The major change in the activity recommendation in 2018 was to no longer require bouts of aerobic activity be of at least 10 minutes in duration — activity bouts of any duration now contribute toward the weekly goal. Although there still is limited published research testing for health benefits of exercise accumulated throughout the day in bouts lasting less than 5 minutes each, data are beginning to emerge that show the potential efficacy (see Figure 1) (5,6).
To include a specific recommendation in the DHHS guidelines, the combined evidence from studies needs to be considered strong. For this reason, some currently popular activities, such as high-intensity interval training, were not included in the primary recommendations because of the lack of sufficient data. Also, the increased health risks caused by too much sitting were addressed with general recommendations to decrease sitting time using almost any type of activity. However, data were inadequate to support specific targets for daily sitting time. Regardless, sitting less is associated with better health, especially in low-fit and high-risk persons.
The 2018 Physical Activity Guidelines report noted important benefits of physical activity that included strong scientific evidence for reduced risk of dementia; improved cognitive function; reduced cancer of the bladder, endometrium, esophagus, kidney, lung, and stomach; health benefits for children ages 3 to 5 years; and reduced risk for adults developing a new chronic condition or risk of progression of a condition they already have (secondary prevention).
BRIEF HISTORY OF THE EVOLUTION OF PHYSICAL ACTIVITY AND HEALTH GUIDELINES
In 1953, the first studies were published that hypothesized moderate/vigorous-intensity physical activity performed on multiple days of the week provided some protection against coronary heart disease clinical events (7). Morris and colleagues reported that more active conductors on double-deck buses in London who walked up and down stairs during much of each shift had lower rates of coronary heart disease events compared with drivers of the same buses. Also, they reported that London postmen who walked routes carrying their mail bags had significantly fewer coronary heart disease events compared with switchboard operators who sat at work all day. Over the next two decades, other investigators who had access to data collected on large employee populations generally reported similar outcomes. Also, several early studies that included brief physical activity questionnaires found that more active men had lower rates of heart disease than their less active counterparts during longitudinal follow-up (8). During this time, several exercise physiology laboratories began reporting the results of exercise training studies on health-related biological markers such as plasma cholesterol and triglycerides, blood glucose, blood pressure (BP), adiposity, and cardiorespiratory fitness. The results of these and an increasing number of similar studies became the early scientific basis for physical activity and health recommendations or guidelines starting in the early 1970s by the American Heart Association (AHA) and the American College of Sports Medicine (ACSM). Between 1972 and 1996, ACSM, AHA, various units within the DHHS, and other organizations published a large number of recommendation or guideline documents for exercise/health professionals or the general public. A list of these recommendations between 1965 and 1996 is available in Physical Activity and Health, A Report to the Surgeon General (9).
Most of the data supporting physical activity recommendations have been derived from two study designs: (a) prospective observational studies with all-cause and cause-specific mortality being the most common outcome and (b) exercise intervention (training) studies where changes in disease biomarkers are key outcomes. It is the combination of different types of results from the observational and interventional studies that lend strength to many of the current physical activity guidelines. Increases in activity produce improvement of some biomarkers in the causal pathways of various chronic diseases supporting the concept of “causality,” whereas observational studies provide clinical outcomes such as mortality generally not obtainable in intervention studies because of the need for a large sample size, a long-term follow-up, and a large number of outcomes.
The earliest guidelines had a major focus on exercise programs to improve cardiovascular and muscular fitness and help decrease the risk of exercise-induced cardiovascular or musculoskeletal injury in adults. For example, in the first AHA recommendations published in 1972 (10), the following precautionary statement was made: “Exercise is a therapeutic agent designed to promote a beneficial clinical effect and, as such, has specific indications and contraindications and possible toxic or adverse reactions” (page 24). Public health guidelines since the 1990s have focused more or the extensive health and performance benefits provided by being appropriately active than on the risks associated with recommended exercise. Especially important were the guidelines by the CDC and ACSM published in 1995 because of their strong public health orientation (11). Also, the “>10-minute bout requirement” was introduced in these guidelines.
The ACSM guidelines provided in their position stands experienced relatively minor changes in the 33 years between 1978 and 2011. In 1978, the key recommendations focused on endurance activity of 15 to 60 minutes per session, 3 to 5 sessions per week of moderate/vigorous intensity (60% to 90% of HRmax reserve or 50% to 80% •VO2max). Activities that help maintain muscle and bone strength, flexibility, and balance were recommended (12). In 2011, key recommendations from ACSM stated that most adults should engage in moderate-intensity cardiorespiratory exercise training for ≥30 minutes per day on ≥5 days per week for a total of ≥150 minutes per week or vigorous-intensity cardiorespiratory exercise training for ≥20 minutes per day on ≥3 days per week (≥75 minutes per week), or a combination of moderate- and vigorous-intensity exercise to achieve a total EE of ≥500–1000 MET-minutes per week. On 2 to 3 days per week, adults were advised to perform resistance exercises for each of the major muscle groups, neuromotor exercise involving balance and agility, and various flexibility and coordination activities (12).
Sidebar 1: METS
METS are used as an indicator of energy expenditure. One MET is the rate of energy expended by a person sitting quietly, and it is considered to be 3.5 ml of oxygen per kilogram of body weight. Four METS means a person is exercising at an intensity that requires four times the energy they expend while sitting at rest. If a person exercises for 30 minutes at 4 METS, they have expended 120 MET-minutes (30 × 4) or 2 MET-hours. If this exercise is performed for 5 days, the total weekly expenditure will be 10 MET-hours or 600 MET-minutes.
The 1990 ACSM Position Stand noted that in the 1978 Position Stand, an important distinction was made between physical activity as it relates to health versus fitness. It indicated that the quantity and quality of exercise needed by physically inactive individuals to attain some health-related benefits may differ from activities recommended for fitness benefits. It is now clear that lower levels of physical activity than those recommended by the ACSM Position Stands may reduce the risk of certain chronic degenerative diseases in low-fit and inactive persons and yet may not be of sufficient quantity or quality (primarily intensity) to significantly improve •VO2max. Data from large prospective observational studies or meta-analysis of such studies have shown that lower mortality rates are associated with small differences in reported moderate- and vigorous-intensity activity, especially in the least active participants — the least active benefit the most from an increase in activity (Figure 2).
As physical activity and health research moves forward, it needs to focus on a number of unanswered questions that will inform the development of new guidelines, including the effect of light-intensity exercise on health risks or benefits, the best pattern of activity over the 24-hour day (how best to balance time spent during sleep, sedentary behavior, and activities of different types, intensities, durations, and frequency), and the efficacy and safety of short-duration HIIT in those with various chronic conditions. This research should greatly benefit from the continued development and innovative application of wearable measurement devices and related software.
1. 2018 Department of Health and Human Services Physical Activity Guidelines for Americans. [cited 2019 April 23]. Available from: https://health.gov/paguidelines/
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6. Diaz KM, Duran AT, Colabianchi N, Judd SE, Howard VJ, Hooker SP. Potential effects of replacing sedentary time with short bouts of physical activity on mortality: a national cohort study. Am J Epidemiology
7. Morris JN, Heady JA, Raffle PA, Roberts CG, Parks JW. Coronary heart-disease and physical activity of work. Lancet
8. Fox SM, Haskell WL. Population studies in an international symposium on physical activity and cardiovascular health. Can Med Assoc J
9. U.S. Department of Health and Human Services. Physical Activity and Health: A Report to the Surgeon General
. Atlanta, GA: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion; 1996.
10. American Heart Association Committee on Exercise. Exercise Testing and Training of Apparently Healthy Individuals: A Handbook for Physicians
. Dallas (TX) and New York (NY): American Heart Association; 1972.
11. Pate RR, Pratt M, Blair SN, et al. Physical activity and public health. A recommendation from the Centers for Disease Control and Prevention and the American College of Sports Medicine. JAMA
© 2019 American College of Sports Medicine.
12. Garber CE, Blissmer B, Deschenes MR, et al. American College of Sports Medicine Position Stand: quantity and quality of exercise for developing and maintaining cardiorespiratory, musculoskeletal, and neuromotor fitness in apparently healthy adults: Guidance for prescribing exercise. Med Sci Sports Exerc