The research, funded by British Heart Foundation and led by scientists at the University of Bristol, revealed that by age 11, children are doing more than an hour less of physical activity a week than they were at age 6.
The study monitored the behaviour of more than 2,000 children from 57 schools across South West England, with participants wearing an accelerometer for five days, including two weekend days.
The data provided researchers with an accurate assessment of how many minutes per day the children participated in moderate to vigorous physical activity (MVPA), which they measured as being enough to get them slightly out of breath and sweaty.
Results, published in the International Journal of Obesity, revealed on average children became 17 minutes less active per week every year they moved through primary school.
This study found that 61% of children in Year 1 did at least an hour of MVPA per day, but by Year 6, that had dropped to just 41%.
The fall was particularly steep for girls, who fell from 54 to 28% by the time they finished primary school.
They found that children lost on average more than an hour of exercise per week between the ages of six and 11.
The amount of time lost was even higher at weekends.
Experts are now calling for more to be done to encourage youngsters keep active as they approach their teenage years.
Commenting on the findings, Professor Jago, lead study author and Professor of Paediatric Physical Activity & Public Health at the University of Bristol, said: “Evaluating patterns of physical activity across childhood is an important way to identify key ages in which to intervene to change behaviour – and establish healthy habits for life.
These numbers prove that more needs be done to ensure children keep active as they approach adolescence. This isn’t about getting children to exercise more, but rather maintaining their activity levels.
“Developing early intervention strategies that help children retain activity levels could include after school physical activity programs, focusing on participation and enjoyment in addition to popular sports – and a greater emphasis on promoting weekend activities.”
Professor Jeremy Pearson, Associate Medical Director at British Heart Foundation said the results had extra significance considering the current childhood obesity rates: “Almost a third of children in the UK weigh more than they should, while one in four primary school children are not meeting the recommended levels of exercise.
“We know that children living with obesity are more likely to become obese adults – putting them at increased risk of developing heart and circulatory diseases and their risk factors, such as type 2 diabetes and high blood pressure, later in life.
“Staying active must be combined with policies that help families make healthy and informed choices, such as a 9pm watershed on junk food marketing and restricting the promotion of unhealthy foods.”
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Previous analysis has found that one in 25 children in England aged 10 or 11 are severely obese.
Measurements on children’s weight and height show the number of children classed as ‘severely overweight’ rose from 15,000 in reception to 22,000 by their final year of primary school.
The data was collected as part of Public Health England figures, and was analysed by The Local Government Association (LGA).
In a bid to help combat the problem, Public Health England encouraged parents to count the calories in their child’s snacks.
Each year, children consume almost 400 biscuits, more than 120 cakes, 100 sweets, 70 chocolate bars and 70 ice creams, washed down with more than 150 juice drink pouches and cans of fizzy drink.
Because of the alarming figures, the health body called on parents to be tougher on their kids snacking of sweets, cakes and fizzy drinks between meals.
The Change4Life campaign urged mums and dads to limit snacks to just two a day of no more than 100 calories each.
But physical exercise also has a huge part to play in combatting childhood obesity.
It isn’t just children who need to up their physical activity either.