June 28 - 29, 2007 Advisory Committee Meeting
Historical Overview of Physical Activity Recommendations
Dr. Haskell asked Bill Kohl, senior epidemiologist, Centers for Disease Control
and Prevention, to provide a historical overview of physical activity
recommendations. He also noted that Dr. Kohl will be leading a CDC group
organizing the literature review for the Committee.
Dr. Kohl opened his remarks acknowledging the importance of "knowing where
you've been to know where you are going" and that his presentation would
summarize the previous 50-years.
Starting in the 1950's a rapidly evolving science base emerged around the
hypothesis of physical activity and benefits to health. Additionally, increased
literature in exercise science surfaced. In the late 1960's to 1970's many
individuals started promoting various health-oriented fitness programs; however,
some confusion began to spring up over what types of physical activity was
really the best? At the same time the medical community expressed concern over
the safety of exercise. While a lot of work and thought happened during this 20
year period there was not a lot of direction or focus.
In 1975 the first form of a recommendation focused on a particular health
outcome was produced by the American Heart Association and included the
"Exercise Testing and Training of Healthy Adults" and "Exercise Testing and
Training of Individuals with Heart Disease or at a High Risk for its
Development" documents. At the same time the American College of Sports Medicine
produced its "Graded Exercise Testing and Exercise Prescription" documents.
These documents represented a growing movement towards very clinically focused
recommendations based on specific health outcomes.
As these recommendations were published and updated they were consistently
rooted in an "athletic" paradigm and stressed vigorous exercise. Recommendations
almost always focused on type of activity, intensity, duration and frequency.
Vigorous activity meant activity involving 60% - 90% of maximum oxygen intake –
VO2 max. All of this revolved around the athletic approach to physical activity
where increases in aerobic capacity are most rapidly achieved by increasing the
intensity of the endurance exercise. In 1990 the athletic approach of
vigorous activity was challenged by a more inclusive population-based approach.
This approach gave credence to the benefits of moderate-intensity activity to
health. Acknowledging this approach the American College of Sports Medicine
produced the following position statement:
"Since 1978 an important distinction has been made between physical activity as
it relates to health versus fitness. It has been pointed out that the quantity
and quality of exercise needed to attain health-related benefits may differ from
that recommended for fitness benefits. It is now clear that lower levels of
physical activity than recommended by this position statement may reduce the
risk of certain chronic degenerative diseases and yet may not be of sufficient
quantity or quality to improve VO2 max."
Through the 1990's more studies emerged about accumulated moderate-intensity
activity. At this time data recognized only a small portion of the population
met the vigorous standard of physical activity and that a portion of the
population was sedentary or totally inactive, leading to the realization of a
possible public health gain for moving people out of the inactive state. In
1995, CDC in conjunction with ACSM released a recommendation that every adult
American should accumulate at least 30 minutes of moderate-to-vigorous intensity
physical activity and, most preferably, all days each week.
As recommendations emerged, problems arose regarding the recommended dose of
activity to different population groups. There was also difficulty in
classifying different population groups, for example, what is an adult, older
adult, youth, infant, etc. Along these lines, is moderate activity for a youth
the same for an older adult? Many different recommendations were produced but
many of them did not say the same thing so how can these recommendations be
harmonized? It became clear one size does not fit all.
Finally, Dr. Kohl summarized the appearance of physical activity guideline
recommendations in the U.S. Dietary Guidelines for Americans. Starting in 1990
one sentence acknowledged the role of physical activity for energy balance.
Later editions included a dose recommendation until finally the current edition
does break out specific doses for some different age groups. Most importantly,
the Dietary Guidelines acknowledged the need for different recommendations for
different population groups.