Guidelines vary in terms of the recommended duration and the number of days per week of physical activity. For example, the US Healthy People 2020 initiative and the USDA Dietary Guidelines for Americans (both based on guidelines issued in 2008 by the US DHHS) counsel approximately 60 min of moderate-to-vigorous intensity activity on most days of the week for adults (U.S. Department of Agriculture (USDA), 2010; U.S. Healthy People, 2011; U.S. Department of Health, Human Services (DHHS) (2008)), whereas the ACSM/AHA more specifically recommend doing moderately intense exercise for 30 min a day, 5 days a week, or vigorously intense exercise for 20 min a day, 3 days a week, and eight to 10 strength-training exercises, with eight to 12 repetitions of each exercise, twice a week (American College of Sports Medicine/American Heart Association, 2007).
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