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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 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)
- 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.
- 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.
- 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.
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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 clearly-established 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 within the
central nervous system which governs levels of activity.
With increasing age, centrally-dictated caloric expenditure
through 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 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 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 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 age-specific 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.
- Bungum & Vincent (1997)
Ethnicity (Caucasian)
Nurture from biological fathers
Participation in organized sports
Friend support
Attitudes toward physical activity
- Janz & Mahoney (1997)
Sexual maturation
Less video game playing
- Gentle et al. (1994)
Satisfaction with amount of activity
Encouragement from others
Desire for competitiveness
- Garcia et al. (1995)
Social support
Access to exercise facilities
- Douthitt (1994)
Feelings of self-competence
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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.
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
group-need 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
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Guest Author:
Dr. Thomas W. Rowland, M.D.
Baystate Medical Center
Department of Pediatrics
Springfield, MA
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Co-edited By:
Drs. Chuck Corbin and Bob Pangrazi
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Arizona State University
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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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