The importance of exercise in childhood

David Owens 歐德維
Deputy Editor, The Hong Kong Practitioner

Diet and exercise are the most important individual risk factors influencing the long term health of the current generation of Hong Kong children. The increase in incidence of childhood obesity and metabolic syndrome, and the associated health risks, have been extensively documented. This was discussed in a recent editorial.¹

An increase in sedentary behaviour and an associated reduction in exercise is directly associated with childhood obesity. Adolescents in western societies typically spend more than five and a half hours per day in media use (watching television, videos or computer use).² One large US study followed a cohort of 9/10 year olds for nine years. By the ages of 18/19 years the median physical activity score dropped by 64% in white girls and by 100% (to zero) in black girls³. The World Health Organization estimates that 'less than one third of young people are sufficiently active to benefit their present and future health'.⁴

There is a significant and growing evidence base for exercise as a positive health intervention. Early evidence for the association between inactivity and mortality came from epidemiological studies comparing occupational groups with different activity levels. In a classic study comparing the drivers of double decker London buses (sedentary) with conductors who walked up and down stairs collecting fares all day (typically in the same role for many years), the drivers had almost twice the incidence of cardiovascular events and mortality.⁵ Subsequently a number of large epidemiological studies have confirmed this association and extended it to include evidence on all cause mortality and morbidity. The Harvard Alumni Health cohort resulted in a number of papers which not only confirmed an inverse relationship between exercise and all cause mortality, but also showed that an increase in exercise levels in sedentary individuals later in life is associated with a reduction in morbidity and mortality. In combination with many other studies comparing physiological parameters of fitness with morbidity and mortality there is now a clear evidence base that individuals can modify their own health risks by modification of levels of activity.

There are many reasons why levels of activity in childhood are reducing. Increasing urbanization has reduced the availability of open areas for play. An increase in transport has led to a corresponding reduction in walking and cycling. There has been a change towards more sedentary pastimes. There are also cultural variables. In Hong Kong, sport and exercise have traditionally been given a relatively low priority. This will need to change if the long term health of the current population is to be optimized. This requires social and political change. It is a public health matter for which there is clear evidence. Again our profession must find a voice in order to lobby for the long term well being of our population.

Children who are active are more likely to maintain activity into later life. Many studies have demonstrated that some physiological benefits acquired early in childhood are retained into adult life.⁶ Furthermore, studies of long term athlete development suggest that there are windows of trainability which if missed can never be recaptured.⁶ A recent leading article in the British Medical Journal demonstrated that a specific exercise programme with a focus on balance and joint proprioception significantly reduces the risk of injury in youth pivot sports.⁷ Although important as an addition to the body of evidence relating to sports medicine, this paper also suggests that improving balance and proprioception under the age of 12 years may reduce the risk of injury in later life. This finding is compatible with our increasing understanding of exercise physiology in children. As an example, the optimal age for training speed in an athlete is around the age of 8 years. At this age training has very little effect on muscle hypertrophy, 'hard wiring' of the nervous system leads to increased efficiency of neural conduction leading to an increase in explosive power. In the same way, a good golf or tennis swing learned in early childhood will be retained into adult life even after prolonged periods of non use. Coaching in both situations concentrates on the development of appropriate biomechanics. Neural plasticity within the maturing brain ensures an increase in the efficiency of proprioception and neuromuscular recruitment which will be retained. Balance and co- ordination, when encouraged and trained in young children, confer long term benefit in terms of injury prevention, in addition to improvement in long term athletic performance.

There is a clear evidence base demonstrating an inverse relationship between activity levels and morbidity and mortality. Children who exercise are healthier. There is also evidence that some aspects of exercise and activity in early childhood confer benefit into adult life. Children who miss this window of opportunity may improve but they will not reach their potential. Children who are active are more likely to be active as adults. As doctors we have a duty to act as advocates for the health of our population. There is a clear evidence base for exercise as a health intervention. We should actively encourage all members of our population to exercise. We should especially educate parents so that they appreciate the long term health benefits of activity and exercise for their children.