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Adolescence: A "Risk Factor" for Physical Inactivity

Exercise is good for your health-a lesson learned from the ancients-but the

recommendations for achieving such benefits has undergone a significant

transition in the closing decades of the Twentieth Century. Most particularly,

there has been a shift away from the importance of developing cardiovascular

(aerobic) physical fitness and toward the promotion of life-long physical activity.

This change has resulted from an understanding that the biological mechanisms

linking exercise to health are not simply related to achieving high cardiovascular

function but also in increasing caloric expenditure (obesity), weight-bearing

activities (osteoporosis), and muscle strength (back problems, physical

incapacity in the elderly).

In addition, it has been recognized that most diseases affected by exercise (such

as coronary heart disease, hypertension, obesity, and osteoporosis) are a result

of life-long processes, surfacing clinically in the older adult years. This

observation has prompted an emphasis on promoting exercise habits in children

and adolescents as the starting point of a life-style of regular exercise that will

be maintained through to adulthood. That is, the introduction of exercise early

in life with the key issue of persistence of activity has replaced an emphasis on

improving physical fitness to threshold levels (Corbin et al., 1994).

This shift in the exercise-promotion paradigm necessitates a parallel change in

focus toward behavior modification rather than exercise training. But in

developing this strategy many questions have arisen. How can young people

best be "turned on" to being physically active? Can it be truly expected that

improving activity habits of an eight-year old girl will cause her to be a more

active adult? Given programmatic and financial constraints, should the

promotional focus be on certain populations of children who are at particular risk

for a sedentary lifestyle (the obese, the athletic "failures")? Or should physical

activity promotion be expanded to the pediatric population at large?

One particularly critical aspect of activity promotion for lifetime health surrounds

its timing. It might be assumed that there are certain periods of development

when efforts to introduce physical activity habits are more likely to 1) be

successful, 2) create an optimal salutary effect on health risk, and 3) be

sustained into adulthood. In fact, when guidelines for physical activity for

children have been created, special attention has been focused on age-specific

recommendations (Corbin & Pangrazi, 1998). There has been a growing

recognition that the adolescent years may, in fact, serve as such a pivotal,

critical period for activity promotion (Sallis & Patrick, 1994; U.S. Department of

Health and Human Services, 1996) and particular guidelines have been

suggested for this age group (Table 1). Epidemiologic evidence suggests that

levels of activity demonstrate a particular decline during the teen years,

especially in females. Adolescence is a key period for changes in certain health
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risk factors such as the appearance of the initial lesions of coronary artery

disease and peak development of bone mineral density. Standing at the

immediate threshold of adulthood, the adolescent's physical activity habits-as

well as health risk factors-are more likely to track into the older years.

Opportunities for participation in organized sports decrease in the teen years,

while factors discouraging physical activity, such as access to automobiles,

become more available. Increases in body fat in the female at puberty may

serve to discourage participation in physical activities. The biological drive for

physical activity wanes during adolescence at the same time that increasing

independence allows teenagers to manage their own lifestyles. They are thus

less influenced by parents and more by their peers, and motivation for physical

activity depends more on social rather than biological or family factors.

Table 1.

Physical Activity Guidelines for Adolescents (Sallis & Patrick, 1994)

1. All adolescents should be physically active daily, or nearly every

day, as part of play, games, sports, work, transportation,

recreation, physical education, or planned exercise, in the context

of family, school, and community activities.

2. Adolescents should engage in three or more sessions per week of

activities that last 20 minutes or more at a time and that require

moderate to vigorous levels of exertion.

3. Special groups deserve particular exercise prescriptions. For

example, obese adolescents may benefit from a program of

increased regular energy expenditure through physical activity.

Blood pressure may be reduced in adolescents with hypertension

by vigorous activity, 3-4 times per week.

These unique features of adolescence provide both risk and opportunities for

exercise-health promotion. The following sections will examine these influences

which affect the present and future physical activity of adolescents. Recognizing

and understanding these factors may prove essential in developing strategies for

exercise promotion at this critical period in life.

Physical Activity in Adolescence

Efforts to improve exercise habits in the population confront the clearlyestablished trend for a progressive decline in individual physical activity

throughout the life span. The daily caloric expenditure (relative to body size) of

an 18-year old is approximately half that of when he/she was 6 years old. (This

is confirmed by life's experience: consider the Brownian motion of a group of

kindergarten children at a birthday party compared to the same individuals at

their high school graduation reception.) In reviewing research data, Sallis (1993)

concluded that during the school-age years, daily physical activity decreases at

a rate of about 2.7% per year in males and 7.4% per year in females. Levels of

activity steadily decline during the adult years as well. The percentage of adults

in the United States who are sedentary generally increases 2-3 fold between the

ages of 20 and 65 years (Stephens, 1987).

It appears that this basic trend for declining activity during life has a biological

basis (Rowland, 1998). Evidence supports the presence of an inherent control

center withinthe central nervous system which

governs levels of activity. With increasing age,

centrally-dictated caloric expenditure through
PCPFS Research Digests

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activity declines, paralleling that of basal

metabolic rate. The decline in physical activity

with age is therefore largely intrinsic, the result

of a fall in central drive as well as other

biological factors, such as a decreasing skeletal

muscle mass in older years. There is no

question, however, that the shape of the

physical activity-age curve, i.e., the rate of decline in activity, is influenced by

extrinsic, or modifiable, factors. And this is where interventional strategies can

be effective in improving habits of physical activity. Critical to this approach is

the identification and manipulation of psychosocial and environmental

determinants which affect the individual's motivation and participation in

physical activity.

Evidence exists to suggest that the rate of decline of physical activity is

particularly accentuated during the teenage years (Pate et al, 1984). The Youth

Risk Behavior Survey indicated that 81% of boys in grade 9 participated in

vigorous activity during 3 or more days in the week before the survey (Heath et

al., 1994). This proportion decreased steadily during the high school years to

only 67% in grade 12. Between the ninth and twelfth grades the percentage

involved in such vigorous activity in girls fell from 61% to 41%.

The survey also revealed a downward trend in enrollment in physical education

during the course of high school. In the ninth grade, 81% of females and 81%

of males were participating in physical education. By their senior year, however,

these numbers had fallen to 39% and 45%, respectively.

Riddoch et al. (1991) reported that 11-13 year old Irish boys participated in an

average of 33 minutes of activity daily while those 14-16 years were active only

7 minutes a day. In females, mean values were 20 and 12 minutes,

respectively. In the Muscatine Study, Janz and Mahoney (1997) used

accelerometers to examine the relationship of sexual maturation and daily

activity levels during adolescence. Average daily movement, expressed as counts

per minute, was 30% less in the postpubertal compared to midpubertal boys. In

girls, counts were 19% less at postpuberty compared to midpuberty.

The longitudinal study of Verschuur and Kemper (1985) involved 233 Dutch male

and female teenagers using heart rate monitoring to assess activity. At age 12-

13 years the boys and girls spent 1.3 and 1.2 hours per day, respectively,

exercising at an intensity equivalent to 50% VO2max. By age 17-18, time had

decreased to 0.5 and 0.8 hours per day, respectively.

It is not difficult to suggest explanations for this inordinate decline in physical

activity during adolescence. A combination of intrinsic and extrinsic factors are

juxtaposed during the teen years which make the adolescent particularly

vulnerable to developing a sedentary life style.

The Decline of Biological Drive and Rise in Psychosocial Influences

During early childhood, daily energy expenditure through physical activity

appears to be largely biologically-driven. That is, the three year-old who zips

about the house does not make a conscious decision to exercise or not. At this

age, motivation for physical activity, access to exercise facilities, and support of

family members are generally not critical to level of habitual physical activity. As

the child grows, the biological drive for exercise energy expenditure declines and

extrinsic factors affecting activity levels become more influential. This reaches a

particularly critical point at adolescence, when the diminished inherent drive for

activity coincides with increasingly important psycho-social factors which

influence involvement in physical activity. Unfortunately, these extrinsic factors
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often act negatively to diminish activity levels during the teen years.

The motivation for physical activity for the typical adolescent, no longer a

biological issue, is shaped by factors that involve peer acceptance, physical

capabilities, sexual attractiveness, and self-concept. For the talented high school

athlete, sports play satisfies these issues. But for the nonathletic teenager,

physical activity may be the antithesis of these goals, which are met by

"hanging out", rebelling from adult forms, and adopting strange dress or hair

styles. For many teenagers, vigorous physical activity is simply not "cool."

These social barriers to regular physical activity are compounded by the growing

need for independence with rejection of adult-oriented health goals. The

adolescent becomes old enough to drive, has more money and access to fast

foods, and increases exposure to cigarette smoking and drugs. All these factors

combine to make regular physical activity and other healthy lifestyles

unattractive options for many adolescents.

Gender and Body Composition

Epidemiologic studies consistently indicate that males are involved in more total

and vigorous daily physical activity compared to females, and this is true during

adolescence as well (U.S. Department of Health and Human Services, 1996). In

addition, as noted above, some reports suggest that the decline in habitual

physical activity during the teen years is more exaggerated in girls. These data

imply that adolescence may be a particularly high risk period for developing

sedentary habits in females.

Females face social pressures that have historically linked physical prowess and

athleticism to maleness, and gender differences in activity have traditionally

been accounted for by perceptions that femininity is not consistent with vigorous

activity and sports play. While significant progress in dispelling this concept has

occurred, detrimental ideas concerning gender-appropriateness in sports play

and physical activity persist. Social issues continue to act as important

impediments to involvement in exercise by girls. In adolescence these influences

are compounded by the burgeoning sexuality at puberty and strong desire for

attractiveness to the opposite sex. In males, sexual desirability is often linked to

physical capabilities in sports participation and physical activity. In females, on

the other hand, attractiveness is focused on physical features, often perceived

as incompatible with vigorous physical activity.

Inevitable changes in body composition at the time of puberty may also work to

the adolescent female's disfavor. Rising estrogen levels in the early teen years

promote an increase in body fat in females, while the androgenic influences of

puberty augment muscle mass in males. A typical 8-year old girl has 16% body

fat, while at age 14 she will be 22% fat (Lohman, 1992). The value will rise to

24-30% by the time she is 35 years old. This increased fat serves as an inert

load that must be transported during weight-bearing physical activity. That

makes exercise more difficult, causing a tendency to avoid physical activity,

which in turn results in increases in body fat and diminished physical fitness. The

end result of this cycle is entrenched sedentary habits in the young female

during the teen years which are difficult to reverse (Bar-Or, 1983).

Limited Access to Sports Play

Participation in organized sports activities is a valuable means of maintaining

high levels of physical activity in a social setting (Katzmarzyk & Malina, 1998).

During the elementary and middle school years, widespread involvement by

youngsters in community sports teams such as soccer, swimming, and baseball

has, in fact, permitted large numbers of otherwise athletic-unskilled children to

become physically active. Once high school is reached, however, such

"everybody plays" programs disappear, supplanted by highly competitive

programs which are designed for the few who are sufficiently skilled to "make
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the team" (Bungum & Vincent, 1997). Opportunities for intramural participation

in high schools is also unusual, as financial constraints channel available funds

to interscholastic programs.

Adolescent Physical Activity and Health Risk Factors

An improved understanding of the natural course of certain chronic diseases of

adulthood has indicated a particular significance for health interventions during

adolescence. Many of these diseases are outcomes of pathological processes

which begin during the teenage years. It follows logically, then, that

interventions designed to prevent or reduce the risks of these diseases are best

introduced during adolescence. In addition, given the proximity of adolescence

to the adult period when such diseases appear clinically, behavioral changes

such as improved physical activity in teenagers should have the best chance of

tracking into the adult years.

Several examples highlight this strategy. Osteoporosis, or decreased bone

density, increases susceptibility to skeletal fractures and is a major cause of

disability and death in elderly individuals, particularly women. The peak gain in

bone mineral density occurs at 13-14 years of age, and 90 percent of adult bone

mineral content is established by the end of adolescence (Bailey et al., 1996).

As weight-bearing activity stimulates bone growth, regular exercise during the

teen years should be expected to be important in decreasing the incidence and

severity of adult steoporosis.

In adults, the risk of coronary artery disease, the major cause of death in the

United States, is reduced by both regular physical activity and increased

physical fitness. This may occur from an ameliorating effect on coronary risk

factors, particularly elevating HDL-cholesterol, or by some unknown direct effect

on the coronary vasculature. The atherosclerotic lesions which lead to coronary

artery obstruction in adults appear initially in these vessels on autopsy

specimens of adolescents (McGill et al., 1963). Physical activity habits which can

reduce the risk of future myocardial infarction are therefore optimally introduced

during the teen years.

Obesity in adults carries an increased risk of atherosclerotic vascular disease,

hypertension, stroke, type 2 diabetes, and other significant diseases. Since

obesity in children and adolescents rarely causes medical complications, the risk

for the overweight youngster lies in the chance that his or her obesity will carry

over into adulthood. As would seem intuitive, the risk of doing so increases with

age during childhood, such that the obese adolescent has an 80% chance of

becoming an overweight adult (Lloyd et al., 1961).

Tracking of Physical Activity

The strategies in this discussion are based on the premise that physical activities

initiated during adolescence will persist, or track, through the adult years. The

extent that this is true has not yet been well clarified. Studies investigating this

question are hampered by lack of an accurate means of measuring physical

activity in large populations, an inability to easily assess intensity of regular

exercise, and the effect of significant subject dropout on study findings.

Glenmark et al. (1994) described correlation coefficients of r=0.64 and 0.48 for

women and men, respectively, between physical activity (by questionnaire) at

age 16 and 27 years of age. In a similar study, Barnekow-Bergkvist et al.

(1998) found that leisure time activity at age 16 years in males decreased the

risk of being sedentary at age 34 years by one-half.

In reviewing both retrospective and longitudinal tracking studies, Telama et al.

(1997) concluded that these reports "indicate that physical activity and sport

participation in childhood and adolescence represent a significant prediction for

physical activity in adulthood. However, the relationship is very low, and, in

some cases insignificant." In their own study of young Finns, they found
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stronger correlations between physical activity in adolescence and 12 years later

in adulthood (r=0.21 to 0.26) than from age 9 to age 21 (r=-.01 to 0.15). The

effectiveness of increasing activity behavior in adolescence as an antecedent to

improving adult activity has not yet been examined.

Tracking of physical inactivity may be more impressive. Raitakari et al. (1994)

reported that the probability of an inactive 12 year old remaining sedentary at

age 18 years was 51-63% for girls and 54-61% for boys.

Strategies for Promoting Activity in Adolescents

Given that physical activity needs to be promoted as a life-long continuum, it is

apparent that different age groups require separate strategies for promoting

regular habits of activity. In all groups, however, creating an enjoyment of

physical activity in which individuals can be successful and receive peer and

family support is key.

Strategies for health promotion are best formulated around recognized agespecific determinants of such activity. Research among adolescents has indicated

a number of factors that have been associated with involvement in physical

activity in this age group (Table 2). In general these center around opportunities

for play, support of friends, and competence in physical activities. It is

apparent, too, that the varying influences of cultural group, socioeconomic

status, race, geography, and season must be considered when formulating

interventional programs (Bungum & Vincent, 1997; Garcia et al., 1995).

Table 2.

Psychosocial Factors Associated With Physical Activity in Adolescents.

1. Bungum & Vincent (1997)

Ethnicity (Caucasian)

Nurture from biological fathers

Participation in organized sports

Friend support

Attitudes toward physical activity

2. Janz & Mahoney (1997)

Sexual maturation

Less video game playing

3. Gentle et al. (1994)

Satisfaction with amount of activity

Encouragement from others

Desire for competitiveness

4. Garcia et al. (1995)

Social support

Access to exercise facilities

5. Douthitt (1994)

Feelings of self-competence

It is possible that the factors which threaten to diminish physical activity habits

during adolescence can be utilized instead as means of exercise promotion. For

example, the educational message that the individual can and should accept

responsibility for his or her own health (and exercise habits) is consistent with

the adolescent's growing need for independence. Similarly, providing the

adolescent with a choice of activities may prove more effective than physical

education programs that dictate a curriculum.
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For instance, using community programs, it might be possible to offer a choice

of activities such as rock climbing, in-line skating, or kayaking that would prove

more appealing to the adolescent than traditional physical education programs.

Taking a cue from anti-smoking programs, efforts could be made to make

physical activities more attractive to teens as opposed to an unpleasant life of

sloth (i.e., "it's cool to sweat"). Such efforts seem particularly pertinent to

females, and the message that vigorous activity is important for girls needs to

be continued to be emphasized. This can be supported through the promotional

efforts of female athlete role models.

If intramural sports cannot be made available within the school program, such

activities should be developed by community recreation departments. The

availability of school gymnasiums, exercise rooms, and pools in evening hours

could be geared specifically to adolescent groups.

The input of adolescents in creating such programs may be critical to their

success. As the "consumers" of preventive efforts, cues as to what "works" may

best come from the teenagers themselves. Providing them independence in

formulating physical activity programs may also provide a means of increasing

participation.

Conclusion

A unique combination of biological and psychosocial factors coincide during

adolescence to create a particular importance for health-related physical activity.

At the same time, many of these factors provide barriers to stimulating

teenagers to adopt regular exercise habits. Innovative physical education

programs and exercise promotional efforts specifically directed to this age group

are important in overall preventive medicine strategies. Success in these

programs may hinge on the ability to utilize characteristics of this age groupneed for independence, peer acceptance, desire for choice and variety-in

formulating exercise initiatives.

Published quarterly by the

President's Council on

Physical Fitness and Sports

Washington, D.C.

http://www.indiana.ed/~preschal

Guest Author:

Dr. Thomas W. Rowland, M.D.

Baystate Medical Center

Department of Pediatrics

Springfield, MA

Co-edited By:

Drs. Chuck Corbin and Bob Pangrazi

Arizona State University

References

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men and women at the ages of 16 and 34 and development of physical activity from

adolescence to adulthood. Scandinavian Journal of Medicine and Science in Sports, 6, 359-370.

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physical activity levels for youth. The President's Council on Physical Fitness and Sports

Research Digest, (8) 1-7, 1994.

7. Douthitt, V.L. (1994). Psychological determinants of adolescent exercise adherence. Adolescence,

711-722.

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and developmental differences in exercise beliefs among youth and prediction of their exercise

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among 14- and 15-year old children. Journal of Public Health, 16, 186-194.
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15. Lloyd, J.K., Wolff, O.H., & Whelen, W.S. (1961). Childhood obesity: a long term study of height

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16. McGill, H.L., Geer, J.C., & Strong, J.P. (1963). Natural history of human atherosclerotic lesions.

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pp. 41-49.

17. Pate, R.R., Long, B.J. & Heath, G. (1984). Descriptive epidemiology of physical activity in

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Physical Activity and Fitness Quote

"A unique combination of biological and

psychosocial factors coincide during adolescence to create

a particular importance for health-related physical activity.

At the same time, many of these factors provide barriers to

stimulating teenagers to adopt regular exercise habits."

Dr. Thomas W. Rowland, M.D.

Baystate Medical Center

Department of Pediatrics

Springfield, MA

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