The results of this longitudinal study showed that the decline in MVPA and increase in SB followed nonlinear functions in children transitioning from primary to secondary school (fourth to eighth grade; 9–14 years), whereas the decline in LPA was linear. This corresponded to an accelerated change in MVPA and SB among secondary school adolescents. Male sex and maintaining normal POW over the five years were associated with higher levels of MVPA. However, the changes in LPA and SB over time were statistically unrelated to sex and POW.
Our key finding of a decline in MVPA was consistent with the recent results reported by Farooq et al. , who examined longitudinal PA data at ages 7, 9, 12, and 15 years in the Gateshead Millennium Cohort Study in England. They found a distinct linear trajectory based on the estimated marginal MVPA means derived from their LMM when adjusted for sex, age, and the interaction between them. Unlike their findings, we demonstrated a nonlinear MVPA trajectory. Although the MVPA of girls in their study was less than the currently recommended 60 min/d  after 9 years of age and continued to decline afterward, the girls in our study showed a slight increase in MVPA until 12 years of age. Moreover, there was an accelerated decline in MVPA when both boys and girls advanced from primary to secondary school.
Several longitudinal studies have examined PA changes using an accelerometer in children transitioning from primary to secondary school [10, 11]. A current systematic review , which included studies from 1990 to 2019, found no further studies reporting SB data since the 2017 review by Pearson et al. . In a longitudinal study similar to ours, Mitchell et al.  examined changes in SB (using a quantile regression model) at ages 9, 11, 12, and 15 years in children transitioning from primary to secondary school in the United States. They found a nonlinear increase in daily SB each year but no sex-specific SB changes over time. Our results provide further support for a nonlinear, sex-independent increase in SB in children transitioning from primary to secondary school. Moreover, the SB increase observed in our study was underpinned by more continuous data and detail. We observed a substantial increase when children transitioned to secondary school (after sixth grade). Compared to the findings of Mitchell et al. , our results revealed a slower SB increase over time. This may be explained by the baseline 442 min/d of SB in our cohort, which was more than the 320 min/d in their study. At ages 14 and 15 years, the gap in SB between our cohort (511 min/d) and their cohort (492 min/d) was considerably narrower. However, our results were comparable to other current studies in that they did not reflect a higher level of SB time among school-aged children [18, 19].
Previous studies found that the time spent performing LPA decreases as children grow into adolescents, but few examined whether this LPA change was linear or nonlinear [17, 20–22]. The LPA decrease in our cohort showed that adolescents in eighth grade spent 55 min less per day performing LPA than they did during fourth grade. This difference was smaller than that reported in a recent study based on pooled data from Europe, the United States, Brazil, and Australia from the International Children’s Accelerometry Database . The pooled data (n = 1088) showed an 85-min decrease in LPA (from 360 min to 275 min) among children aged 12–15 years. In addition, another longitudinal study  indicated an 86-min decrease in LPA (from 287 to 201 minutes) among 75 Spanish children aged 8.5–13.8 years.
In our cohort, this LPA decrease was linear over the five years with annual data collection, whereas the MVPA and SB changes were nonlinear. Thus, we conclude that the linear decline in LPA in children begins in primary school. The increased time spent in SB corresponded to the decrease in LPA in our analysis, indicating that time spent performing LPA is replaced by time spent in SB as children grow. Future studies should determine whether interventions focused on LPA may prevent an increase in SB.
Our results showed a temporal relationship between obesity and the changes in MVPA and SB, consistent with the conclusions from a previous systematic review by Elmesmari et al.  who concluded that accelerometer-measured MVPA for obese children and adolescents was less than that for non-obese children aged 10–19 years without marked differences in SB. In contrast, a previous longitudinal study in children aged 9–15 years indicated a possible association between time spent in SB and the body mass index (BMI) . Subjects in the 90th BMI percentile showed that the time spent in SB was associated with a curvilinear increase in BMI. However, there were weak or non-existent associations between SB duration and BMI for individuals in the 10th, 25th, and 50th BMI percentiles. Although there was no significant difference in SB duration between non-obese and obese participants in our cohort, negative estimates were shown in the LMM, implying that less time was spent in SB among non-obese participants than among obese participants. Whether obesity is affected by an increase in time spent in SB as children advance from primary to secondary school needs further investigation since our cohort included few obese children. An overview evaluating interventions for preventing or treating pediatric and adolescent obesity found conflicting results on whether a decrease in BMI could be achieved by reducing SB time .
Our results need to be interpreted within the limitations of this study. First, this study was conducted in a limited geographic area in Tokyo, Japan. Further studies are needed to establish whether our findings can be replicated in more diverse populations. Second, although our findings were adjusted for covariates, residual confounding factors might have remained. Environmental, socioeconomic, and sociocultural confounding factors [25–27] were not considered in the analysis. Third, children did not maintain a record of the periods during which they wore the accelerometer. Hence, their activities might have been underestimated. However, the accelerometry methods, epoch length, non-wear time, and valid wearing minutes and days used in this study are widely used and accepted in child and adolescent populations .
In summary, we found that the time spent in MVPA, LPA, and SB changed as children transitioned from primary to secondary school. The strengths of this study include the identification of linear and nonlinear changes with repeated objective annual data collection at five time points in a cohort of children progressing from fourth to eighth grade. MVPA and SB showed accelerated changes after the transition to secondary school, whereas changes in LPA were linear. In addition, we found that MVPA changes may be associated with obesity among children and adolescents; however, our cohort included few obese children.