Modern humans are frequently “sitters”–at work, in the car, and at home. Sitting may be the most common human behavior and for many adults exceeds the time spent sleeping! Furthermore, changes in transportation, communication, workplace, and entertainment technologies continue to reduce human activity level and energy requirements.
National physical activity and health recommendations from the American College of Sports Medicine and the American Heart Association include 30 minutes or more of moderate-intensity physical activity of most days of the week as the minimum requirements. The U.S. recommendations also state that the amount of aerobic activity should be in addition to routine activities of daily living, such as walking to the grocery store or taking out the trash. Although doing more of these activities would result in less time spent sitting, this is not addressed explicitly in the recommendations.
In people who spend about one-third of each 24-hour period sleeping, the 16 hours remaining allow for a large amount of time to be spent sedentary, or alternatively a large amount of time engaged in low-intensity physical activity (LIPA). On average, most people with sedentary occupations are probably spending approximately 9 to 11 hours per day sitting or reclining. In leisure time, sedentary time can include four hours or more, lying down to watch TV or reading in bed. People with desk jobs can sit for about 75% of the workday (~6-7 hours), and students can sit for more than 95% of each class period.
Thus, it is not difficult to understand why able-bodied peopled sometimes sit or lie down for all but 2 hours of a 24-hour period! Although the relatively small amount of recommended moderate to vigorous exercise is known to be beneficial, there are too many hours in the day for it to displace this vast amount of sedentary time.
To put the amount of energy the body expends during sitting versus more active activities, consider the following:
- The resting metabolic rate (defined as the amount of energy used when no activity is occuring) is about 1 MET (which is about 1 calorie per kg of body weight per hour)
- Moderate to vigorous physical activity (walking, bicycling, swimming, or running) requires 3-10 METs.
- Sitting or lying down to watch TV, read a book, type on a computer, or talk on the phone all require about 1.0-1.2 METs
Ongoing Research In Sedentary Lifestyles
An important tool in researching sedentary behavior is an accelerometer, a small eletronic device typically worn on the hip that measures the frequency and intensity of movement. Data from accelerometers may be analyzed to identify sedentary, low-intensity, and moderate- to vigorous-intensity activity time. Findings from the U.S. National Health and Nutrition Examination Survey (usually referred to by its acronym, NHANES), in which many thousands of people wore accelerometers for up to a week, suggest that the most common patterns in the adult population are those involving large amounts of sedentary time.
Studies examining sedentary time and its relationships with mortality, disease outcomes, and clinical biomarkers have consistently suggested that limiting the vast amount of daily time spent sitting or reclining could potentially provide healthful benefits, independent of exercise participation. Thus, it is critical to examine what types of activity people routinely perform when they are not sitting.
It is not unreasonable to expect that most able-bodied people are capable of spending many hours per week engaged in LIPA, because this is already what many people do in their every-day lives. For example, recent data show that some nonexercising adults (e.g., active housewives with children) engage in LIPA for the majority of the waking day.
Research on sedentary behavior and inactivity physiology will be an important influence on future public health initiatives in chronic disease prevention. Furthermore, future physical activity guidelines for adults could include explicit recommendations on reducing prolonged sitting time.
Implications For The Future
The potential public health actions that might be taken in relation to sedentary behavior include the following:
- Public information campaigns to emphasize reducing sitting time, as well as increasing physical activity
- Innovative technologies that can provide more opportunities to reduce sitting time (e.g., height-adjustable desks, telephone headsets)
- Regulations in workplaces for reducing or breaking up extended periods of job-related sitting
- Promotion of active transport modes as alternative to prolonged periods of time spent sitting in automobiles
In conclusion, further evidence is needed from population-based studies and intervention trials to demonstrate the feasibility and health outcomes of reducing and breaking up sedentary time. A more in-depth and broader understanding of sedentary behavior will provide a strong basis for pursuing novel public health initiatives and innovative lifestyle modifications.
Marc Hamilton is an inactivity physiologist at the Pennington Biomedical Research Center in Baton Rouge, Louisiana. He oversees an internationally renowned research program into the metabolic and health effects of physical inactivity and sedentary time. His pioneering research has led to the emergence of the new research discipline – ‘inactivity physiology’. In addition to basic science research, his program is assisting a wide range of groups from schools to industries and governmental groups with a stake in disease prevention.
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