Appendix 1. Translating Scientific Evidence About Total Amount and
Intensity of Physical Activity Into Guidelines
This appendix discusses two issues that arise when translating
scientific evidence into physical activity guidance for the public:
- In scientific terms, total weekly physical activity in the range of
500 to 1,000 MET-minutes produces substantial health benefits for adults. How
should this finding be simplified and translated into Guidelines that are
understandable by the public?
- Two methods are used to assess the intensity of aerobic physical
activity, termed "absolute intensity" and "relative
intensity." Should the Guidelines specify one method or allow both?
After discussing background information related to these questions, this
appendix explains the approach taken on these two issues in the Physical
Activity Guidelines for Americans.
Background
The Guidelines are derived from an evidence-based report on the health
benefits of physical activity, written by the Physical Activity Guidelines
Advisory Committee. As background, this appendix first briefly explains the
concept of METs and MET-minutes. It then discusses three key findings of the
Advisory Committee report, and finally discusses the difference between
absolute and relative intensity.
METs and MET-minutes
A well-known physiologic effect of physical activity is that it expends
energy. A metabolic equivalent, or MET, is a unit useful for describing the
energy expenditure of a specific activity. A MET is the ratio of the rate of
energy expended during an activity to the rate of energy expended at rest. For
example, 1 MET is the rate of energy expenditure while at rest. A 4 MET
activity expends 4 times the energy used by the body at rest. If a person does
a 4 MET activity for 30 minutes, he or she has done 4 x 30 = 120 MET-minutes
(or 2.0 MET-hours) of physical activity. A person could also achieve 120
MET-minutes by doing an 8 MET activity for 15 minutes.
MET-Minutes and Health Benefits
A key finding of the Advisory Committee Report is that the health
benefits of physical activity depend mainly on total weekly energy expenditure
due to physical activity. In scientific terms, this range is 500 to 1,000
MET-minutes per week. A range is necessary because the amount of physical
activity necessary to produce health benefits cannot yet be identified with a
high degree of precision; this amount varies somewhat by the health benefit.
For example, activity of 500 MET-minutes a week results in a substantial
reduction in the risk of premature death, but activity of more than 500
MET-minutes a week is necessary to achieve a substantial reduction in the risk
of breast cancer.
Dose Response
The Advisory Committee concluded that a dose-response relationship
exists between physical activity and health benefits. A range of 500 to 1,000
MET-minutes of activity per week provides substantial benefit, and amounts of
activity above this range have even more benefit. Amounts of activity below
this range also have some benefit. The dose-response relationship continues
even within the range of 500 to 1,000 MET-minutes, in that the health benefits
of 1,000 MET-minutes per week are greater than those of 500 MET-minutes per
week.
Two Methods of Assessing Aerobic Intensity
The intensity of aerobic physical activity can be defined in absolute or
relative terms.
Absolute Intensity
The Advisory Committee concluded that absolute moderate-intensity or
vigorous-intensity physical activity is necessary for substantial health
benefits, and it defined absolute aerobic intensity in terms of METs:
- Light-intensity activities are defined as 1.1 MET to 2.9 METs.
- Moderate-intensity activities are defined as 3.0 to 5.9 METs.
Walking at 3.0 miles per hour requires 3.3 METs of energy expenditure and is
therefore considered a moderate-intensity activity.
- Vigorous-intensity activities are defined as 6.0 METs or more.
Running at 10 minutes per mile (6.0 mph) is a 10 MET activity and is therefore
classified as vigorous intensity.
Relative Intensity
Intensity can also be defined relative to fitness, with the intensity
expressed in terms of a percent of a person’s (1) maximal heart rate, (2)
heart rate reserve, or (3) aerobic capacity reserve. The Advisory Committee
regarded relative moderate intensity as 40 to 59 percent of aerobic capacity
reserve (where 0 percent of reserve is resting and 100 percent of reserve is
maximal effort). Relatively vigorous-intensity activity is 60 to 84 percent of
reserve.
To better communicate the concept of relative intensity (or relative
level of effort), the Guidelines adopted a simpler definition:
- Relatively moderate-intensity activity is a level of effort of 5 or
6 on a scale of 0 to 10, where 0 is the level of effort of sitting, and 10 is
maximal effort.
- Relatively vigorous-intensity activity is a 7 or 8 on this scale.
This simplification was endorsed by the American College of Sports Medicine and
the American Heart Association in their recent guidelines for older adults.1 This approach does create a minor difference
from the Advisory Committee Report definitions, however. A 5 or 6 on a 0 to 10
scale is essentially 45 percent to 64 percent of aerobic capacity reserve for
moderate intensity. Similarly, a 7 or 8 on a 0 to 10 scale means 65 percent to
84 percent of reserve is the range for relatively vigorous-intensity activity.
Developing Guidelines Based on Minutes of Moderate- and
Vigorous-Intensity Activity
Physical activity guidelines expressed using MET-minutes are not useful
for the general public. The concept of METs is difficult to understand and few
people are familiar with it. It is challenging for the public to know the MET
values for all the activities they do.
As long as people who follow the Guidelines generally achieve 500 to
1,000 MET-minutes per week (or more), it is appropriate to express the
Guidelines in simpler terms of minutes of moderate-intensity activity, and
minutes of vigorous-intensity activity. Because not all the benefits of
physical activity occur at 500 MET-minutes per week, Guidelines that help
people exceed this minimum are desirable.
Information in the Advisory Committee Report lays the basis for
expressing physical activity guidelines in minutes. The Advisory Committee
indicated that 150 minutes (2 hours and 30 minutes) of moderate-intensity
activity per week could be regarded as (roughly) equivalent to 500 MET-minutes
per week. In fact, 3.3 METs for 150 minutes per week is equal to 500
MET-minutes per week. By recommending that adults do at least 150 minutes of
moderate-intensity activity per week, adults will achieve 500 to 1,000
MET-minutes per week if the intensity is 3.3 METs or greater. As indicated by
the Advisory Committee Report, people who do 150 minutes of a 3.0 to 3.2 MET
activity are acceptably close to achieving 500 MET minutes. As noted earlier,
walking at 3.0 miles per hour is a 3.3 MET activity. Hence, it is appropriate
to communicate to the public that a “brisk walk” is walking at 3.0
miles per hour or faster.
By recommending at least 75 minutes (1 hour and 15 minutes) per week of
vigorous-intensity activity, adults who choose to do vigorous-intensity
activity will also generally achieve 500 to 1,000 MET-minutes per week. The
lower limit of vigorous–intensity activity (6.0 METs) is twice the lower
limit of moderate-intensity activity (3.0 METs). So, 75 minutes of
vigorous-intensity activity per week is roughly equivalent to 150 minutes of
moderate-intensity activity per week. As the MET range for vigorous-intensity
activity has no upper limit, highly fit people can even exceed 1,000
MET-minutes in 75 minutes by doing activities requiring 13.4 MET or more. It is
not of concern that the vigorous-intensity Guideline "misleads"
people with a high degree of fitness into doing more activity than is really
required to meet the Guidelines. Highly fit people have already decided to do
large amounts of physical activity, as this is the only way to achieve this
degree of fitness.
Finally, the Guidelines needed to address the issue that some people do
both moderate-intensity and vigorous-intensity activity in a week. To determine
whether they are doing enough activity to meet the Guidelines, these people
need a "rule of thumb" as to how vigorous-intensity minutes
substitute for moderate-intensity ones. Because 150 minutes of
moderate-intensity activity and 75 minutes of vigorous-intensity activity are
the minimum amounts, the rule of thumb becomes that 1 minute of
vigorous-intensity activity counts the same as 2 minutes of moderate-intensity
activity.
Using Relative Intensity To Meet Guidelines Expressed in Terms of
Absolute Intensity
The intent of the aerobic Guidelines for adults is to ensure that people
who follow them generally achieve 500 to 1,000 MET-minutes or more. For this to
occur, the definition of intensity in the Guidelines needs to be in terms of
METs (i.e., absolute intensity). However, the Guidelines for Adults indicate
that relative intensity can also be used as a means of assessing the intensity
of aerobic activities. And the Guidelines for Older Adults require the
use of relative intensity. How can this be appropriate?
For many adults it does not matter a great deal whether they use
relative or absolute intensity. That is, following the Guidelines means they
attain 500 to 1,000 MET-minutes per week using either absolute or relative
intensity to guide level of effort. Their level of fitness is such that, when
they do absolute moderate-intensity activities in the range of 3.0 to 5.9 METs,
they generally are also doing relatively moderate-intensity activity.
Similarly, absolutely vigorous and relatively vigorous activities overlap a
great deal.
For adults with higher levels of fitness, using relative intensity
means they will do higher amounts of activity than intended by the Guidelines.
For example, a 3.5 MET activity can be relatively light for these adults, and
perhaps 6.0 MET activities are relatively moderate. By doing 150 minutes of a
6.0 MET activity, they exceed the amount of activity intended in the Guideline.
But this is acceptable for two reasons: First, the Guidelines encourage people
to do higher amounts of activity, as higher amounts have greater health
benefits. Second, people with higher levels of fitness generally can only
achieve this level of fitness by doing higher amounts of activity, and thus
have already chosen to do more activity.
Some adults have low levels of fitness, particularly older adults. For
these adults, activities in the range of 3.0 to 5.9 METs are either relatively
vigorous, or physiologically impossible. The Advisory Committee Report stated
that for older adults, who commonly have low levels of fitness, the level of
effort should be guided by relative intensity (as opposed to absolute). The
report also stated that inactive adults should not do relatively
vigorous-intensity activity when they start to increase their activity level.
In other words, it is not intended or appropriate for people with low levels of
fitness to meet a moderate-intensity guideline by routinely doing relatively
vigorous-intensity activity.
Allowing the Use of Either Relative Intensity or Absolute Intensity in
Children
The Guidelines for Children and Adolescents do not require carefully
tracking of the intensity of the activity. The mix of moderate- and
vigorous-intensity activity is flexible, as long as some vigorous-intensity
activity is done at least 3 days per week. This flexibility means that relative
and absolute intensity are both appropriate ways to track intensity.
Relative intensity is appropriate for several reasons. The exercise
studies on which the Guidelines are based commonly prescribed aerobic activity
using relative intensity. Children and adolescents who follow the Guidelines
should have improvement in cardiorespiratory fitness, and the relative
intensity of the activity is a major determinant of its fitness effects. The
intent of the Advisory Committee Report is that, when a child breathes rapidly
during physical activity (an indicator of relatively vigorous-intensity
activity for that child), this activity should count as vigorous intensity.
However, it is not always feasible to observe children closely enough to
determine their level of effort. In this case, absolute intensity can be used
to judge whether the child is doing activity that counts toward the Guidelines.
Brisk walking (as opposed to slow walking) counts as moderate-intensity
activity, and running counts as vigorous-intensity activity, based on the
typical level of effort required for these activities.
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1Nelson, M. E., Rejeski, W. J.,
Blair, S. N., Duncan, P. W., Judge, J. O., King, A. C., et al. (2007, August).
Physical activity and public health in older adults: Recommendation from the
American College of Sports Medicine and the American Heart Association.
Medicine & Science in Sports & Exercise 8, 1435–1445.
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