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The UK is facing an inactivity time bomb. Research suggests half of seven year olds don't get enough exercise. The outlook is even worse for girls. Whilst nearly two out of three boys do an hour's physical activity a day, for girls it is around one in three.

The results come from the biggest UK-wide scientific analysis of primary school children's activity. Many previous studies were based on survey results whereas this research, by University College London's Institute of Child Health, recorded the activity of 6,500 youngsters for a week.

Each wore an accelerometer - a gadget which measures both the duration and intensity of exercise. The observation that children are not getting enough exercise is hardly new. But this research, funded by the Wellcome Trust, gives perhaps the most accurate picture yet of the scale of the problem at primary school level.

It is especially concerning because children tend to do less exercise as they get older. If two-thirds of girls are not active enough at the age of seven, then that proportion will get even higher - unless action is taken.

So how do you motivate children to be active? Clearly school sports are vital. But parental input is crucial. Seven year olds today face more distractions, more incentives to be sedentary, than any previous generation. For many of them, the computer tablet, apps or smart phones are a part of daily life. Sadly, many regard playing outside as a dull alternative to using their thumbs to control a computer game.

From my experience there is limited value in telling them that they need to do exercise to be healthy - the concept of future heart, bone or weight problems cuts little ice with your average primary or secondary school child.

Two things are crucial. First is making physical activity routine - part of daily life. Walking to school is one obvious example. Parents who set an example and do exercise tend to have more active children. The second is finding a sport or activity which engages your child. Getting children more involved in sport was one of the legacy aims of the 2012 Olympics.

The is aimed at encouraging people of all ages to lead healthier, more active lives. Its motto, "Eat well, move more, live longer", pretty much sums up the aim of the campaign. It already has half a million members and is doing another recruitment drive linked to the start of the new school year.

Using apps and email messages it encourages children to take several small steps that will help lead to a permanent improvement in their health. This includes breaking down the perhaps daunting task of being active for at least 60 minutes into 10 minute chunks, plus suggestions for games which will tempt kids away from their screen.

From today's research such campaigns are vital if primary school children are to be motivated to lead an active life.