Are children fit and active?

Neil Armstrong reviews the evidence on whether children are fit and active, and looks at how the situation has changed in recent years

A SEDENTARY ADULT LIFESTYLE IS associated with increased mortality. The effect of regular physical activity and physical fitness in improving blood lipid profile, reducing high blood pressure, helping to control blood glucose, countering obesity, delaying the onset of osteoporosis and promoting psychological well-being is evidence-based and extensively documented. Although the evidence is less compelling than in adults, appropriate physical activity and high levels of fitness during youth have been associated with the promotion of skeletal health, the improvement of elements of metabolic syndrome, the reduction of body fatness and enhanced mental health. Furthermore, there is a growing conviction that adults’ health and well-being has its origins in behaviour established during childhood and adolescence. Inactive and/or unfit children are unlikely to become physically active and  fit adults. But, are our children fit and active?

What we know
● 60-75% of young people do not meet current physical activity guidelines.
● Youth habitual physical activity has not declined over the last two decades.
● There is no evidence to suggest that young people have low levels of aerobic fitness, and children and adolescents are as aerobically fit as previous generations.
● There is no meaningful relationship between current levels of habitual physical activity and aerobic fitness.
● Low levels of habitual physical activity and a decline in maximal performance involving the transport of body mass are major issues in the promotion of youth health and well-being.

Are children fit and active?

Physical activity and physical fitness are often used interchangeably but they are not synonymous.

Physical activity consists of behaviours which contribute to total energy expenditure and involve bodily movements. In the context of health and well-being habitual physical activity (HPA) is the behaviour of prime interest. HPA can be defined as the physical activity that is carried out in normal daily life.

Physical fitness is a complex phenomenon that can be described in terms of its health-related or skill-related components. Health-related fitness includes physiological attributes such as aerobic fitness, muscle strength, muscle power and flexibility. Aerobic fitness (or cardiovascular fitness) is the component most frequently associated with health and well-being. Aerobic fitness is defined as the ability to deliver oxygen to the muscles and to use it to generate energy to support muscle activity during exercise. The best single measure of aerobic fitness is maximal oxygen uptake or maximal aerobic power (MAP), the highest rate at which oxygen can be consumed during exercise.

Are children active?

There are numerous methods of estimating children’s HPA but currently the two most popular methods are through questionnaires and using accelerometers. Questionnaires (self-report) allow the study of very large samples of young people. Accelerometers provide more objective data but tend to be used with smaller samples than questionnaires.

Several expert groups have reviewed the literature that links physical activity to health-related outcomes and have proposed evidence-informed physical activity guidelines for the promotion of children’s health and well-being. Currently the most influential guideline is the recommendation of the UK Health Education Authority (UKHEA) – now part of the National Institute for Health and Clinical Excellence – that all young people should participate in physical activity of at least moderate intensity for an hour each day. Moderate intensity physical activity is equivalent to brisk walking.

Self-reported physical activity data are available on large, representative, national and international samples of children and adolescents. The results of these surveys are remarkably consistent. They show that 60–70% of young people do not satisfy the UKHEA guidelines. Similarly, accelerometer data indicate that <25% of young people accumulate an hour of moderate intensity exercise on a daily basis. Both self-report and accelerometry studies show that fewer girls than boys meet the requirements of health-related physical activity. For both genders HPA decreases as young people move through childhood and adolescence. However, data consistently indicate that children’s and adolescents’ HPA has not declined over time, at least not in the last 20 years.

Are children fit?

Boys’ MAP rises almost linearly with age and girls’ data show a similar but less consistent trend with a tendency to plateau at about age 14. Boys’ values are about 10% higher than those of girls at age 10 and the difference increases to about 35% by the time they are 16. When MAP is expressed in ratio with body mass a different picture emerges. Boys’ values remain constant from age 8–18 while girls’ values decline by about 25% over the same period, largely reflecting a sex-specific increase in body fat.

There is no compelling evidence to suggest that as a population young people’s aerobic fitness is low or that there is a threshold level of aerobic fitness which is associated with youth health and well-being. Nevertheless, several publications have advocated health-related threshold levels of MAP (usually in ratio with body mass) either extrapolated from adult data or based on expert opinion.

Few studies have reported their results in sufficient detail to estimate the number of young people achieving health-related thresholds of MAP. The most comprehensive database probably lies within the Amsterdam Growth and Health Longitudinal Study (AGHLS). These data reveal that the percentage of adolescents falling below expert-derived, health-related thresholds increases from 1–8% in boys and 3–17% in girls between the ages of 13 and 17. The higher percentage of girls not meeting the threshold is explained by the sex-specific increase in body fat during puberty. Re-analysis of large data sets from my laboratory shows that of 220 11–16 year-olds, 3% of boys and 3% of girls failed to meet a health-related threshold and of 164 pre-pubertal 11 year-olds none fell below the threshold value.

Laboratory determinations of aerobic fitness dating back over 70 years indicate that young people’s MAP has remained remarkably stable over this time period. Several field tests designed to estimate aerobic fitness have reported conflicting data. The most extensively documented test is the 20m shuttle run test and large international databases of children’s performance on the shuttle run have been published. These show a significant deterioration in young people’s maximal performance, with a mean decline of 13% since 1975. Shuttle run performance is, however, strongly influenced by the body mass that the participant has to carry over the distance of the run. Increases in body fat explain 50–70% of the decline in shuttle run performance. Other factors, such as reduced experience with maximal sustained efforts and an unwillingness to continue running when fatigued, also play a role. As most health-related activities, including sport participation and aspects of play, involve moving body mass, the increase in body fatness without a corresponding increase in aerobic fitness is a cause for concern.

Are fitness and physical activity related during youth?

Appropriate exercise training will increase aerobic fitness but the duration and intensity of exercise required to enhance MAP is seldom, if ever, experienced during youth HPA. Studies have consistently shown that there is either a very weak or no relationship between HPA and MAP. The AGHLS reported non-significant relationships between HPA and aerobic fitness over 23 years of observation and concluded that no clear relation can be proved between physical activity and MAP.

Conclusions

There is no compelling evidence to suggest that low levels of aerobic  fitness are common or that it has decreased over the last 70 years. However, young people’s performance on tests involving the transport of body mass has markedly declined over the last 40 years.

The percentage of young people reported to satisfy current physical activity guidelines varies from 25–40%. Youth HPA appears to have stabilised, at least over the last two decades. Boys are generally observed to be more active than girls and the PA levels of both genders decline as young people move through adolescence. There is no meaningful relationship between current levels of HPA and MAP.

The low levels of habitual physical activity for the majority of children and adolescents and the finding that an increase in body fatness is not being accompanied by a corresponding increase in aerobic fitness are serious concerns in the context of youth health and well-being.

About the author

Professor Neil Armstrong is Senior Deputy Vice-Chancellor at the University of Exeter. He has written or edited 13 books and 600 other publications. His research on children’s health and well-being won the Queen’s Anniversary Prize for Higher Education in 1998.

Further reading

Armstrong N and Van Mechelen W (eds.) (2008), Paediatric Exercise Science and Medicine (2nd edition). Oxford: Oxford University Press, pp. 1–650.

Biddle S, Sallis J, and Cavill N (eds.) (1998), Young and Active? London: Health Education Authority, pp. 1–176.

British Journal of Sports Medicine (Special Issue) (2011), Sports: Keeping Young People Healthy. British Journal of Sports Medicine, 45 (11), 837–942.

Published

June 2012