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5
Physical Activity and Special Considerations
for Children, Adolescents, and Pregnant
and Postpartum Women
A number of population groups merit special attention with regard to
the relationship between physical activity and health. This chapter pre-
sents evidence of the effects of physical activity on the health of school-
age children and adolescents and of pregnant and postpartum women.
For convenience, the term youth was sometimes used by the presenters to
refer to school-aged children and adolescents. The discussion section
presents promising evidence relating physical activity to specific out-
comes in children and adolescents, information on physical activity and
skeletal growth in children and adolescents, and a brief summary of
points raised during the group discussion.
PHYSICAL ACTIVITY AND CHILDREN
AND ADOLESCENTS
Presenter: Robert M. Malina
This presentation included background information, evidence related
to the benefits of physical activity for children and adolescents, and con-
sideration of the amount of physical activity needed by youth.
Background
It is generally presumed that regular physical activity is essential to
support the normal growth and maturation, health, and fitness of children
and adolescents. Fairly recently, an expert panel was formed to review
95
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96 PHYSICAL ACTIVITY WORKSHOP
the effects of physical activity on health and behavioral outcomes in
school-aged children and adolescents ages 6 to 18 years. The panel also
was charged with developing a recommendation for physical activity for
children and adolescents. The effects of physical activity on indicators of
growth and maturation were not considered. In particular, the panelists
addressed the following questions:
• What are the health, fitness, and behavioral benefits of regular
physical activity for school-aged children and adolescents?
• In terms of frequency, intensity, and time (duration), what
amount and type of activity is needed to bring about beneficial
effects on the selected indicators?
• What amount of physical activity should be recommended?
The indicators considered included measures of health, fitness, behavior,
and injury risk. Panelists used a standardized evaluation format to exam-
ine the evidence. The panel report is published in the Journal of Pediat-
rics (Strong et al., 2005).
Evidence of Specific Outcomes
Adiposity
Normal weight youth Enhanced physical activity programs appear to
have a minimal effect on adiposity among youth of normal weight.
Cross-sectional and longitudinal comparisons of active and less active
youth give equivocal results regarding skinfold measurements. Two ex-
perimental studies suggest that youth need a considerable amount of
physical activity to maintain a healthy weight.
Obese youth Experimental studies of obese youth indicate a reduction in
total body and visceral adiposity with regular activity. In this case, regu-
lar activity is defined as a variety of moderate to vigorous activity for 30
to 40 minutes per day on 3 to 5 days per week.
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CHILDREN, ADOLESCENTS, AND PREGNANT AND POSTPARTUM WOMEN
Lipids and Lipoproteins
In a variety of intervention studies of youth, weak beneficial effects
of physical activity are reported for reducing the level of high-density
lipoprotein cholesterol and triglycerides. The amount of physical activity
needed to achieve beneficial effects appears to be a minimum of 40 min-
utes per day of moderate to vigorous activity for 5 days per week over a
4-month period. A sustained volume of physical activity may be a key
factor. School-based programs generally were not effective in improving
the lipid and lipoprotein profiles of youth.
Blood Pressure
Normotensive youth No clear association was found between physical
activity and blood pressure in normotensive youth.
Hypertensive youth Experimental aerobic training programs have dem-
onstrated beneficial effects on the blood pressure of hypertensive youth.
Thirty-minute sessions of sufficient intensity to improve aerobic fitness
on 3 days per week for 12 to 32 weeks provided benefit. Two studies
showed no change in blood pressure with resistance training. Among
youth with mild essential hypertension, some evidence suggests that
aerobic programs may reduce blood pressure.
Other Indicators of Cardiovascular Health
In general, the data that physical activity may improve other indica-
tors of cardiovascular health among youth are very limited. However,
data suggest that aerobic training increases resting vagal tone as reflected
in higher heart rate variability, which is a marker of cardiac parasympa-
thetic activity. The implications of this change for the development of
cardiovascular disease are uncertain.
Metabolic Syndrome and Type 2 Diabetes
Two studies indicate that insulin and triglyceride concentrations and
adiposity are favorably influenced by physical activity in obese youth.
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98 PHYSICAL ACTIVITY WORKSHOP
The physical activity level was moderate to vigorous three times per
week. Essentially no useful data are available regarding associations of
physical activity and type 2 diabetes in children and adolescents.
Skeletal Health
Evidence from a variety of studies suggests a beneficial effect of
regular physical activity on bone mineral content and bone mineral den-
sity. Most of the studies have been conducted on prepubertal children or
adolescents in the early stages of puberty. Typically, the studies involve
moderate- to high-strain physical activity for 10 to 60 minutes per day at
least 2 to 3 times per week. Some have used 10 minutes of impact activ-
ity and 45 to 60 minutes of general weight-bearing activity. Limited evi-
dence suggests little or no positive effect after puberty.
Aerobic Fitness
In both cross-sectional and longitudinal studies, data suggest higher
aerobic fitness in active youth than in less active youth. In experimental
studies of children ages 8 years and older, the evidence consistently
shows a favorable effect of physical activity on aerobic fitness—namely,
about a 10 percent increase in peak VO2. The programs that produced
benefits involved continuous vigorous aerobic activity of various types
for 30 to 45 minutes per day at least 3 days per week.
Muscular Strength and Endurance
Although the cross-sectional and longitudinal data are equivocal re-
garding the association of physical activity with muscular strength and
endurance among youth, experimental data show significant gains with
weight training 2 to 3 days per week with a rest day between sessions.
The key appears to be a variety of progressive resistance activities, adult
supervision, and the involvement of both reciprocal and large muscle
groups.
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CHILDREN, ADOLESCENTS, AND PREGNANT AND POSTPARTUM WOMEN
Behavioral Outcomes
Examining evidence relating physical activity to behavioral out-
comes poses a number of challenges, including a large number of out-
come measures, problems with sampling, quasi-experimental studies, and
various modes and combinations of activity. Physical activity is fairly
strongly associated both with global self-esteem and physical self-
concept. Aerobic activity generally has a small positive effect on symp-
toms of depression and anxiety, but the addition of cognitive behavioral
modification produces a stronger positive effect.
The data on effects of physical activity on academic performance are
limited, with very mixed results that tend toward the positive. Notably,
quasi-experimental data indicate that the allocation of more curricular
time to physical education or activity programs did not negatively influ-
ence academic achievement—even when less time was allocated to other
subjects.
Risk of Injury
As discussed earlier by Drs. Jones and Hootman (see Chapter 4),
physical activity increases the risk of injury. Data are limited, however,
regarding associations of school, recreational, and free-time activities
with injury. Comparability among studies is limited because of the chal-
lenges of accurate surveillance and reporting, the definition of injury, and
exposure statistics, among other reasons.
Evidence from six studies of physical education indicates very low
risk of injury in programs involving moderate to vigorous activity for 10
to 40 minutes on 3 days per week. In a study of self-reported injuries in
recreational and sporting activities among Australian adolescents, about
one in four students reported an injury, and the rate of injury was one per
three instances of participation (Grimmer et al., 2000). Most (72 percent)
of the injuries were classified as minor (e.g., bruises, aches, strains) and
5 percent as major (e.g., fracture, dislocation, concussion).
Summary of the Evidence on Physical Activity and Youth
The expert panel that addressed the benefits of physical activity in
children and adolescents (Strong et al., 2005) determined that dose–
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100 PHYSICAL ACTIVITY WORKSHOP
response data are limited. The evidence base is strongest for skeletal
health, aerobic fitness, muscular strength and endurance, adiposity in the
obese, and blood pressure in the hypertensive. Evidence is suggestive for
adiposity and lipids in nonobese youth; for blood pressure in normoten-
sive youth; and for self-concept, anxiety and depression symptoms, and
academic performance. Data on associations of physical activity with
metabolic syndrome, type 2 diabetes, and some indicators of cardiovas-
cular health currently are very limited for youth.
Most of the experimental intervention protocols used a continuous
activity program, but the activities of children, especially young children,
are primarily intermittent. Figure 5-1 illustrates how activity needs vary
with age during childhood and adolescence. This suggests the need to
develop activity protocols that examine the effects of high-intensity, in-
termittent activity on indicators of health, fitness, and behaviors.
A difficulty in evaluating the effects of activity on indicators of
health and fitness in children and adolescents relates to the fact that the
outcome variables change with normal growth, maturation, and devel-
opment, whether or not the individual is physically active on a regular
basis (Malina et al., 2004; Malina and Katzmarzyk, 2006). Available data
do not consider interindividual differences in maturity status and pro-
gress, especially during the adolescent years. Data also are needed on the
persistence of activity-induced beneficial effects and on the amount of
activity needed to maintain these beneficial effects.
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CHILDREN, ADOLESCENTS, AND PREGNANT AND POSTPARTUM WOMEN
Dashed line: general physical
100 activity, emphasis on motor skills
90
80
Solid line: prescriptive physical
Relative emphasis, %
70 activity, emphasis on health,
fitness, behavioral outcomes
60
50
Childhood:
activities largely
40
intermittent
Adolescence:
30 increased capacity for
continuous activities
20
10
0
2 4 6 8 10 12 14 16 18
Age (years)
FIGURE 5-1 Changes in types of activity needs with increasing age of children
and youth.
SOURCE: From R.M. Malina, 1991, Fitness and Performance: Adult Health and
the Culture of Youth. In New Possibilities, New Paradigms? American Academy
of Physical Education No. 24, edited by R.J. Park and H. M. Eckert, pages 30-
38. © 1991 by American Academy of Physical Education. Adapted with per-
mission from the American Academy of Kinesiology and Physical Education
and Human Kinetics (Champaign, IL).
Concluding Remarks
In closing, Dr. Malina highlighted the importance of investigating
how to prevent unhealthy weight gain in children and adolescents, con-
sidering physical inactivity as well as physical activity. Inactivity and
activity have different meanings and contexts in children and youth, and
they are generally independent of each other.
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102 PHYSICAL ACTIVITY WORKSHOP
PHYSICAL ACTIVITY DURING PREGNANCY
AND THE POSTPARTUM PERIOD
Presenter: James M. Pivarnik
This presentation addressed the historical concern with potential
harm from maternal physical activity, short-term benefits from maternal
physical activity for the mother and fetus, and the role of maternal physi-
cal activity on chronic disease risk of the mother later in life.
Historical Concerns and Guidelines
Early studies on physical activity and pregnancy were concerned
with harm rather than benefit. Most studies used animal models. Human
studies examined cardiorespiratory responses and thermoregulation in
the mother, heart rate of the fetus, and outcomes of pregnancy such as
birth weight, gestational length, and adverse events.
The American College of Obstetricians and Gynecologists (ACOG)
developed the first exercise guidelines for pregnant women in 1985
(ACOG, 1985). Those guidelines were based on the early studies and
were conservative. They included a maximum heart rate of 140 beats per
minute and time limits for physical activity. The potential need for indi-
vidualization of the recommendations was noted.
Between 1985 and 1994, nearly 600 relevant studies were published,
most of which focused mainly on doing no harm. Many of these were
laboratory studies with small sample sizes, and most involved acute ma-
ternal responses to exercise. The data suggested (1) no detrimental ef-
fects of the exercise to the mother or fetus, (2) possible reduced length of
labor, (3) possible improvement in gestational diabetes mellitus (GDM),
and (4) relatively little loss of fitness by chronic exercisers. The use of a
target heart rate was found to be somewhat problematic because of in-
consistent influences of pregnancy on heart rate among women. ACOG
updated its guidance for exercise during pregnancy in 1994 (ACOG,
1994) and again 8 years later (ACOG, 2002).
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CHILDREN, ADOLESCENTS, AND PREGNANT AND POSTPARTUM WOMEN
Current Evidence Relating Physical Activity to Outcomes
More recently, a number of studies have examined whether maternal
physical activity affects birth weight. Perkins and colleagues (2005) per-
formed a meta-analysis of the effect of maternal physical activity on
birth weight, stratified by exercise intensity. They found a slightly lighter
(but not low) mean birth weight for infants born to women who took part
in vigorous activity (see Figure 5-2). The topic merits attention because
in the year 2000 more than two-thirds of women in the childbearing
years took part in some type of physical activity (Evenson, 2004), and
many women want to continue these activities during pregnancy.
Information on vigorous activity during pregnancy is very limited. In
2005, an expert panel was assembled to examine the impact of physical
activity during pregnancy and the postpartum period on chronic disease
risk (Pivarnik et al., 2006). That panel addressed a number of topics, in-
cluding the association of physical activity with the risk of preeclampsia
and with the risk of GDM. Regular physical activity in early pregnancy
is associated with a reduced risk of preeclampsia in two case–control
studies (Marcoux et al., 1989; Sorenson et al., 2003) and one cohort
study (Saftlas et al., 2004). The evidence is not strong, but it is in the
same direction. Although pilot data from Mottola and colleagues (2005)
show a slightly reduced rate of GDM in overweight pregnant women
who began a program of physical activity, data are insufficient to de-
velop specific optimal physical activity guidelines for GDM prevention.
Some evidence is available regarding the role of physical activity in
postpartum weight reduction. Larson-Meyer (2002) reviewed approxi-
mately 60 cross-sectional and randomized trials on this topic, looking at
postpartum exercise but not activity performed during pregnancy. When
compared to sedentary women, those who performed moderate physical
activity without caloric restriction did not appear to show greater weight
or fat loss. In a later report, Rooney et al. (2005) evaluated nearly 800
women early in pregnancy and followed them for 15 years. Disease and
risk factor development (i.e., diabetes, heart disease, dyslipidemia, and
hypertension) were positively related to weight gain over the 15 years.
The women who continued to perform aerobic exercise postpartum were
less likely to become obese than those who did not. Among factors that
can create barriers to postpartum physical activity are lack of time and
lack of child care (Beilock et al., 2001).
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104 PHYSICAL ACTIVITY WORKSHOP
Moderate intensity PA
Vigorous intensity PA
Pooled Effect Size 0.05; Heterogeneity P = .49
Pooled Effect Size -0.27; Heterogeneity P < 0.01
-2.00 0 2.00
Standardized mean difference (birth weight)
FIGURE 5-2 Effect of maternal physical activity on birth weight, stratified by
exercise intensity. The sizes of the boxes reflect the relative sample sizes.
NOTE: PA = physical activity. The studies represented by the boxes in the fig-
ure are listed in descending order as follows: Beckmann and Beckmann, 1990;
Bell et al., 1995; Botkin and Driscoll, 1991; Clapp, 1990, 1996; Clapp and Dick-
stein, 1984; Clapp et al., 1998, 1999, 2000; Collings et al., 1983; Homs et al.,
1996; Lewis et al., 1998; Magann et al., 1996, 2002; Marquez-Sterling et al.,
2000; Pivarnik et al., 1994; Rice and Fort, 1991; Stemfeld, 1997.
SOURCE: Perkins et al. (2005).
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CHILDREN, ADOLESCENTS, AND PREGNANT AND POSTPARTUM WOMEN
Concluding Remarks
Dr. Pivarnik identified the effects of maternal recreational exercise
on the health, growth, and development of the offspring as an exciting
new topic of research. Dr. Jim Clapp is a lead investigator in this area,
but few studies are available and sample sizes are small. Initial studies
indicate that beginning or continuing recreational exercise during preg-
nancy has no identifiable acute or chronic adverse effects on the off-
spring and may have some positive effects. There is a great need for
prospective, randomized exercise-intervention studies in diverse popula-
tions.
DISCUSSION
Promising Lines of Evidence Relating Physical
Activity to Outcomes in Children and Youth
Discussant: Russell R. Pate
There is much less evidence supporting the development of physical
activity guidelines for children and youth than there is for adults. The
young age group poses enormous research design challenges for two ma-
jor reasons: (1) most young persons are healthy and remain so during
childhood and adolescence, and (2) young people are the most active
segment of our society. These two factors make it very difficult to dem-
onstrate positive health effects. In view of these factors and the data pre-
sented by Dr. Malina, Dr. Pate suggested directing attention to the
following three topics:
1. The amount of activity sustained over time that minimizes the
risk of excessive weight gain over that period. This focus could
be valuable in part because the increased prevalence of obesity is
of concern to society at large. A few U.S. studies and more
European studies have addressed activity and weight gain over
time in children and youth of normal weight.
2. Activity related to fitness. Children and youth with low fitness
levels tend to have low fitness during adulthood, and evidence
shows a strong association of low adult fitness with a range of
chronic disease outcomes (see Chapter 2). Using data from the
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106 PHYSICAL ACTIVITY WORKSHOP
National Health and Nutrition Examination Survey, Dr. Pate’s
group recently found that about one-third of U.S. children and
youth fail to meet aerobic fitness standards. Moreover, Dr.
Malina reported that the literature on this topic is fairly
extensive.
3. Activity and the multiple metabolic syndrome. This is a rapidly
developing area, and it is possible that evidence will be available
soon regarding protective levels of physical activity.
Physical Activity and the Skeleton in Children and Youth
Discussant: Heather McKay
Topics addressed in this discussion include the optimal period for a
response of bone to exercise, bone geometry related to bone strength,
modes and intensity of exercise, and the benefits of muscular strength for
the skeleton.
Timing
Childhood is an essential time to introduce physical activity in terms
of healthy growth of the skeleton. The key time appears to be early pu-
berty. Bone mineral content velocity lags behind peak growth and peak
linear growth by about 8 months. Approximately 26 percent of one’s
adult skeleton is accrued in the 2 years around the age of peak skeletal
growth (age 11.8 years for girls and 13.4 years for boys). This amount of
accrual represents as much bone as most people will lose in their entire
adult lives (Arlot et al., 1997).
Mechanical Loading and Bone Geometry
Stress causes strain on bones. Bone is accrued primarily on the pe-
riosteal (outer) surface, meaning that the bone is getting larger (Turner
and Robling, 2003). An increase of as little as one millimeter in the outer
surface of bone increases strength substantially. Bone may alter its geo-
metric properties independently of changes in bone mass (Jarvinen et al.,
1999). Adding bone to the outside increases strength, and adding bone to
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CHILDREN, ADOLESCENTS, AND PREGNANT AND POSTPARTUM WOMEN
the endosteal (inner) surface increases strength also. Dual-energy X-ray
absorptiometry (DEXA) does not detect these responses. Bone mineral
density by DEXA may be the same across different configurations of a
cross section of bone, even though strengths may differ by four or eight
times. Thus there is a huge gap between knowing the amount of bone
mineral in a given bone and knowing its structural capacity. The whole
bone structure substantiates the mechanical competence of bone
(Jarvinen et al., 2005).
Modes and Intensity of Exercise
Data are lacking on what mode or intensity of physical activity is os-
teogenic (builds bone). Studies with rats indicate that 5 jumps per day
were as effective as 40 jumps per day in increasing bone strength
(Umemura et al., 1997). Preliminary data from Dr. McKay’s group
suggest that taking five jumps each of three times per day during school
increases distal tibia muscular strength in boys but not in girls.
Muscular Strength and the Skeleton
In both males and females, peak lean mass accrual occurs about 4
to 6 months before peak bone mass accrual (Rauch et al., 2004). Data
show a very close relationship between a surrogate for muscle strength,
total lean body mass, and bone strength. Similarly there is a close asso-
ciation between an increase in muscle mass and an increase in bone
mass. Thus interventions that increase muscular strength also may bene-
fit the skeleton.
Concluding Remarks
Dr. McKay highlighted the following points:
• Early puberty is the period when the response of bone to exercise
is optimized.
• Small changes in bone geometry result in substantial increases in
bone strength.
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108 PHYSICAL ACTIVITY WORKSHOP
• Frequent episodes of short exercise bouts may be as effective as
sustained exercise.
• Further studies are needed.
• Schools may be the key avenue to having an impact on chil-
dren’s health.
• The benefits of exercise do not persist if exercise is withdrawn.
GROUP DISCUSSION
Moderator: Patty S. Freedson
Among the points raised during discussion were the following:
• The quantification of intermittent bouts of physical activity may
pose a challenge. Accelerometry may be helpful, as may other
new technological approaches to assess behavior objectively.
• Questions were raised about the appropriateness of an exercise
prescription model for children. Considering a diversity of ac-
tivities may be helpful, along with setting appropriate targets for
the duration of moderate to vigorous intensity activity.
• More data are needed about the risk of physical activity among
children and youth.
• Work in animals suggests that dividing loads over several ses-
sions per day substantially increases the osteogenic response.
Evidence regarding this relationship in humans is needed.
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Representative terms from entire chapter:
uncorrected proofs
