Videos and computers for entertainment stopped children from running around and playing as they once did and parents were more conscious of child safety. "Neither of these factors is going to change," Mr Jess said.
Growing evidence showed that since the 1970s children had become worryingly obese. "The only reason why more are getting obese is to do with the lack of exercise. If you get pretty obese early on, it's difficult to shift it," he said.
Mr Jess, a leading academic in physical activity and movement, said previous fit-for-life programmes had failed to persuade adults to change their behaviour and they were now even less active. Research and intervention programmes had highlighted complex reasons why adults shunned regular, physical activity - important for several health aspects, including avoidance of heart attacks and strokes.
Harry Black, Fife's health promotion manager, said 70 per cent of adults do not take enough exercise to benefit their health, although eight out of ten say they are fit or very fit. "But children who regularly participate are much more likely to participate in adulthood," he said.
Mr Jess added: "Slowly the focus is going on children. People are becoming aware that unless we start early, it's a difficult task. Although it's complex, it's nowhere near as complex as in adults. Habit-forming in childhood is important."
Ten years ago, people believed children were naturally active but all the evidence now provedthey were not. "They are the most active age group in society, but they are less and less active," he said.
"We need children to have the tools to access and choose a range of different activities," he continued. "We need them to be active but they also need to learn the appropriate movements. Some of the benefits are in childhood and some in adulthood."
David Maiden, an education adviser in Fife and physical education specialist, said he had seen first-hand a decrease in movement competence in pupils moving up to secondary school.
"There's a general decrease in the levels of fitness of children. It used to be that we could do cross-country with loads and loads of the school. Then it became three miles to two miles to one mile, and then a walk round the track became a bit of a problem," he said.
When he became an adviser he was approached by occupational therapy services about the high number of referrals from primaries of pupils classed as clumsy. "The child we would have termed with two left feet in the past. Maybe they had not been exposed to the movement experience they should have had," he said.
There was also a big decrease in the amount of time children were playing in the playground due to lack of space and underfoot conditions. Games had gone out of fashion. "There is peer pressure. It's not cool to be active," he suggested.
Mr Maiden said the Play@home initiative, based on a New Zealand scheme, was showing strong results with parents, babies and pre-fives since it was introduced in stages over two years by health and education services. Three booklets, aimed at different age ranges, suggest simple games and activities for parents to use daily with children.