Gender and grade differences in objectively measured physical activity and sedentary behavior patterns among Japanese children and adolescents: a cross-sectional study

The present study examined differences in objectively measured activity levels and attainment of physical activity guidelines in Japanese children and adolescents according to gender and school grade. Boys and younger students were more active than girls and older students. Moreover, approximately half of the preschool and elementary school boys and girls attained the recommended physical activity levels, while 70 % of junior high school students did not. These findings indicate that strategies to promote greater physical activity among Japanese children and adolescents should include special emphasis on physical activity for girls and on maintaining activity throughout adolescence.

The present results indicate that students engaged in less moderate and vigorous physical activity on weekends than weekdays, despite the time spent on mandatory sedentary weekday activities such as sitting in class. Similar trends were reported among young children and youth in Hungary [22], Germany [23], Portugal [24], China [25], France [26], Spain [27, 28], and Canada [29]. Thus, to promote moderate to vigorous physical activity, it may be effective to consider not only school-based approaches but also home and neighborhood-based approaches. Children have more time for physical activity on weekends at home or in a neighborhood setting than at school because there are time restrictions such as class on weekdays. Therefore, an approach that considers neighborhood environment, social support from family, and the community may increase physical activity among Japanese school children [30].

On both weekdays and weekends, boys were more likely to spend time on moderate and vigorous physical activities and less likely to spend time on sedentary behaviors than girls. The same was true of younger students compared to older students. Previous studies have shown that boys are more active than girls worldwide [9, 23, 24, 26‐29, 31‐40]. In Japan, the gender difference among elementary school children is especially pronounced. Therefore, promoting physical activity for elementary school girls is urgently needed. Moreover, the present results show children’s physical activity declines from elementary school to junior high school even though physical activity level was higher in elementary school than preschool. Previous studies from England [37], nine European countries [41, 42], France [26], U.S. [9, 34, 43], Canada [33], Spain [27], Denmark [7], and Mexico [44] have also shown that physical activity levels decline with age. Since physically active children are more likely to become physically active adults [45], it is important to identify ways to prevent the decline and to promote activity during the childhood years.

Time spent on sedentary behavior increased with school grade, and girls were more sedentary than boys on both weekdays and weekends. Similar results have been reported in previous studies [23, 26, 27, 43]. Moreover, children who spent less time on sedentary behavior spent more time on light physical activity, and vice versa, on both weekdays and weekends. These reciprocal relationships were found in previous studies as well [21, 33, 43], suggesting that a strategy focused on reducing sedentary behavior would concomitantly promote light physical activity. Therefore, in addition to strategies for promoting moderate to vigorous physical activity, strategies for reducing sedentary behavior and promoting light physical activity may benefit sedentary or slightly active children. On weekdays, sedentary behavior increases with grade because children in higher grades spend more time at school. On weekends, children are not restricted by class time. Therefore, a targeted plan to reduce sedentary behavior on weekends or outside of class time on weekdays, such as during recess or after school, may be effective.

Boys and younger students also took more steps per day than girls and older students on both weekdays and weekends. Boys were more likely to take the recommended number of steps [6] on both weekdays and weekends, whereas girls were likely to be 1000 steps per day under the recommended number [6] with the exception of preschool girls on weekends. Children take fewer steps per day on weekends than weekdays in other countries as well [46, 47]. A positive association was found between children’s physical activity on weekends and family encouragement or family social support [48]. Moreover, the Canadian Physical Activity Levels Among Youth (CANPLAY) study found that every 1000-step increase in a parent’s step count per day was associated with 200–450 extra steps per day in the child’s count [49]. Therefore, to increase steps on weekends, a family-based education program or intervention could be effective.

The present study indicated that boys were more than twice as likely to meet the WHO guidelines than girls, in agreement with previous studies on gender differences in activity goal attainment [23, 27, 35, 40‐43, 50, 51]. For instance, fourth- through sixth-grade boys in the U.S. [43] were 5.85 times more likely than girls to attain recommended levels of physical activity. Also consistent with the present results, goal attainment also fell with age; fourth-grade children in the U.S. were 1.53 times more likely than fifth- and sixth-grade children to attain the guidelines. There are, however, substantial differences in goal attainment levels across studies. In the present study, less than 50 % of the children met the recommendation for at least 60 min of moderate to vigorous physical activity per day. However, a study in Europe found that more than 90 % of children attained the guidelines [43], while other studies in England [35], China [25], and European countries [50] found guideline attainment under 10 %. In the present study, 75.0 % of elementary school students in the higher grades met the guidelines, higher than the 24.3 % reported in the aforementioned U.S. study [43]. Guinhouya et al. reviewed several studies of European children and suggested that reported differences in guideline attainment could be attributed to different accelerometer cutoff points [52]. Up to 100 % of youth may be considered sufficiently active when using approximately >1000–1500 cpm, the most widely used cut-off point, up to 87 % when >2000 cpm is used, but no more than 3–5 % when a cut-off point of >3000 cpm is used. In the present study, the accelerometer was only capable of measurements for 2-min epochs, and it estimated rather than counting the minutes for each activity level and steps. Therefore, these percentages should be compared and interpreted carefully.

In 2012, the Ministry of Education, Culture, Sports, Science and Technology of Japan recommended that preschoolers participate in 60 min or more of physical activity daily [21]. Physical activity guidelines for older children have not yet been established in Japan. The present study indicates that the percentage of Japanese children attaining the guidelines is low. A better understanding of the sociodemographic, psychological, social, and environmental variables that affect guideline attainment is needed in order to develop effective strategies to encourage physical activity.

Some limitations of the current study should be considered. First, the accelerometer was capable of activity measurement only in 2-min epochs, while a shorter epoch is strongly recommended for children because their physical activity is often intermittent [53]. Therefore, the amount of sedentary time and vigorous physical activity may be underestimated while light and moderate physical activity may be overestimated. Second, the participants were living in a relatively narrow area of Japan, which could limit generalizability. Despite these limitations, the present study is meaningful as it is the first to indicate gender and age (school grade) differences in physical activity, sedentary behavior, and guideline attainment among children and adolescents in Japan. The present data can be used to inform interventions for promoting physical activity in Japanese children and adolescents.