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 .
Fatness leads to inactivity, but inactivity does not lead to fatness: a longitudinal study in children (EarlyBird 45)
Objective To establish in children whether inactivity is the cause of fatness or fatness the cause of inactivity . Design A non-intervention prospect i ve cohort study examining children annually from 7 to 10 years . Baseline versus change to follow-up associations were used to examine the direction of causality . Setting Plymouth , Engl and . Participants 202 children ( 53 % boys , 25 % overweight/obese ) recruited from 40 Plymouth primary schools as part of the EarlyBird study . Main outcome measures  Physical activity ( PA ) was measured using Actigraph accelerometers . The children wore the accelerometers for 7 consecutive days at each annual time point . Two components of PA were analysed : the total volume of PA and the time spent at moderate and vigorous intensities . Body fat per cent ( BF% ) was measured annually by dual energy x ray absorptiometry . Results BF% was predictive of changes in PA  over the following 3 years , but PA levels were not predictive of subsequent changes in BF% over the same follow-up period . Accordingly , a 10 % higher BF% at age 7 years predicted a relative decrease in daily moderate and vigorous intensities of 4 min from age 7 to 10 years ( r=−0.17 , p=0.02 ) , yet more PA at 7 years did not predict a relative decrease in BF% between 7 and 10 years ( r=−0.01 , p=0.8 ) . Conclusions Physical inactivity appears to be the result of fatness rather than its cause . This reverse causality may explain why attempts to tackle childhood obesity by promoting PA have been largely unsuccessful
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 .