This is an excerpt from Promoting Physical Activity-2nd Edition by Centers for Disease Control.
Current Physical Activity Guidelines for Adults
The four key guidelines for adults are shown in the sidebar on page 25. The first guideline is to avoid being inactive; some health benefits are gained with even small amounts of physical activity. Aerobic and muscle-strengthening activities, however, comprise the key types of activities specified in the guidelines.
Aerobic Physical Activity Guidelines
The guidelines recommend that to achieve substantial health benefits, a person should undertake 150 minutes per week of moderate-intensity physical activity, 75 minutes per week of vigorous-intensity physical activity, or an equivalent combination of both. To achieve more extensive health benefits, a person should perform 300 or more minutes per week of moderate-intensity activity, 150 minutes per week of vigorous-intensity activity, or an equivalent combination of both. Although both moderate- and vigorous-intensity activities count toward meeting the aerobic guidelines, time spent in vigorous-intensity activity counts roughly twice that spent in moderate-intensity activity; for example, engaging in 150 minutes of vigorous-intensity activity is equivalent to engaging in 300 minutes of moderate-intensity activity. Healthy adults who find it difficult to meet the guidelines because of time constraints may wish to substitute vigorous-intensity aerobic activity for some moderate-intensity aerobic activity.
Physical Activity Prescription
Some key concepts of the physical activity prescription (frequency, intensity, duration) are important in implementing the guidelines. First, the prescription for aerobic physical activity in the guidelines is based on a total weekly volume of moderate- or vigorous-intensity physical activity. Physical activity volume is the product of frequency (episodes per week—often expressed as days per week), intensity (level of effort—often expressed as an individual's perception of effort as being light, moderate, or vigorous intensity or as a multiple of resting energy expenditure, known as a MET), and duration (time per episode). After reviewing the scientific evidence, a federal advisory committee determined the total volume of aerobic activity to be most related to health—more so than any one component of the physical activity prescription. Next, in the older adult guidelines, intensity is defined in two ways: absolute intensity and relative intensity. Absolute intensity is based on the rate of energy expenditure during the activity, without taking into account a person's cardiorespiratory fitness. Absolute intensity is commonly based on the type of activity a person is doing. For example, jogging is generally considered a vigorous-intensity activity, whereas brisk walking is typically considered to be moderate-intensity activity. Relative intensity uses a person's level of cardiorespiratory fitness to assess level of effort. One way to gauge relative intensity is to use a scale of 0 to 10 where sitting is 0 and the highest level of effort possible is 10. On this scale, the level of effort for performing moderate-intensity activity is a 5 or 6 and for vigorous-intensity activity is a 7 or 8. Another way to gauge relative intensity is by using the “talk test.” A person engaging in moderate-intensity aerobic activity should be able to talk, but not sing, during the activity. A person undertaking vigorous-intensity activity should not be able to say more than a few words without pausing for a breath. In terms of relative intensity, brisk walking may be a vigorous-intensity activity for an unfit person or may be a moderate-intensity (or lower intensity) activity for a fit person.
Muscle Strengthening Guidelines
Muscle-strengthening activities enhance skeletal muscle mass, strength, power, and neuromuscular activation (PAGAC, 2008). For these reasons, muscle strengthening is an important physical activity guideline for adults (sidebar). Adults should strengthen all seven muscle groups (legs, hips, back, chest, abdomen, shoulders, and arms) on at least 2 days of the week. The guidelines do not specify that muscle strengthening be undertaken on nonconsecutive days. Participants should use the overload principle to strengthen muscles, that is, make the muscles do more work than they are accustomed to doing. To overload the muscle, a person must lift more weight than she is accustomed to and continue lifting until unable to perform another repetition without help. The intensity of muscle strengthening refers to how much weight or force is used relative to how much a person is able to lift.
Scientific evidence shows that muscle strengthening can occur with one set (or series) of 8 to 12 repetitions (number of times a person lifts a weight) per muscle group. One set is sufficient to increase muscular strength, although performing two or three sets may be more effective. There are many ways to strengthen muscles: working with resistance bands, using weight machines, or performing exercises that use one's body weight (such as push-ups, pull-ups, and sit-ups). Increases in the amount of weight lifted or the number of days of training per week will result in stronger muscles.
What's New About the 2008 Physical Activity Guidelines for Americans?
Compared with previous recommendations and guidelines developed over the years (described earlier), the 2008 guidelines offer people flexibility in ways they can meet their activity requirements. For example, the adult guidelines do not specify a minimum number of days per week for aerobic activity, although it is recommended that adults participate in activity that is spread throughout the week. Why is this? Because there is insufficient scientific evidence to recommend a minimum frequency (days per week) of activity associated with health benefits, but, in terms of behavior, it is important to encourage people to develop a habitual pattern of physical activity participation. The guidelines strongly endorse muscle strengthening but not some features typical of training programs; for example, there is no requirement for strength training on nonconsecutive days. There is also a clear statement in the guidelines that healthy children and adults do not need physician approval or consultation prior to engaging in physical activity.
More Is Better
The guidelines for aerobic physical activity for adults emphasize that there is not a minimal amount of physical activity for which all health benefits will accrue. Doing some physical activity is better than none. Meeting the minimal aerobic guideline goal of 150 minutes per week of moderate-intensity physical activity (or 75 minutes of vigorous-intensity activity, or the combined equivalent of moderate- and vigorous-intensity activity), however, will provide substantial health benefits like a lower risk of premature death, coronary heart disease, stroke, hypertension, type 2 diabetes, and depression. Achieving 300 minutes per week of moderate-intensity physical activity (or 150 minutes of vigorous-intensity activity, or the combined equivalent of moderate- and vigorous-intensity activity) will provide additional and more extensive health benefits, such as lowering one's risk of colon and breast cancer and preventing unhealthy weight gain.
Overweight and Obesity
Physical activity plays a role (along with intake of food and beverages) in energy balance and is important in maintaining a healthy body weight, losing excess body weight, or maintaining weight loss. There is variability, though, in the amount of physical activity a person needs to achieve and maintain a healthy weight. Some people need more physical activity than others to maintain a healthy body weight, to lose weight, or to keep weight off once it has been lost.
Strong scientific evidence shows that physical activity will help people maintain a stable weight over time; however, the amount of physical activity needed to maintain an optimal weight is unclear. To maintain weight for the long term, many people need more physical activity than the minimal amount recommended to achieve substantial health benefits—that is, the equivalent of 150 minutes per week of moderate-intensity physical activity. Scientific evidence shows over short periods of time (up to 1 year), performing the equivalent of 150 to 300 minutes per week of moderate-intensity physical activity may help a person maintain his weight.
To lose a substantial amount of weight (5 percent or more of body weight) or to keep a significant amount of weight off once it has been lost, many people need to perform more than the equivalent amount of 300 minutes per week of moderate intensity aerobic activity to meet weight loss or weight control goals. Most of these people will need to reduce caloric intake as well as increase physical activity.
Scientific evidence shows neither a harmful nor a beneficial effect of engaging in flexibility activities. For this reason, undertaking flexibility activities and warm-up or cool-down activities is reasonable and acceptable but is not a specific guideline.
Physical activity must be of at least a moderate intensity to achieve health benefits. Time spent in light-intensity activities (such as light housework) and sedentary behaviors (such as watching TV) do not count toward meeting the aerobic physical activity guidelines. In addition, moderate-intensity activities must be done for at least 10 or more minutes at one time. For example, climbing flights of stairs, although usually of vigorous intensity, is typically done for less than 10 minutes at one time and therefore does not count toward meeting the aerobic guidelines. Efforts have been made to increase stair walking as part of a community-based intervention, as discussed in the Point-of-Decision Prompts subsection of chapter 3 (pp. 57-62), and similar efforts and other types of longer-duration activities are needed to help people achieve recommended amounts of physical activity.