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 .
Effect and cost of an after-school dance programme on the physical activity of 11–12 year old girls: The Bristol Girls Dance Project, a school-based cluster randomised controlled trial
Background The aim of this study was to examine the effectiveness and cost of an after-school dance intervention at increasing the physical activity levels of Year 7 girls ( age 11–12 ) . Methods A cluster r and omised controlled trial was conducted in 18 secondary schools . Participants were Year 7 girls attending a study school . The Bristol Girls Dance Project ( BGDP ) intervention consisted of up to forty , 75-minute dance sessions delivered in the period immediately after school by experienced dance instructors over 20-weeks . The pre-specified primary outcome was  accelerometer assessed mean minutes of weekday moderate to vigorous physical activity ( MVPA ) at time 2 ( 52 weeks are T0 baseline assessment s ) . Secondary outcomes included accelerometer assessed mean minutes of weekday MVPA at time 1 ( while the intervention was still running ) and psychosocial outcomes . Intervention costs were assessed . Results 571 girls participated . Valid accelerometer data were collected from 549 girls at baseline with 508 girls providing valid accelerometer data at baseline and time 2 . There were no differences between the intervention and control group for accelerometer assessed physical activity at either time 1 or time 2 . Only one third of the girls in the intervention arm met the pre-set adherence criteria of attending two thirds of the dance sessions that were available to them . Instrumental variable regression analyses using complier average causal effects provided no evidence of a difference between girls who attended the sessions and the control group . The average cost of the intervention was £ 73 per girl , which was reduced to £ 63 when dance instructor travel expenses were excluded . Conclusion This trial showed no evidence that an after-school dance programme can increase the physical activity of Year 7 girls . The trial highlighted the difficulty encountered in maintaining attendance in physical activity programmes delivered in secondary schools . There is a need to find new ways to help adolescent girls to be physically active via identifying ways to support and encourage sustained engagement in physical activity over the life course . Trial registration IS RCT
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 .