The decline in physical activity with age may be the most consistent finding in physical activity epidemiology (1). Age is inversely associated with physical activity in studies of children (7), children and adolescents (4), U.S. adults (9), and adults in international studies (1, 2). Declines throughout youth have been documented with both self-report and objective measures of physical activity (5). Although this phenomenon is well documented and well accepted, it is not well understood. For example, it is not known whether the mechanism of the decline is primarily environmental or biological. Few studies have attempted to determine whether all types and intensities of physical activity decline in similar ways. Identifying ages of greatest decline may be useful in targeting interventions to critical periods in life. The symposium described in this section was organized to extend knowledge in this area by presenting new analyses of existing data.
A related issue is the finding from a review of studies of young people that the decline in physical activity is greater in female than male subjects (5). This pattern was seen with both self-report and objective measures. The presenters in the symposium were asked to describe sex differences in their findings. Thus, the symposium was organized to address three questions regarding the age-related decline in physical activity. Those questions are listed below, and this paper synthesizes the findings related to each question.
1. What is the age of greatest decline for male and female subjects?
2. What are differences in declines for various types and intensities of physical activity?
3. How do results of human studies compare with those of animal studies?
The four papers in this section present very diverse data of unusually high quality. Caspersen et al.(2) summarize results of large, nationally representative surveys of U.S. adolescents and adults. This paper covers the ages of 12 through 75 plus, and it reports on several types and intensities of physical activity. Although cross-sectional data are useful in examining age-related declines, longitudinal studies can both confirm and extend cross-sectional findings. This section contains reports from the two largest and longest longitudinal studies of physical activity in young female and male subjects. Telama and Yang (8) report longitudinal data collected from 1980 to 1989 on a large sample of Finnish young people that ranged in age from 9 to 27 yr. Van Mechelen et al. (10) report new analyses from the Amsterdam Growth and Health Study. Almost 200 young people have been followed for 15 yr, from age 13 to 27. All of these three studies describe the decline in multiple types and intensities of physical activity. All of these papers present new analyses that provide new insights.
The consistency of the age-related decline in multiple countries raises the question of biological mechanisms. One way to address this issue is to examine data from animal studies. If the decline is seen in animals, it is likely to be biological in origin. Because the animal literature on this topic is not well known to physical activity epidemiologists, Donald Ingram, from the National Institute of Aging, was invited (and graciously agreed) to review the animal literature. He surpassed expectations by offering a clear and well-documented summary of the animal literature and presenting strong evidence for a major biological mechanism for the decline (3).
What is the age of greatest decline for male and female subjects?
The three studies of humans produced strikingly similar results. Summarizing across measures, the ages of greatest decline were 13–16 in the Dutch study (10), 12–15 or 15–18 in the Finnish study (8), and 15–18 in the U.S. study (2). The Caspersen et al. study (2) clearly showed that the annual rate of decline is much greater during adolescence than during adulthood.
Although these ages of greatest decline were similar for male and female subjects, there were important sex differences. On most measures, the decline was greater in male than female subjects. For example, an overall energy expenditure score in the Amsterdam study (10) declined 42% in male and 17% in female subjects from age 13 to 27. Similarly, the physical activity index in the Finnish study (8) declined 55% in male and 20% in female subjects from age 12 to 27. In the U.S. study (2), regular, sustained activity declined 16 percentage points for male subjects and 10 percentage points for female subjects from age 12 to 21. However, regular, vigorous activity in the U.S. study (2) had no sex difference, with a 34-point decline for male and a 38-point decline for female subjects from age 12 to 21. The current finding of greater decline among young male than female subjects conflicts with findings from an earlier review (5), and explanations for this discrepancy are not apparent.
A limitation of these studies is the absence of comparable results for younger children. It can be concluded that teenagers are at high risk of physical activity decline in these industrialized countries, but it is possible that the decline is equal or greater at earlier ages. More studies using comparable measures across the entire age range are needed. The other major limitation of these studies is the reliance on self-report data. It appears from other studies that self-reports underestimate the age decline and the sex differences (5), so future studies should use objective measures whenever possible.
What are differences in declines for various types and intensities of physical activity?
Because each study used different measures and definitions, limited comparisons can be made, but intriguing patterns were observed. Van Mechelen et al. (10) reported vigorous activity (defined here as 7+ METs) decreased dramatically with age, whereas moderate intensity activity (defined here as 4–7 METs) increased from ages 13 to 27. Caspersen et al. (2) showed a large decline in regular vigorous activity of youth and a smaller decline in regular, sustained activity that is not vigorous.
Different types of activity also showed very different patterns of change with age. Van Mechelen et al. (10) classified activities as organized sport, nonorganized sport, and other. Organized sport was at low levels at every age. Nonorganized sport was the primary component of activity that declined. Other activities actually increased significantly with age. Thus, examining only summary measures of activity seems to obscure a variety of underlying patterns.
Caspersen et al. (2) report activities designed to promote various components of physical fitness. During youth, all categories of physical activity declined, with peak declines in the 15- to 18-yr age group in virtually every case. Adults tended to have different patterns of decline for different activities. For strengthening and stretching exercises, declines were usually largest in young adulthood and smaller at later ages. For regular, sustained activity, there were declines in early and late adulthood, but stability during the middle years. The pattern for regular, vigorous physical activity is anomalous, because it increases in the later years. This is largely due to changes in the definition of vigorous activity in older age groups.
Based mainly on the findings of Van Mechelen et al. (10), it appears that most of the physical activity decline during adolescence is due to decreases in nonorganized sport and vigorous physical activity. If these patterns generalize across countries, they provide a rationale for designing interventions to counter these specific trends.
How do results of human studies compare with those of animal studies?
Ingram’s (3) review of animal studies shows a great deal of consistency with the human studies. There is evidence of a physical activity decline in many species, ranging from insects and rodents to monkeys. These results are seen in both cross-sectional and longitudinal studies. Over the adult age range, rodents decrease their overall activity levels by approximately 50%. Although some studies seemed to show a linear decline in activity levels with age, others found greater decline at young ages, similar to the human studies. Unfortunately, the animal data did not permit firm conclusions about activity patterns during youth or about sex differences, because most studies were of adult male animals. Nevertheless, the striking similarities between human and animal physical activity patterns with age support an interpretation that the decline is at least partly a biological phenomenon.
The biological hypothesis is further strengthened by findings that dopamine, acting on specific brain areas, is related to the motivation for locomotion. It may be useful to take these findings from animal studies and determine how they can be applied to understanding the age-related decline in human physical activity. Even though the physical activity decline appears to be strongly influenced by biological factors, nonbiological factors are also involved. There is substantial evidence that psychological, social, and physical environmental variables are related to physical activity in both cross-sectional and longitudinal studies (6,9). An important research issue is to determine how environmental variables can modify the biological tendency to decline in physical activity with age.
The utility of the animal studies for enhancing understanding of human physical activity suggests that animal models could be used to address other questions related to the epidemiology of human physical activity. One of Ingram’s (3) most fascinating sections briefly reviewed the evidence that physical activity extends longevity in animals. Thus, animal models may be useful for establishing dose-response relations for various physiological outcomes that could then be evaluated in humans.
This symposium confirmed the near universality of the decline in physical activity. The teen years (i.e., 13–18) were identified as the age of greatest decline in physical activity, although it is possible that large declines will also be seen at younger ages. Declines during adulthood occur at a much slower rate than during adolescence. A surprising finding was that male subjects decline more in physical activity than female subjects, especially during youth.
This set of papers identified more specific patterns of decline. The decline is greater in vigorous activities and in nonorganized sport activities. These results can lead to hypotheses that can be tested in intervention studies. A high priority should be placed on developing effective interventions for adolescents. Although male subjects are decreasing most in their activity, male subjects continue to be more active than female subjects during the teen years. This pattern suggests interventions are needed for both sexes. Strategies can include promoting vigorous activities to halt their decline or stimulating further increases in nonsport activities.
The goal of extending understanding about the decline in physical activity was achieved by the symposium. Another goal was to stimulate research on this fundamental dimension of the epidemiology of physical activity, and numerous hypotheses can be derived from the papers that follow. The decline in physical activity with age is antithetical to public health goals, so methods of countering the decline need to be developed, based upon an improved understanding.
1. Caspersen, C. J., R. K. Merritt, and T. Stephens. International activity patterns: a methodological perspective. In:Advances in Exercise Adherence
, R. K. Dishman (Ed.). Champaign, IL: Human Kinetics, 1994, pp. 73–110.
2. Caspersen, C. J., M. A. Pereira, and K. M. Curran. Changes in physical activity patterns in the United States, by sex and cross-sectional age. Med. Sci. Sports Exerc. 32: 1601–1609, 2000.
3. Ingram, D. K. Age-related decline in physical activity: generalization to nonhumans. Med. Sci. Sports Exerc. 32: 1623–1628, 2000.
4. King, A. J. C., and B. Coles. The Health of Canada’s Youth: Views and Behaviours of 11–13, and 15-Year-olds from 11 Countries. Ottawa, Canada: Minister of National Health and Welfare, 1992, p. 29.
5. Sallis, J. F. Epidemiology of physical activity and fitness in children and adolescents. Crit. Rev. Food Sci. Nutr. 33: 405–408, 1993.
6. Sallis, J. F., and N. Owen. Physical Activity and Behavioral Medicine. Thousand Oaks, CA: Sage, 1999, pp. 110–133.
7. Sallis, J. F., J. J. Prochaska, and W. C. Taylor. A review of correlates of physical activity of children and adolescents. Med. Sci. Sports Exerc. 32: 963–975, 2000.
8. Telama, R., and X. Yang. Decline of physical activity from youth to young adulthood in Finland. Med. Sci. Sports Exerc. 32: 1617–1622, 2000.
9. U. S. Department of Health and Human Services. Physical Activity and Health: A Report of the Surgeon General
. Atlanta, GA: Centers for Disease Control, 1996, pp. 215–217, 234–236.
10. Van Mechelen, W., J. W. R. Twisk, G. B. Post, J. Snel, and H. C. G. Kemper. Habitual activity of young people: the Amsterdam Growth and Health Study. Med. Sci. Sports Exerc. 32: 1610–1616, 2000.