Jakob Tarp, Knut Eirik Dalene and Ulf Ekelund
As will be obvious to anyone on a bus or train or waiting for coffee, access to screen-based media has been revolutionised over the past two decades. In 2020, there were more than six billion smartphone users worldwide and almost global penetration. Yet, our understanding of how these devices have affected other behaviour, such as sedentary time, is limited.
In our study, recently published in the IJE, we used accelerometer measurements of sedentary time collected for the Norway-wide physical activity monitoring system to estimate that a 9-year-old boy or 15-year-old girl or boy spent, on average, 20 to 30 minutes more each day being sedentary in 2018 than in 2005. We also found that children and adolescents accumulate more of their sedentary time in longer uninterrupted periods, compared with a more fragmented pattern of activity in 2005. Children and adolescents now spend less time sitting for short periods of 5 minutes or less at a time.
The public health consequences of this increase in sedentary time are uncertain. Sedentary time appears to be unrelated to metabolic health in youth, when moderate- to vigorous-intensity physical activity is taken into consideration. However, prolonged time spent sitting, particularly screen time, might have detrimental influences on other health dimensions, such as mental health, and on social interactions.
Modern societies are highly automated, with little need to expend energy in physical activity. It is possible to get by with as few as 2000 steps a day. Still, we don’t know how active people were before the Industrial Revolution, or even 40 years ago, before the start of the obesity epidemic in the 1980s. These data simply don’t exist. We can only rely on comparisons with modern hunter–gatherer communities, such as the Hadza in Tanzania, or with religious groups like the Amish in North America to get a sense of how much physical activity people used to do. But these indirect comparisons give only suggestive evidence of what could be considered (in a historical context) as a uniquely inactive human lifestyle. Integration of the internet into many aspects of daily life might represent a landmark in the evolution of how we move — or don’t move. On the flip side, these technological developments can be used to monitor and understand these changes.
Our group has previously shown that declines in higher-intensity activities can’t explain the 20- to 30-minute increase in sedentary time. This means that the increase in inactive time has come at the expense of lower-intensity physical activity, implying that children and adolescents have reduced their engagement in informal activities of daily living (not exercise) since 2005. Although our data don’t tell us which activities are being replaced, they do identify a shift in time-use that might be amplified by further integration of internet-based solutions into daily living.
We had access to time-stamped data through the Norwegian physical activity monitoring system, a collaboration between the Norwegian Institute of Public Health and the Norwegian School of Sports Sciences. This unique system is not nested within a framework covering multiple health behaviours (like the Health Behaviour in School-aged Children survey) but focuses strictly on assessing physical activity. The advantage is that the time and resources used to measure physical activity do not have to be balanced against measurement of other factors.
However, few countries have similar physical activity surveillance systems, and even fewer have data from before the 2007 advent of smartphones. Many countries rely on people’s own perceptions of how much time they are active and inactive, which are both prone to bias and too crude to detect changes in the patterns of how sedentary time accumulates. Most current physical activity surveillance systems are therefore unlikely to fully capture real changes in human movement behaviour across the lifespan.
Without appropriate surveillance systems, countries will be unable to track the kinds of changes we found and could fail to initiate preventive countermeasures. Integrating device-measured physical activity into general health surveys is, however, possible and something we hope and expect to see more of.
KE Dalene, E Kolle, J Steene-Johannessen, et al. Device-measured sedentary time in Norwegian children and adolescents in the era of ubiquitous internet access: secular changes between 2005, 2011 and 2018. Int J Epidemiol 2022; 1 April. doi: 10.1093/ije/dyac063
Dr Jakob Tarp (@JakobTarp) is a postdoctoral fellow at the Department of Clinical Epidemiology, University of Aarhus, Denmark. His research interests focus on the health effects of physical activity on non-communicable diseases.
Dr Knut Eirik Dalene (@knuteirik47) is a researcher at the Norwegian Institute of Public Health, Oslo. His research interests focus on the health effects of physical activity on non-communicable diseases.
Professor Ulf Ekelund (@Ulf_Ekelund) is a professor of physical activity epidemiology at the Norwegian School of Sports Sciences and a senior researcher at the Norwegian Institute of Public Health, Oslo. His research interests focus on the associations between sedentary time, physical activity and non-communicable diseases throughout the life course.