. However, it is difficult for un-exercisers to increase their physical activity independently. As most previous studies have indicated, teachers and coaches who educate children and adolescents on sports should comprehend that birth month is associated with not only the physical fitness of children and adolescents but also future sports activity [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. Additionally, based on our results, which show that the relative age effect on physical activity emerges among elementary school students, it is necessary to provide opportunities to freely participate and succeed in sports activities. The Japan Association Athletics Federation emphasized that elementary and middle school students should experience various kinds of sports without limiting them to only one sport and that high school students can start to specialize in a few kinds of sports without sticking to their sports performance [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. Furthermore, teachers and coaches should delay the determination of player selection and specialization. Seasonal sports systems may be effective in avoiding relative age effects. In seasonal sports systems, children and adolescents can participate in different sports depending on the season, and seasonal sports systems can prevent them from concentrating on only one sport [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e, \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]. Seasonal sports systems are accepted in countries, such as New Zealand [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e] and the U.S. [\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e]. However, Japan has not introduced a seasonal sports system, and most Japanese adolescents participate in only one sports activity [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]. Jayanthi et al. concluded that training specializing in a single sport should be delayed until late adolescence based on evidence that training specializing in a single sport is associated with injury, psychological stress, and dropout [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]. Moreover, Brenner recommends that athletes take a break from their specialized sports for 1 month, 6 months, or 2 months annually [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]. Introducing a seasonal sports system is expected to minimize the inequity of player selection between relatively younger and older adolescents, as well as to prevent both adolescents from injury and dropping out.\u003c/p\u003e \u003cp\u003eRegarding sedentary behavior, it may be necessary to pay attention to the potential effects of birth month on the sedentary behavior of elementary and middle school students. We found that the relative age of sedentary behavior may emerge later than the relative age of physical activity. However, an umbrella review of interventions to reduce sedentary behavior reported that five of the seven reviews supported significant improvement in children\u0026rsquo;s sedentary behavior but that only one of the four reviews for adolescents assisted significant improvement in reducing sedentary behavior [\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e]. Previous studies have clarified that childhood sedentary behavior continues until adolescence and adulthood [\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e]. The sedentary behavior of adolescents increases with aging [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Therefore, it is desirable to prevent extreme sedentary behavior, especially during recreational screen time, at an earlier school stage. Furthermore, it is difficult to demand that middle and high school students who do not have exercise habits increase their physical activity and reduce sedentary behavior. Recently, \u0026ldquo;green time (outdoor time to attach nature environments such as greenspace, park, and forest)\u0026rdquo; has been remarked as the opposite of screen time. Better access to greenspaces and more outdoor activity in nature reduce mental health risks such as depression, anxiety, and stress [\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e]. Of course, it remains unknown whether an increase in green time can mitigate the relative age effect on screen time because a few studies have simultaneously examined both screen time and green time [\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e]. Nevertheless, an increase in green time could naturally reduce the relative age effect on sedentary behavior, although increasing VPA time is challenging. Therefore, we expect that the school and educational field will inform children and adolescents of green time benefits in health education and that policymakers will encourage the provision of a neighborhood environment where anyone, including the relatively younger, can freely and easily interact with nature.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e4.4. Strength and limitations\u003c/h2\u003e \u003cp\u003eWe examined the relative age effect on physical activity and sedentary behavior in children and adolescents after adjusting for grade, physique, and school differences. Various aspects of physical activity, such as intensity, frequency, time, and cut-off value, were considered, and evidence of the relative age effect among high school students was added. Furthermore, we analyzed the relative age of sedentary behavior based on weekday and weekend differences and according to the type of screen time, which has not been examined in previous studies. Our study clarified that the relatively younger individuals showed insufficient physical activity (especially higher intensity) and more sedentary behavior (especially weekend total sedentary behavior and Internet use), regardless of age and physique. Additionally, we analyzed the effect of the school stage on the relative age effect, which was apparently observed. Consequently, we found that the relative age effect differed from the school stage in terms of the intensity and type of screen time. Moreover, we extend the target school stage to high school. Particularly, it is unique in suggesting that high school students are at the school stage where the relative age effect on physical activity remains, and the relative age effect on sedentary behavior emerges.\u003c/p\u003e \u003cp\u003eHowever, our study had some limitations. First, we could not clarify causal relationships because our study was cross-sectional. For example, although sedentary behavior showed an opposite relative age effect compared to physical activity, we could not perfectly clarify whether relatively younger children and adolescents dropped out of physical activity and replaced them with sedentary behavior. Future longitudinal studies should follow the same person for a longer time to ensure the relative age effect on physical activity and sedentary behavior change. Second, we cannot perfectly deny the overestimation of physical activity and underestimation of sedentary behavior because of the characteristics of the questionnaire survey. Nonetheless, we made our maximum efforts to ensure that participants could comprehend the questionnaire\u0026rsquo;s intention and easily remember it (physical activity in the last 1 week and usual sedentary behavior on average). Third, we could not investigate the places where children and adolescents engaged in physical activity or sedentary behavior and the details of physical activity or sedentary behavior. Therefore, we could not distinguish between organized physical activity and unorganized recreational behavior. Additionally, due to the lack of information on exercise habits, we assumed that participants with 60 or more min per week are \u0026ldquo;exercisers.\u0026rdquo; Regarding screen time, we could not specify the detailed purpose (for example, playing games, web search, and communication). The relative age effect emerges not only in behavioral development but also in social development (e.g., number of friends and prosocial emotions) [\u003cspan additionalcitationids=\"CR13\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Hence, future studies should focus on the locations and purposes of physical activity and sedentary behavior. Moreover, future studies should clarify how the relative age effect influences the interaction between behavioral and social development.\u003c/p\u003e \u003c/div\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eWe observed the relative effects of age on physical activity and sedentary behaviors among children and adolescents living in various areas of Japan. Additionally, the relative age effect on physical activity had already emerged in elementary school students and was observed in middle and high school students. Particularly, relatively younger individuals engaged in less high-intensity physical activity than relatively older individuals. Conversely, relatively younger male high school students spent more sedentary time, especially on weekends, playing video games and Internet use. Meanwhile, we could show the relative age effect on television viewing for female middle school students. Hence, it is suggested that the relative age effect on physical activity remains after middle school and that relatively younger individuals are more likely to replace physical activity with sedentary behavior.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003eMVPA: moderate-to-vigorous physical activity, VPA: vigorous physical activity, MPA: moderate physical activity, WHO: World Health Organization, IPAQ-JEA: International Physical Activity Questionnaire for Japanese Early Adolescents, BMI: body mass index, AME: average margin effect,\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eEthics approval was obtained from the Ethics Committee of Doshisha University (approval number: 17095). The purpose, methods, benefits, and risks of our study were explained to the principals of the schools. We also obtained participants and their guardians by explaining that we would protect their personal information and that their answers were never related to their school records.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe datasets are publicly unavailable because the publication of datasets, including schools and personal information (e.g., birth month), is prohibited in Japan. Nevertheless, the datasets are available from the corresponding author upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors do not have any competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by JSPS KAKENHI Grant Number JP 18H01000, JSPS KAKENHI Grant Number JP 19K21797, and JSPS KAKENHI Grant Number JP 21H00848.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026rsquo; Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTM and KI conceptualized and designed this study. TA (Aoki) and KI acquired funding. TA (Aoki), TH (Harada), CT, ST, HT, KF, YK, NT, KK, SK, NM, KS, MW, RK, TH (Hara), RM, TA (Abe), KY, DK, HA, NY, TT and KI conducted the survey. TA, KO, and KI sorted out the data. TT and TN provided technical information about statistical analysis. TM performed statistical analysis and drafted the manuscript. TM and KI reviewed and edited the manuscript. All authors read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe appreciate all participants and their teachers for their cooperation with the questionnaire survey. We would also like to thank Editage (http://www.editage.jp/) for English language editing.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003ePoitras VJ, Gray CE, Borghese MM, Carson V, Chaput JP, Janssen I, et al. Systematic review of the relationships between objectively measured physical activity and health indicators in school-aged children and youth. 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Int Rev Sports Exerc Psychol. 2020;13:187-204.\u003c/li\u003e\n\u003cli\u003eJapan association athletics federations. JAAF athlete development model (in Japanese). https://www.jaaf.or.jp/development/model/. Accessed 8 Mar 2024.\u003c/li\u003e\n\u003cli\u003eMori T, Ishii K. Relative age effect among Japanese athletes who participated in the World Athletics Championships and Olympic Games. Jpn J Hum Growth Dev Res. 2023;2023:18-24.\u003c/li\u003e\n\u003cli\u003eNishio T. The plan for the reform of Japanese school club sports with reference to the New Zealand context. J Jpn Soc Sports Ind. 2017;27:69-72. \u003c/li\u003e\n\u003cli\u003eNakazawa A. Post World War Ⅱ and the present of extracurricular sports activity -Why sports are connected with school education?- Tokyo: Seikyusha; 2014. p. 46-51.\u003c/li\u003e\n\u003cli\u003eJayanthi N, Pinkham C, Dugas L, Patrick B, Labella C. Sports specialization in young athletes: evidence-based recommendations. 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PLoS One. 2020;15:e0237725. \u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Tables","content":"\u003cp\u003eTables are available in the Supplementary Files section.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"bmc-public-health","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"pubh","sideBox":"Learn more about [BMC Public Health](http://bmcpublichealth.biomedcentral.com/)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/pubh/default.aspx","title":"BMC Public Health","twitterHandle":"@BMC_series","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"em","reportingPortfolio":"BMC Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"physical activity, sedentary behavior, relative age effect, school, adolescence","lastPublishedDoi":"10.21203/rs.3.rs-4937775/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4937775/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003eBackground\u003c/h2\u003e \u003cp\u003eA few studies have shown that relatively younger children and adolescents (those born later in the same school year) were less likely to engage in physical activity in a phenomenon termed the relative age effect. Although these studies mainly targeted elementary and middle school students, limited studies have reported on the relative age effect on physical activity in ordinary high school students. Moreover, the relative age effect on sedentary behavior might show an opposite association with physical activity. Therefore, we aimed to clarify the relative age effects on physical activity and sedentary behavior across different school stages in Japanese children and adolescents.\u003c/p\u003e\u003ch2\u003eMethods\u003c/h2\u003e \u003cp\u003eA cross-sectional questionnaire survey was conducted with 21,491 children and adolescents (elementary, middle, and high school students aged 10\u0026ndash;18 years) in various Japanese regions from January 2018 to July 2019. Overall, 18,281 children and adolescents (10,299 boys and 7,982 girls) were finally included in our analysis. Data on the birth month, as well as the frequency and duration of their physical activity (vigorous, moderate, and moderate-to-vigorous intensity) and sedentary behavior (weekday and weekend total sedentary time, television viewing, playing video games, and Internet use) were obtained. We utilized two-part model regression analyses Statistically significant association with birth month indicated that a relative age effect was observed.\u003c/p\u003e\u003ch2\u003eResults\u003c/h2\u003e \u003cp\u003eThe relatively younger individuals were less likely to engage in physical activity (especially vigorous physical activity); this association was observed in middle and high school students. Conversely, relatively younger boys spent more time on the weekend with sedentary behaviors and Internet use. Because of the school stage, the relative age effect on physical activity was observed in middle and high school students, whereas the relative age effect on sedentary behavior was observed on the weekend with sedentary behaviors, playing video games and Internet use for male high school students and on television viewing for female middle school students.\u003c/p\u003e\u003ch2\u003eConclusion\u003c/h2\u003e \u003cp\u003eThe relative age effect on physical activity can persist after middle school, and relatively younger individuals are more likely to replace physical activity with sedentary behavior.\u003c/p\u003e","manuscriptTitle":"Relative age effect on the physical activity and sedentary behavior in children and adolescents aged 10 to 18 years old: A cross-sectional study in Japan","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-10-15 04:55:41","doi":"10.21203/rs.3.rs-4937775/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-09-24T08:10:46+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-09-22T14:25:20+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"190465742013397899399662240074535163332","date":"2024-09-16T15:21:07+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-09-15T17:16:42+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"183143423387237496360349269842913924422","date":"2024-09-15T16:38:17+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"28483635585825867161158506342326899965","date":"2024-09-14T16:03:11+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-09-14T15:18:55+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2024-08-21T07:29:39+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-08-20T23:53:11+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-08-20T23:52:06+00:00","index":"","fulltext":""},{"type":"submitted","content":"BMC Public Health","date":"2024-08-19T10:05:47+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"bmc-public-health","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"pubh","sideBox":"Learn more about [BMC Public Health](http://bmcpublichealth.biomedcentral.com/)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/pubh/default.aspx","title":"BMC Public Health","twitterHandle":"@BMC_series","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"em","reportingPortfolio":"BMC Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"6eb10fdc-43e7-4128-befe-3be7978f6060","owner":[],"postedDate":"October 15th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-12-02T16:02:24+00:00","versionOfRecord":{"articleIdentity":"rs-4937775","link":"https://doi.org/10.1186/s12889-024-20659-7","journal":{"identity":"bmc-public-health","isVorOnly":false,"title":"BMC Public Health"},"publishedOn":"2024-11-25 15:57:37","publishedOnDateReadable":"November 25th, 2024"},"versionCreatedAt":"2024-10-15 04:55:41","video":"","vorDoi":"10.1186/s12889-024-20659-7","vorDoiUrl":"https://doi.org/10.1186/s12889-024-20659-7","workflowStages":[]},"version":"v1","identity":"rs-4937775","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4937775","identity":"rs-4937775","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}