The physical, economic and social environments in which modern humans sit or move within the contexts of their daily lives have been changing rapidly, and particularly so since the middle of the last century. These changes — in transportation, communications, workplace and domestic-entertainment technologies — have been associated with significantly-reduced demands for physical activity. However, these reductions in the environmental demands for being physically active are associated with another class of health-related behaviors.
Sedentary behaviors (typically in the contexts of TV viewing, computer and game-console use, workplace sitting, and time spent in automobiles) have emerged as a new focus for research on physical activity and health (18
). Put simply, the perspective that we propose is that too much sitting
is distinct from too little exercise
. Research findings on sedentary behavior and health have proliferated in the 10 years following publication of our first Exercise and Sport Sciences Reviews
paper on this topic (32
). As we will demonstrate, initial findings on the metabolic correlates of prolonged TV viewing time (TV time) have since been confirmed by recent objective-measurement studies, which also show that breaking up sedentary time can be beneficial. Furthermore, we describe recent studies from Canada, Australia, and the United States, which show prospective relationships of sedentary behaviors with premature mortality. Importantly, adults can meet public-health guidelines on physical activity, but if they sit for prolonged periods of time, their metabolic health is compromised. This is a new and challenging area for exercise science, behavioral science, and population-health research. However, many scientific questions remain to be answered before it can be concluded with a high degree of certainty that these adverse health consequences are uniquely due to too much sitting
, or if what has been observed so far can be accounted for by too little light, moderate, and/or vigorous activity.
The updated recommendation for adults on Physical Activity and Public Health
from the American College of Sports Medicine and the American Heart Association (ACSM/AHA) “clearly states that the recommended amount of aerobic activity (whether of moderate- or vigorous-intensity) is in addition to routine activities of daily living which are of light intensity, such as self care, casual walking or grocery shopping, or less than 10 min of duration such as walking to the parking lot or taking out the trash” ((20
) p. 1426). Logically, doing such daily activities differently could involve reductions in sitting time, but sitting
per se is not addressed specifically in the recommendations. In this context, the key question to be asked about the strength of the evidence on sedentary behavior and health that we present in this paper is: Would one expect to see a statement on reducing sitting time
included in future physical activity recommendations?
Sedentary behaviors (from the Latin sedere
, “to sit”) include sitting during commuting, in the workplace and the domestic environment, and during leisure time. Sedentary behaviors such TV viewing, computer use, or sitting in an automobile typically are in the energy-expenditure range of 1.0 to 1.5 METs (multiples of the basal metabolic rate)(1
). Thus, sedentary behaviors are those that involve sitting and low levels of energy expenditure. In contrast, moderate-to-vigorous physical activity such as bicycling, swimming, walking, or running may be done in a variety of body positions, but require an energy expenditure of 3 to 8 METs (1
). In this perspective, light intensity activity behaviors are those done while standing, but that requires expenditure of no more than 2.9 METS.
Addressing research on the health consequences of sedentary behavior requires some initial clarification of terminology. We refer to sedentary behaviors (different activities, for different purposes in different contexts; see above). We refer also to sitting time, a generic descriptor covering what these sedentary behaviors primarily involve. As we demonstrate below, adults spend the majority of their waking hours either sitting, or in light intensity activity (predominantly standing with some gentle ambulation).
Time in sedentary behaviors is significant, if only because it displaces time spent in higher intensity physical activity — contributing to a reduction in overall physical activity energy expenditure. For example, displacement of two hours per day of light intensity activity (2.5 METS) by sedentary behaviors (1.5 METS) would be predicted to reduce physical activity energy expenditure by about two MET-hrs/d, or approximately the level of expenditure associated with walking for 30 min per day (0.5 hrs * 3.5 METs = 1.75 MET-hrs).
Research on physical activity and health has concentrated largely on quantifying the amount of time spent in activities involving levels of energy expenditure of 3 METs or more, characterizing those with no participation at this level as “sedentary” (33
). However, this definition neglects the substantial contribution that light intensity (1.9 to 2.9 METs) activities make to overall daily energy expenditure (8
), and also the potential health benefits of participating in these light-intensity activities, rather than sitting. Furthermore, although individuals can be both sedentary and physically inactive, there is also the potential for high sedentary time and being physically active to co-exist (the Active Couch Potato
phenomenon, which we explain below). An example would be an office worker who jogs or bikes to and from work, but who then sits all day at a desk and spends several hours watching TV in the evening.
Common behaviors in which humans now spend so much time — TV viewing, computer use and electronic games, sitting in automobiles — involve prolonged periods of these low levels of metabolic energy expenditure. It is our contention that sedentary behavior is not simply the absence of moderate-to-vigorous physical activity, but rather is a unique set of behaviors, with unique environmental determinants and a range of potentially-unique health consequences (43
). Our population-health research perspective is on the distinct role of sedentary behavior, as it may influence obesity and other metabolic precursors of major chronic diseases (type 2 diabetes, cardiovascular disease, and breast and colon cancer).
Sedentary Behavior and Health: A Unique Underlying Biology?
Physiologically, distinct effects are observed between prolonged sedentary time and too little physical activity (17
). There are broad consistencies between the patterns of findings from epidemiologic studies on the cardio-metabolic correlates of prolonged sitting that we will describe, and recent evidence on biological mechanisms — “inactivity physiology” — identified in animal models. It seems likely that there is a unique physiology of sedentary time, within which biological processes that are distinct from traditionally-understood exercise physiology are operating. The groundbreaking work of Hamilton and colleagues (3
) provides a compelling body of evidence that the chronic, unbroken periods of muscular unloading associated with prolonged sedentary time may have deleterious biological consequences. Physiologically, it has been suggested that the loss of local contractile stimulation induced through sitting leads to both the suppression of skeletal muscle lipoprotein lipase (LPL) activity (which is necessary for triglyceride uptake and HDL-cholesterol production) and reduced glucose uptake (3
). A detailed account of findings and implications from Hamilton’s studies has been provided in recent reviews (17
Hamilton’s findings suggest that standing, which involves isometric contraction of the anti-gravity (postural) muscles and only low levels of energy expenditure, elicits EMG and skeletal muscle LPL changes. However, in the past, this form of standing would be construed as a “sedentary behaviour” because of the limited amount of bodily movement and energy expenditure entailed. This highlights the need for an evolution of the definitions used for sedentary behavior research. Within this perspective, standing would not be a sedentary activity and our approach (subject to revision as further findings accumulate) is to equate “sedentary” with ”sitting.”