Evolution of physical activity guidelines reflects changing body of evidence
This is an excerpt from Physical Activity and Health-2nd Edition by Claude Bouchard,Steven N. Blair & William Haskell.
Evolution of Physical Activity Guidelines
Scientific knowledge about physical activity and health is of little value if people cannot understand it and apply it to their lives. For the past three decades, there has been a gradual but steady development in the effort to present information on physical activity and health to the general public. This has come through public health messages known as physical activity guidelines.
Health scientists and practitioners have long believed that regular physical activity is essential to maintain good health. So it is not surprising that for a very long time, individual health professionals and health organizations have been making recommendations regarding the types and amounts of physical activity needed for health and fitness. As emphasized in the preceding section, scientific support for the impact of physical activity on health has developed rapidly in recent decades. As the relevant knowledge base has grown, physical activity recommendations for the public have been modified to maintain consistency with the existing research evidence. In this section, we track the evolution of physical activity guidelines as they have been presented to the public since the 1950s.
In 1957, Finnish researcher Marti Karvonen and his colleagues published the findings of a study that has become a classic in exercise science. Karvonen observed the effects of exercise training via treadmill running on endurance fitness in a small number of male medical students. He reported that training intensity corresponding to a heart rate of at least 60% of the heart rate range (maximum heart rate minus resting heart rate) was required to produce significant gains in cardiorespiratory fitness. Although Karvonen's study was very small and quite limited in its research design, his findings became the platform for exercise guidelines for the ensuing three decades. Karvonen's program was presented in terms of minimums for frequency, duration, and intensity of training. A half century later, it seems remarkable that such a small and limited study could have had such a powerful influence on health practice.
“Heart rate during training has to be more than 60% of the available range from rest to the maximum attainable by running . . . in order to produce a change in the WR [working heart rate]. . . . A decrease of the WR is understood to indicate an increase of the maximum oxygen uptake.”
Karvonen, Kentala, and Mustala 1957, p. 314
Initial Attempts to Speak to the Public
During the 1960s, two American men, one a track coach and the other a physician, published books that brought practical physical activity guidelines to the masses. In 1963, Bill Bowerman, coach of the University of Oregon's track team, visited a coaching colleague in New Zealand, where he witnessed many middle-aged adults running for health and fitness. He was so impressed by what he had seen that on his return to the United States, he wrote Jogging, a small paperback volume that has often been credited with launching a fitness revolution (Bowerman and Harris 1967). Bowerman described a slow running program that emphasized gradual, progressive increases in distance and frequency of exercise. His basic recommendation was that almost everyone can benefit from “an exercise program of relaxed walking and running” and that jogging is something that almost everyone can do.
In 1968, only a year after Bowerman popularized jogging as a specific form of exercise, Dr. Kenneth Cooper, then an Air Force physician, published Aerobics, a book in which he laid out a simple point system for determining how much exercise should be accumulated on a weekly basis. With his Aerobics Point System he recommended that adults accumulate a minimum of 30 points per week. Cooper recommended that sedentary adults begin an exercise program by starting at a level compatible with their current fitness (perhaps earning as few as 10 points per week for the first few weeks, for those at the lowest levels of fitness), choose an activity they enjoy, and exercise with others when at all possible. Table 2.1 provides examples of point values assigned to exercises by Cooper. Although neither Bowerman nor Cooper was able to base these recommendations on extensive bodies of directly relevant scientific evidence, both were talented practitioners and gifted communicators who were able to draw on their extensive experience in educating the public about how much physical activity is needed for health and fitness.
Exercise Prescription for the Public
Concurrent with the popularization of the exercise-for-fitness movement and the so-called running boom of the late 1960s and 1970s, exercise scientists began to systematically explore the effects of various types, intensities, durations, and frequencies of endurance exercise on cardiorespiratory fitness. A leader of this extensive scientific effort was Dr. Michael Pollock. During the 1970s, Pollock and his colleagues undertook a series of experimental exercise training studies that, when considered collectively along with the work of other researchers, produced the knowledge needed to recommend exercise in a precise, detailed, and individualized manner. This method became the central dogma in efforts to communicate to the public the types and amounts of exercise needed to promote health and fitness. The American College of Sports Medicine (ACSM) first formally endorsed this detailed approach to recommending exercise in its exercise guidelines book in 1975 (ACSM 1975) and in a position statement issued in 1978 (ACSM 1978). The key recommendations presented in the ACSM Position Statement are summarized in table 2.2.
During the same period in which exercise scientists were systematically studying endurance exercise training and its impact on cardiorespiratory fitness in healthy adults, cardiologists and clinical exercise physiologists were studying the effects of exercise training in patients with cardiovascular disease. This research demonstrated the critical and now well-accepted role that exercise can play in rehabilitation of patients with compromised cardiovascular function. But furthermore, this research and the clinical guidelines that it spawned established a medical approach to recommending exercise that came to be referred to as “exercise prescription.” This technique drew on the research on normal healthy adults as well as research performed on heart patients. In 1975, the AHA published guidelines on exercise prescription for patients with cardiovascular disease. This document helped to establish a place for exercise in the practice of medicine and was influential in communicating to the public the significant health benefits that accrue to physically active persons, even those with already established cardiovascular disease. Table 2.3 summarizes the AHA's first guidelines for physical activity in people with heart disease or at risk for heart disease (AHA 1975).
Importance of Moderate-Intensity Physical Activity
ACSM's Guidelines for Exercise Testing and Prescription has undergone revision approximately every five years since its initial publication in 1975 (ACSM 1975). Note that the first version of this was called Guidelines for Graded Exercise Testing and Prescription, but subsequent revisions dropped the word graded. Each volume included a primary recommendation on prescription of exercise that reflected the current body of knowledge regarding the types of exercise needed to provide health and fitness benefits to initially sedentary adults. Between the first edition published in 1975 and the eighth edition released in 2009, an interesting trend is evident. As shown by table 2.4, most elements in the exercise prescription guideline remained unchanged. The exception is the recommended range for exercise intensity, the lower end of which decreased from 60% .VO2max to 40%.VO2max. The earlier editions of this influential book indicated that rather vigorous exercise was needed to provide benefits, and this concept was widely communicated to the public during the 1970s and 1980s.
Recognition of the importance of moderate-intensity physical activity, as reflected by the changing exercise prescription guidelines of ACSM, evolved gradually during the 1980s and early 1990s as the result of a growing and changing body of research evidence. Two lines of research led to the conclusion that moderate-intensity physical activity (the equivalent of brisk walking) provided important benefits to health and fitness. First and most importantly, the science of physical activity epidemiology matured during the 1980s and produced a series of important investigations. These studies strongly suggested that regular performance of moderate-intensity physical activity provided important health benefits. Not only did these studies show that regularly active persons were less likely than sedentary persons to develop or die from cardiovascular disease, but they also demonstrated that much of the active population's physical activity came from walking and other forms of moderate-intensity physical activity. For example, results of the Third National Health and Nutrition Examination Survey (Crespo et al. 1996) showed that most of the physical activities preferred by American adults were moderate-intensity lifestyle activities, such as walking, gardening, and cycling (table 2.5).
As discussed previously, the exercise prescription method for recommending physical activity to the public was based primarily on the findings of a large number of experimental exercise training studies. The results of these studies had generally been interpreted as indicating that vigorous physical activity (6 METs or 60% or more individual functional capacity) was required to produce benefits. That moderate-intensity physical activity did not provide those benefits became the assumption. However, the epidemiological studies published in the 1980s and early 1990s forced a reexamination of the experimental studies. A closer look revealed that, in studies that compared moderate- and vigorous-intensity physical activity, the moderate level produced increased fitness, although often not to the same extent as the vigorous level. Also, it was seen that moderate-intensity physical activity often provided comparable or even greater beneficial effects on health outcomes such as blood pressure and high-density lipoprotein (HDL) cholesterol. For example, Duncan and colleagues (1991) found that both women who participated in a vigorous exercise program and those who participated in a moderate exercise program had significant improvements in their lipoprotein profiles (figure 2.3). Although women in the vigorous program had significantly greater gains in fitness, as measured by .VO2max, increases in HDL were similar in the two groups.
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