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 .
Strength exercises during physical education classes in secondary schools improve body composition: a cluster randomized controlled trial
Background Metabolic health in people with obesity is determined by body composition . In this study , we examined the influence of a combined strength exercise and motivational programme –embedded in the school curriculum– on  adolescents body composition and daily physical activity . Methods A total of 695 adolescents ( 11 - 15y ) from nine Dutch secondary schools participated in a one year cluster r and omised controlled trial ( RCT ) . In the intervention schools , physical education teachers were instructed to spend 15–30 min of all physical education lessons ( 2 × per week ) on strength exercises . Monthly motivational lessons were given to stimulate students to be more physically active . Control schools followed their usual curriculum . The primary outcome measure was body composition assessed by the deuterium dilution technique . Daily physical activity and sedentary behaviour measured by accelerometry served as a secondary outcome . Results After 1 year , a 1.6 % fat mass difference was found in favour of the intervention group ( p = .007 ) . This reflected a 0.9 kg difference in fat free mass ( intervention > control ; p = .041 ) and 0.7 kg difference in fat mass ( intervention < control ; p = .054 ) . Daily physical activity decreased from baseline to posttest in both groups , but less so in the intervention group ( p = .049 ) . After 1 year , a difference of 0.4 % was found for moderate to vigorous physical activities in favour of the intervention group ( p = .046 ) . No differences in sedentary behaviour , or light physical activity were found between groups . Conclusion In 11–15 year olds , the combination of strength exercises plus motivational lessons contributed to an improvement in body composition and a smaller decrease in physical activity level . Trial registration ID(NTR5676 – retrospectively registered 8 February 2016 ; enrolment of first participant : 2 March 2015 )
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 .