In adult life the effect of regular physical activity is widely accepted. We are told it promotes aerobic fitness, lowers blood lipids, reduces high blood pressure, counters obesity, retards osteoporosis and improves psychological well-being.
And there is general agreement that adults' health and well-being has its origins in behaviour established during childhood. So, young people should be encouraged to adopt active lifestyles which can be sustained into adult life.
Yet children have surprisingly low levels of habitual physical activity. Our analyses of the activity patterns of 750 l0 to 16-year-olds have shown that less than half of the girls monitored did the equivalent of a l0-minute brisk walk during three days of 12-hour heart rate monitoring (9am to 9pm). Boys were significantly more active than girls, and girls' activity levels decreased as they moved through secondary school.
Why do we fail to promote girls' physical activity? In our view, the national curriculum discriminates against girls and promotes inactive lifestyles; evidence suggests that these early lifestyles will have some profound effects on future generations of women.
In his speech to last year's Conservative party conference, the Prime Minister said that many schools offered at least two hours a week for sport and physical education. "That should be the minimum. I hope schools will offer more. "
The reality, of course, is shown by the recent OPCSSports Council survey which reported that fewer than half of six to 16-year-olds experienced two hours a week of physical education. The European Physical Education Associations reported that less time is devoted to PE in England and Wales than in any of the other European countries surveyed. Since 1987, there has been a 97 per cent increase in the number of 14-year-olds who do not receive two hours of PE.
The Secondary Heads Association acknowledges that "the main cause of the decrease is the national curriculum" and predicts a further decrease "in each of the age groups when the national curriculum is fully implemented, despite the recent decision to reduce its requirements".
But what of the content of national curriculum physical education? Politicians appear convinced of the value of competitive team games in "building character", despite the lack of any supporting evidence.
The Prime Minister proclaimed to his 1994 conference that the national curriculum would "put competitive games back at the heart of school life. Sport will be played by children in every school from 5 to 16, and more time must be devoted to team games."
In his response to the national curriculum draft proposals the then Secretary of State for Education wrote: "I am particularly pleased to see emphasis given to competitive team games in key stages 1 to 3 of the PE order, and your recommendation that games should be made a requirement of key stage 4 for the first time."
It appears that politicians have little understanding of the need for a balanced physical education curriculum and the promotion of physical activities which are likely to be sustained into adult life.
The OPCSSports Council survey has confirmed not only the dominance of the curriculum by competitive team games but also the reluctance of girls to participate in games outside school. Competitive team games traditionally have a strong focus on male-defined standards for exercise and male-defined expectations for leisure.
The dominance of these games in the PE curriculum and social life of the school may reinforce perjorative messages regarding the relative unimportance of female athletic interests. The dislike of many girls for team games should not be confused with inability or disinterest in physical activity.
Women's leisure interests have been shown to be quite different to those of men and competitive team games may have little meaning in most of their lives. Individual activities are far more popular with girls and, as they progress through key stages 3 and 4, competitive team games become even less popular as interest in fitness activities pursued more actively by adult women, particularly aerobics, increases.
Some girls will enjoy team sport and natural talent must be nurtured. However, many girls are discouraged from participating through lack of success - which may be simply due to the fact that their biological clock is running at a different rate from those of their classmates.
It should be demonstrated clearly that physical activity can be enjoyable and that competition and athletic excellence are not necessary for the promotion of health. Children need to be exposed to a wide variety of individual, partner and team activities, and the emphasis should be placed upon developing a sound foundation of motor skills that can contribute to successful and enjoyable activity experiences.
Motivations vary. "Looking good" remains a significant motivator for girls' participation in physical activity. The activity and leisure preferences of children and adolescents of different sexes and of different physical and intellectual capabilities must be taken into consideration in national curriculum physical education.
Only this week the Prime Minister said he wanted to re-establish sport as "one of the great pillars of education" and a White Paper on school sport is imminent. Will the importance of promoting lifetime physical activity be emphasised? Will the decline in the time scheduled for physical education since the implementation of the national curriculum be addressed? Will it be recognised that significant gender differences exist in the sports and activities that boys and girls enjoy and participate in? The future health and well-being of girls may depend upon the answers.
Neil Armstrong is director of the children's health and exercise research centre at the University of Exeter. Alison McManus is postdoctoral research fellow in the centre for physical education and sports science at the University of Hong Kong.