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Educating the Student Body: Taking Physical Activity and Physical Education to School
42
Educating the Student Body
BOX 2-2
Dose of Physical Activity
The dose of physical activity, sometimes referred to as volume or amount,
is a function of the type, frequency, duration, and intensity of the activity.
Until
recently, benefits appeared to derive almost exclusively from
vigorous- or moderate-intensity physical activity, and almost all epide-
miologic, clinical, and intervention research focused on that intensity
range. Evidence derived from that research indicated that, for both adults
and youth, health benefits are more closely related to the total dose of
vigorous- or moderate-intensity physical activity than to any of the com-
ponents of dose (i.e., type, frequency, duration, intensity) (PAGAC, 2008).
Research has consistently demonstrated an inverse
relationship between
the dose of regular vigorous- or moderate-intensity physical activity and
a wide variety of adverse health outcomes (e.g., heart disease, diabetes,
functional limitation, depressive symptoms) (PAGAC, 2008).
Dose of vigorous- or moderate-intensity physical activity has been mea-
sured in a variety of ways, including minutes per week, kilocalories or
kilojoules of energy expenditure, or even miles or minutes of running
per week. A method frequently used to describe
the dose of vigorous- or
moderate-intensity physical activity is to multiply the time spent in the
activity by its metabolic equivalent (MET) value (see Box 2-1). For exam-
ple, running at 5 miles per hour (about 8 METs) for 20 minutes provides
a dose of 160 MET minutes.
Accelerometers permit estimates of the total dose of physical activity
performed in a day regardless of intensity (see also Box 2-3). By estimat-
ing intensity for every minute, accelerometers can provide an estimate
of the total dose of vigorous- or moderate-intensity physical activity.
Accelerometers are not yet able to “know” exactly what type of physical
activity is being performed (e.g., cycling, stair
climbing, raking, sweep-
ing) and as a result may over- or underestimate the rate of energy
expenditure. Nevertheless, the products are improving rapidly, and their
capability to measure sedentary and light-intensity as well as vigorous- or
moderate-intensity activities makes them an increasingly valuable tool.
The inverse relationship between dose of vigorous- or moderate-intensity
physical activity and reduced risk of adverse health outcomes clearly
demonstrates that larger doses provide more health benefits. Selecting a
single dose to recommend has been more difficult, especially
for youth,
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Educating the Student Body: Taking Physical Activity and Physical Education to School
Status and Trends of Physical Activity Behaviors and Related School Policies
43
because less research is available on this question, and the pattern
of the dose-response curve is less well established (PAGAC, 2008). A
composite of the findings for adults from 10 studies of regular physi-
cal activity and all-cause mortality indicates a curvilinear relationship,
with the risk of mortality decreasing as the dose of regular vigorous- or
moderate-intensity physical activity increases (PAGAC, 2008; Powell et
al., 2011) (see the figure below). Four points
are worth noting about the
dose-response curve. First, there is no lower threshold for benefit; some
vigorous- or moderate-intensity physical activity is preferable to none.
Second, the slope is steepest at the left side of the curve, suggesting that
the most rapid reduction in risk occurs when the least physically active
people become slightly more active. Third, there is no obvious best dose,
making a range of doses (e.g., 150-300 minutes per week of vigorous-
or moderate-intensity physical activity for adults) as in the current U.S.
guidelines (PAGAC, 2008) preferable to a single dose. Fourth, at levels of
activity commonly achieved in the general population, there
is no upper
threshold above which risk ceases to decline, although the rate of decline
decreases.
Risk of all-cause mortality by hours per week of vigorous- or moderate-
intensity physical activity.
SOURCE: Adapted from PAGAC, 2008.
The available evidence indicates that the dose-response curves for the
beneficial effect of regular vigorous- or moderate-intensity physical activ-
Figure for Box 2-2.eps
0
0.5
1
0
1
2
3
4
5
6
7
Relativ
e Risk
Hours/Week of Moderate- to Vigorous-Intensity Physical Activity
No
Lower Threshold for Benefit
Steep Early Slope
No Upper Threshold
No Obvious “Best” Amount
Risk reduction for some
conditions accrues
more rapidly and for
others more slowly
continued
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