While there has been some success with these public health campaigns, evident through the population-wide increases in leisuretime physical activity being observed in some countries over the past 10 years, this success has also coincided with a rapid rise in the prevalence of overweight and obesity in several countries over the same period.2 Several factors may explain this apparent paradox. The most plausible explanation is the sole focus on an important, but limited, element of the overall physical activity spectrum: moderateto vigorous-intensity activities. Focusing on this single component does not address the health consequences of participation in the plethora of sedentary behaviors that occupy the waking hours of most adults.
For instance, for a person who typically sleeps for eight hours per day, meeting the minimum public health physical activity levels of 30 minutes of moderate- to vigorous-intensity activity each day constitutes only a small proportion of the remaining 16 hours in his or her day (see Figure 1). Indeed, recent studies that have used accelerometers to objectively measure daily physical activity among Australian adults have identified that, on average, the majority of adults’ non-sleeping hours (up to 60%) is spent in sedentary time, with the remainder being disproportionally distributed to light-intensity (incidental movement; 35%), and only a small fraction of time to moderate to vigorous physical activities (usually less than 5%; see Figure 2).3
Sedentary time, derived from the Latin word ‘sedere’ meaning ‘to sit’, represents the time that individuals spend in various behaviours that require low energy expenditure such as working on the computer, watching television or driving a car. Sedentary behaviour, often used interchangeably with sedentary time, is the term now used to collectively characterise those behaviours that people encounter at home, at work and during leisure and transportation that involve prolonged sitting rather than ambulatory movement.4–6
Recent evidence indicates that time spent in sedentary behaviours, independent of time spent in moderate- to vigorous-intensity activity, is related to health outcomes and cardio-metabolic biomarkers of chronic disease risk among adults.4 The independence of these two behaviours is further reinforced in studies that have demonstrated detrimental cardio-metabolic health outcomes for ‘active couch potatoes’ (i.e. those individuals who meet the physical activity guidelines but also have high sedentary time; see Figure 3).7–9 These findings have led to the emergence of a strong scientific interest in understanding and influencing sedentary behaviour.
In this article, we posit that sedentary behaviour (too much sitting) may be at least as important a public health problem as the lack of moderate- to vigorous-intensity physical activity (too little exercise). We put forward an argument for an expanded perspective on physical activity and health where behaviour (both sedentary and physical activity) across the day and at all intensities should be considered. We present a brief overview of recent evidence that identifies too much sitting as an important ingredient of the physical activity and health equation, particularly in relation to cardio-metabolic risk. We emphasise that the impacts of too much sitting need to be considered as influences that are additional to the still very important clinical and public health concerns about too little exercise.