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Physical Activity Guidelines

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

For More Information

See Chapter 1—Introducing the 2008 Physical Activity Guidelines for Americans, for details on the Advisory Committee and its report.

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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