Physical Activity Guidelines
In 1975, the American Heart Association began to recognize the need for more Americans to become physically active. The American Heart Association recommended 3 to 4 days/week of aerobic activity at 70% to 85% of the HRmax for 20 to 30 minutes. With those guidelines established, the American College of Sports Medicine (ACSM) presented their first recommendation in 1978 with a similar prescription that included 3 to 5 days a week at 50% to 80% VO2max (60% to 90% HRmax), for a duration of 20 to 60 minutes. Over the next 30 years, the activity guidelines evolved based on the available epidemiological evidence. In 1995, the ACSM lowered the intensity recommendation to 40% to 85% of the VO2max, suggesting that Americans could exercise at a lower intensity and receive a similar health benefit. In 1996, the Surgeon General’s Report on Physical Activity attempted to explain what type of exercise represented moderate intensity. For instance, the report stated that the average person who engaged in brisk walking for 30 minutes or dancing for 30 minutes would use approximately 150 kcal/session, with the added suggestion of being active most days of the week, suggesting that everyone expend 1000 kcal/week. Based on brisk walking, that is roughly 210 minutes/week of moderate physical activity. Current ACSM guidelines from 2011 state that one should perform at least 150 minutes/week of moderate physical activity, performing 30 to 60 minutes 5 days/week, or 20 to 60 minutes of vigorous exercise 3 days/ week (see Table 2-1). So, how much physical activity is truly needed to gain the health benefits? A historical view of the guidelines shows an evolution on the best available evidence. Although the epidemiological evidence is clear that physical activity has a dose-response relationship with health, it is less clear as to what specific intensity and volume is best. As researchers continue to gather information, guidelines are being written more clearly and practically than ever before.
The next section reviews some key studies used in developing the current physical activity guidelines. Each new study shaped the messages that physicians, public health professionals, educators, coaches, and athletic trainers provided on the health benefits of exercise. Two consistent themes can be seen regardless of the specific disease studied. The first is that more physical activity is associated with less risk of disease. The second is that many forms and intensities of physical activity can provide health benefits. However, the physical activity guidelines of 150 minutes/week of moderate physical activity provides each individual with a starting framework to participate in enough physical activity to confidently assume an improvement in overall health.
Exercise and All-Cause Mortality
Physical activity has a clear inverse association with all-cause mortality. In 2010, heart and cerebrovascular disease, cancer, and diabetes mellitus represented more than 50% of all deaths in the United States. Each disease is a related to low levels of physical activity. Lee et al33 reported that there would a 9.9% reduction in all-cause mortality in the United States if physical activity levels increased as little as 25%. The following section begins with the relationship of physical activity with all-cause mortality and then reviews individual disease’s links to physical activity. Physical activity benefits do not just impact the incidence of disease but, rather, life as a whole.
Research on physical activity and all-cause mortality has a long history. Epidemiologists used longitudinal cohort studies with large numbers of participants and followed them over the courses of their lives to establish links between the amount of physical activity participants reported and their life expectancies. Starting in 1976, the Nurse’s Health study enrolled female registered nurses aged 30 to 55 years. Participants completed a baseline questionnaire regarding their medical histories and physical activity and were sent follow-up questionnaires every 2 years. Between 1982 and 1996, 4746 of the 80,348 women had died. Physical activity was inversely related to allcause mortality. The most active women had the lowest risk. However, moderate physical activity showed approximately the same risk reduction as more vigorous activity, and the sharpest decrease in mortality was for those who participated in 1 to 1.9 hours/week of physical activity compared to those who had less than 1 hour/week of physical activity. The relationship was strongest with respiratory disease and CVD.57
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One of the most important research studies on physical activity and risk of disease was the Harvard Alumni Health Study published in 1986. Paffenbarger et al58 followed roughly 17,000 men for 12 to 16 years until 1978, during which time 1413 men died. They assessed physical activity using questionnaires, and the self-reported data were converted by the researchers into kcal expended per week. The age-adjusted death rates showed a steady decrease as energy expenditure increased. Those who expended the least energy ( 2000 kcal/week had the lowest death rate (54/10,000 person-years; P 3 days/week) had the fewest coronary risk factors; however, those who engaged in physical activity once a week had fewer risk factors compared to the sedentary group.60
A 2008 report presented by an expert panel appointed by the federal government concluded that the evidence from 73 studies showed a significant inverse relationship between physical activity and all-cause mortality in 67 of those studies.38 The median risk reduction was 31% for those who were the most active compared to those who were the least active, and the inverse association was shown for all ages. The panel also suggested the Physical Activity Guidelines for Americans based on its research.38 One year later, a meta-analysis of 38 studies reported findings of a 22% risk reduction in men who were highly active compared to low-active men and a 31% risk reduction in highly active women.61
The Aerobics Center Longitudinal Study is an ongoing cohort study. In 2011, Lee et al62 reported 31,818 men and 10,555 women who received a medical examination between 1978 and 2002. The subjects were followed until December 2003. During that time, 1492 men and 230 women died. Again, physical activity had a significant inverse association with mortality. More importantly, the researchers also presented data that showed a stronger relationship between cardiorespiratory fitness and mortality. Although physical activity was proven to be helpful, this research suggests that improving an unfit person’s cardiorespiratory fitness would have the greatest impact on the risk of all-cause mortality. Participants who did not reach 500 metabolic equivalent of task (MET) minutes/week (approximately 150 minutes/week) but were cardiovascularly fit and those who met the MET cut-off and were fit had a significantly lower risk those who did not meet the guidelines and were unfit. A MET minute represents the amount of time in an activity multiplied by the MET intensity of that activity. For example, a moderate-intensity activity has a MET range between 3 and 5.9 METs, and performing it for 30 minutes would yield between 90 and 177 MET minutes of activity. Also, 150 minutes/week of a 3.3-MET activity would equal 500 MET minutes. In the study above, the 2 fit groups showed no difference in their mortality risk. Meeting the physical activity guideline was not as important as being cardiovascularly fit.62
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