The relative decline in MVPA affects both sexes from an early age ; however , it is greater among girls .
Moderate-to-vigorous-intensity  physical activity ( MVPA )  is important for childhood obesity prevention and treatment , yet declines with age .
Timing and magnitude of the decline in MVPA in children and adolescents are unclear but important for informing effective obesity intervention development .
This systematic review aim ed to determine and compare the year-to-year changes in MVPA among children and adolescents .
Assessment of a two-year school-based physical activity intervention among 7-9-year-old children
Background  Physical activity ( PA )  in children has declined in recent decades , highlighting the need for effective intervention programs for school-aged children . The main objective of this study was to assess to what extent PA during and after school hours changed among children who received a progressive two-year long intervention vs. that of children who only received general curriculum-based PA .  Methods A cluster r and omized intervention study was conducted and six elementary schools r and omly assigned to serve as control- or intervention schools . All children attending second grade ( mean age = 7.4 years - born in 1999 ) were invited to participate in the fall of 2006 ( N = 320 , 82 % participated ) , again in 2007 ( midpoint ) and 2008 ( end of intervention ) . The intervention consisted of multi-component PA-intervention during school hours and was conducted by teachers at each intervention school . PA  was assessed by means of accelerometers and subjectively at the intervention schools via teachers ' PA log-books . Results There was no difference in PA intensity ( minutes of moderate-to-vigorous physical activity - min of MVPA ) between the two study groups at baseline , but children in the intervention schools were more physically active at moderate-to-vigorous intensity compared to those in control schools after one year of intervention ( mean difference of MVPAlog-minutes : 0.61 , 95%CI : 0.02 , 1.20 , p = 0.04 ) . Moreover , the model for minutes of MVPA during school hours , showed a significant three-way interaction between time at mid-point , group and gender ( mean difference of MVPAlog-minutes : 1.06 , 95%CI : 0.15 , 1.97 , p = .02 ) , indicating a significantly greater increase among the boys in the intervention schools compared to girls . No difference in PA was detected between the study groups at the end of the study period after two years of intervention . Conclusions The results suggest that the objective of increasing PA at school was met after one year of intervention , and it was more pronounced among boys . The lack of increase at the end of the study period suggested that any increase in  PA  during school may highly depend on both motivation and training of general teachers . Boys may respond better to PA interventions such as the one described in this study
The relative decline in MVPA affects both sexes from an early age ; however , it is greater among girls .
Moderate-to-vigorous-intensity  physical activity ( MVPA )  is important for childhood obesity prevention and treatment , yet declines with age .
Timing and magnitude of the decline in MVPA in children and adolescents are unclear but important for informing effective obesity intervention development .
This systematic review aim ed to determine and compare the year-to-year changes in MVPA among children and adolescents .