Exercise sessions have 'little impact on child activity'
Extra-curricular exercise sessions have little impact on children's overall daily physical activity, research in the British Medical Journal suggests.
Researchers from Plymouth University found attending these sessions was only equivalent to doing an extra four minutes walking or running per day.
Children who attended did less physical activity at home afterwards, they said.
They looked at 30 studies, each lasting for at least four weeks, that monitored the bodily movements of the under-16s.
Eight studies involved overweight and obese children only, while the rest involved children who were a range of different weights.
Their results came from controlled trials that took place between 1990 and 2012.
The researchers looked at the effects of extra exercise sessions on total physical activity during waking hours as well as on overall time spent doing moderate exercise.
Despite the extra sessions, they found there were only "small to negligible" increases in children's total activity and small improvements in time spent doing moderate or vigorous exercise.
They calculated this would have minimal impact on children's body fat or BMI (Body Mass Index), equivalent to a reduction of 2mm (0.07in) in waist circumference.
"It could be that the intervention specific exercise sessions may simply be replacing periods of equally intense activity," the study said.
"For example, after-school activity clubs may simply replace a period of time that children usually spend playing outdoors or replace a time later in the day/week when the child would usually be active."
Brad Metcalf, lead author of the study and a medical statistician from the department of endocrinology and metabolism at Plymouth University, said extra-curricular PE lessons could often be far from energetic.
"A PE lesson can be 10 minutes of running, 10 minutes of walking and 20 minutes of standing in a queue waiting for your turn."
Children may also end up eating or snacking more at home afterwards because they feel they have been more active - or parents may decide not to take their children to the park because they believe they have already had their exercise for the day.
Obesity is estimated to cost the NHS £4bn a year.
Mr Metcalf suggests any initiatives to combat obesity should emphasise diet and healthy eating, rather than relying solely on physical activity to solve the problem.
In an editorial in the same issue of the BMJ, Mark Hamer and Abigail Fisher, from the department of epidemiology and public health at University College London, said the devices used to measure the total activity of the children were a more reliable tool than questionnaires.
"The small effects reported by Metcalf and colleagues are probably more realistic and provide the best evidence to date on the effectiveness of activity interventions in childhood."
But they added that the accelerometers could not measure activities like swimming or cycling.
Looking to the future, they said it was important to identify how best to promote exercise to children, "because a wealth of evidence supports the association between an active lifestyle and many facets of child health".