1. FACULTY OF HEALTH AND MEDICAL SCIENCES
UNIVERSITY OF COPENHAGEN
EXPLORING SEDENTARY BEHAVIOUR
HEALTH IMPLICATIONS AND
SOCIO-ECOLOGICAL DETERMINANTS
PhD Thesis
Madina Saidj
2015
2. SUMMARY
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SUMMARY
Background: A growing body of research is accumulating on the topic of sedentary behaviour. Recent
evidence suggests that the health effects of this behaviour may be independent of physical activity, but
knowledge of the mechanisms involved is fragmentary. The evidence regarding sedentary behaviour at work
is sparse compared to that of leisure-time sedentary behaviour, albeit appointed in many studies as the
prime domain to target. A variety of factors are likely to influence an individual’s ‘choice’ to engage in
sedentary behaviours. Based on a socio-ecological conceptual framework, sedentary behaviour may be
dependent on the dynamic relationships between a person’s individual characteristics and the surrounding
(social and built) environment.
Aims: To explore the cross-sectional and longitudinal associations of self-reported sedentary behaviour
during leisure-time and at work with health risk markers (cardiometabolic, cardiorespiratory and muscular)
(paper I-III), and to explore the association with occupation type and perceptions towards physical activity
(paper V) and with housing characteristics (in relation to leisure-time sitting) (paper IV).
Methods: The thesis is based on data from the Danish Health2006 cohort, baseline (N=2,544) and 5-year
follow-up (N=1,403 in paper III; N=2,128 in paper IV), and the French ACTI-Cités/Nutrinet Santé study
(N=35,444). Information on sedentary behaviour, physical activity and socio-ecological determinants were
questionnaire-based, and health markers were collected through health examinations. Statistical analyses
were carried out using multiple linear regression analyses, with and without restricted linear splines.
Results: Sedentary behaviour during leisure-time, rather than at work, appeared as a potential health risk in
relation with cardiometabolic, cardiorespiratory and muscular health markers, taking into account moderate-
to-vigorous physical activity (paper I-III). Each type of sedentary behaviour was differently associated with
occupation type and one’s perceptions attached to physical activity (paper V). Housing characteristics and
especially household size seemed predictive of sedentary behaviour during leisure-time, pointing to the
home environmental influences on sedentary behaviour outside work (paper IV).
Conclusions: Findings encourage knowledge on how much the effects of sedentary behaviour are being
driven by physiological processes linked to sitting and how much the broader context (where, when, with
whom, under what circumstances) of the specific behaviour (leisure vs. work) matters.
For now, the implications for public health policy and physical activity counselling should be to join together
the whole spectrum of behaviours from sleep to high-intensity physical activity, rather than merely focus on
the value of moderate-to-vigorous activity on top of an indefinite baseline, that has moved towards zero
activity.
3. SUMMARY
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PREFACE
It seems inevitable not to introduce this thesis by referring to Jerry Morris’ study of London’s sedentary bus
drivers, presumably the first epidemiological study of sedentary behaviour, although in its time considered as
lack of physical activity. In 1949 Morris found that sedentary drivers of London's double-decker buses had
higher rates of cardiovascular disease than the conductors who climbed the stairs (Morris et al., 1953; Blair
et al., 2010). He extended the study and noticed that postmen who delivered the mail by bike or foot had
fewer heart attacks than sedentary men who served behind counters or as telephonists and clerks (Ibid.).
Here 66 years later, the level of scientific and popular interest in sedentary behaviour has escalated
immensely, and is today no longer addressed as a synonym for physical inactivity but rather as a posture
with low energy expenditure. While this phenomenon has been more evident in the Western world, the level
of concern has also begun to rise globally (Ng & Popkin, 2012). With reason: Over the past decades, our
lives have become increasingly technologised and remote. Although these societal changes have had some
pleasant effects, they have also led to a decrease in everyday movement. Historically, our ancestors walked
briskly throughout a typical day (Bramble & Lieberman, 2004), estimated to be in the range of 6 to 16 km
(O’Keefe et al., 2010), while today adults in westernized societies spend approximately 5% of their waking
hours in moderate-to-vigorous physical activity, whereas 50-70% is spent in sedentary pursuits,
approximating 7-10 hours per day (European Commission, 2014; Bauman et al., 2011; Aadahl et al., 2013a;
Matthews et al., 2008). From an evolutionary perspective, some have thus declared that we, Homo Sapiens,
have turned into Homo Sedentarius (Levine, 2014).
Until recently, public health efforts have primarily focused on physical activity and the lack thereof, while the
extent and impact of sedentary behaviour has been less recognized. A growing body of research indicates
that it is not only necessary to be physically active as recommended, but it is also important to limit the hours
spent in a sedentary state for better health. Sedentary behaviour is currently thought to be related to chronic
disease-related risk factors, poor health and premature death, giving it a reputation as the “New Smoking” of
the 21st century. A number of countries have hence produced public health guidelines that include
recommendations on time spent in sedentary behaviour. Consideration has been extended to include
environmental, regulatory and educational interventions to reduce prolonged periods of sitting; for adults,
especially in workplaces, where this “mandatory sitting” has prompted an even more urgent concern
regarding sedentary behaviour and health. That said the field of sedentary behaviour research is still in its
relative infancy (even more so when I started working with this thesis!), and there are (still) many important
scientific questions to be addressed and answered. Among them are the need of a better understanding of
sedentary behaviour accumulated in terms of domains (e.g. by work and leisure/domestic) and in a broader
socio-ecological context of the social/built environment, along with understanding which health marker
profiles are actually affected by sedentary behaviour in different domains. This thesis has aimed to explore
on these aspects, with the overall objective to describe the epidemiology of sedentary behaviour in adults, of
relevance to inform the scientific sedentary behaviour debate, and potential public health and clinical
guidelines in this regard.
4. SUMMARY
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The thesis constitutes a summary based on the following five papers, conducted at the Research Centre for
Prevention and Health (DK), and at ICAN Institute for Cardio-metabolism and Nutrition (FR).
Paper I: Saidj M, Jørgensen T, Jacobsen RK, Linneberg A, Aadahl M. Separate and joint associations
of occupational and leisure-time sitting with cardio-metabolic risk factors in working adults: A
cross-sectional study. PLoS ONE 2013;8(8):e70213.
Paper II: Saidj M, Jørgensen T, Jacobsen RK, Linneberg A, Aadahl M. Differential cross-sectional
associations of work- and leisure-time sitting, with cardiorespiratory and muscular fitness
among working adults. Scand J Work Environ Health 2014;40(5):531-38.
Paper III: Saidj M, Jørgensen T, Jacobsen RK, Linneberg A, Oppert JM, Aadahl M. Work and leisure-
time sitting, inactivity, and cardiorespiratory and -metabolic health. Submitted to Eur J Prev
Cardiol.
Paper IV: Saidj M, Jørgensen T, Jacobsen RK, Linneberg A, Aadahl M. The influence of housing
characteristics on leisure-time sitting. A prospective cohort study in Danish adults. Prev Med
2015;81:58-62.
Paper V: Saidj M, Menai M, Charreire C, Weber C, Enaux C, Aadahl M, Kesse-Guyot E, Hercberg S,
Simon C, Oppert JM. Descriptive study of sedentary behaviours in 35,444 French working
adults: cross-sectional findings from the ACTI-Cités study. BMC Public Health 2015;15:379.
5. SUMMARY
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Title Exploring Sedentary Behaviour: Health Implications and Socio-ecological
Determinants
Author Madina Saidj, MSc Public Health, m@saidj.dk
Institution Research Centre for Prevention and Health, Rigshospitalet – Glostrup, Denmark
Graduate School of The Faculty of Health and Medical Sciences, University of
Copenhagen, Denmark
Supervisors Mette Aadahl, Senior researcher, Associate professor, PT, MPH, PhD, Research
Centre for Prevention and Health, Rigshospitalet – Glostrup, Denmark
Torben Jørgensen, Professor and Director, DrMedSc, Research Centre for Prevention
and Health, Rigshospitalet – Glostrup, Denmark
Assessment
committee
Bente Klarlund Pedersen (Chairperson), Professor and Director, MD DMSc, Centre of
Inflammation and Metabolism (CIM) and Centre for Physical Activity Research (CFAS)
Janne Schurmann Tolstrup, Research Programme Director, Professor, DMSc, PhD,
Master of human biology, National Institute of Public Health, University of Southern,
Denmark
Ulf Ekelund, Professor, Department of Sport Medicine, Norwegian School of Sport
Sciences, Oslo, Norway
Thesis submitted This thesis has been submitted to the Graduate School of The Faculty of Health and
Medical Sciences, University of Copenhagen, September 15, 2015
