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Original article
The independent associations of sedentary behaviour and physical activity on cardiorespiratory fitness
  1. Rute Santos1,2,
  2. Jorge Mota1,
  3. Anthony David Okely3,
  4. Michael Pratt4,
  5. Carla Moreira1,
  6. Manuel João Coelho-e-Silva5,
  7. Susana Vale1,
  8. Luis B Sardinha6
  1. 1Research Centre in Physical Activity, Health and Leisure, Faculty of Sport, University of Porto, Porto, Portugal
  2. 2Maia Institute of Higher Education, Maia, Portugal
  3. 3Interdisciplinary Educational Research Institute, University of Wollongong, Wollongong, New South Wales, Australia
  4. 4National Center for Chronic Disease Prevention and Health Promotion, Atlanta, Georgia, USA
  5. 5Research Centre of Anthropology and Health, Faculty of Sport Sciences and Physical Education, University of Coimbra, Coimbra, Portugal
  6. 6Exercise and Health Laboratory, Faculty of Human Kinetics, Technical University of Lisbon, Lisbon, Portugal
  1. Correspondence to Dr Rute Santos, Research Centre in Physical Activity, Health and Leisure, University of Porto, Rua Dr Plácido Costa, 91, 4200-450 Porto, Portugal; rutemarinasantos{at}hotmail.com

Abstract

Background During childhood and adolescence, both physical activity (PA) and sedentary behaviour seem to influence cardiorespiratory fitness (CRF); however, the combined association of PA and sedentary behaviour remains to be understood. We analysed the combined association of objectively measured sedentary behaviour and moderate-to-vigorous intensity PA (MVPA) on CRF in Portuguese children and adolescents.

Methods The sample comprised 2506 Portuguese healthy children and adolescents aged 10–18 years, from a cross-sectional school-based study (2008). PA and sedentary behaviour were assessed with accelerometry. Participants were classified as meeting current PA guidelines for youth versus not meeting, and as low versus high sedentary (according to the median value of sedentary time/day by age and gender), and then grouped as follows: Low active—high sedentary; low active—low sedentary; high active—high sedentary; high active—low sedentary. CRF was assessed with the FITNESSGRAM 20 m shuttle-run test. Binary logistic regression models were constructed to verify the relationship between high CRF and the combined influence of MVPA/sedentary behaviour, adjusting for age, gender, body mass index and accelerometer wear time.

Results Participants classified as high active/low sedentary (OR=1.81; 95% CI 1.21 to 2.69), as well as those classified as low active/low sedentary (OR=1.27; 95% CI 1.01 to 1.61) were more likely to be fit, compared with those from the low-active/high-sedentary group.

Conclusion MVPA and sedentary behaviour may act independently in their relation with CRF, and that MVPA levels may not overcome the deleterious influence of high-sedentary time in maximising CRF.

  • Aerobic fitness/Vo2 Max

https://doi.org/10.1136/bjsports-2012-091610

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    Introduction

    Sedentary behaviour refers to activities that do not increase energy expenditure substantially above the resting level, such as sitting, lying down or viewing TV.1 It has been suggested that sedentary behaviour should be explicitly measured either for surveillance purposes or for research studies, instead of being defined as lack of physical activity (PA) (physical inactivity).1 ,2 In fact, defining a sedentary population, based on either low levels or lack of PA, might be inaccurate because sedentariness and PA are two independent and not mutually exclusive behaviours with different effects on health outcomes.3 ,4

    Cardiorespiratory fitness (CRF) reflects the overall capacity of the cardiovascular and pulmonary systems to supply oxygen during sustained exercise, as well as the ability to perform such exercise.5 Higher levels of CRF appear to delay all-cause mortality primarily because of decreased rates of cardiovascular disease (CVD) and cancer6 and provide strong and independent prognostic information about the overall risk of illness and death, especially related to cardiovascular causes.7 Cross-sectional studies indicate that high CRF, during childhood and adolescence, is associated with a favourable plasma lipid profile,8 total and central body fat,9 features of the metabolic syndrome,10 ,11 novel CVD risk factors12 and with arterial compliance.13 Longitudinal studies also suggest that higher CRF, during childhood and adolescence, is associated with a healthier cardiovascular profile14 and a healthier body composition15 later in life.

    CRF is influenced by several factors, including body fatness, growth, sexual maturation, age, sex, health status and genetics, yet its principal modifiable determinants are moderate-to-vigorous PA (MVPA) and sedentary behaviour.16 Further, it is known that sedentary behaviour increases and MVPA decreases from childhood to adolescence,17 and that sedentary behaviour, PA and CRF in youth track into adulthood.18–20

    In children and adolescents, self-reported leisure-time sedentary behaviours such as TV viewing and overall screen time (ie, TV viewing, videogames and computer use) have been commonly studied; however, although they may represent a substantial part of the time spent in total sedentary behaviours,21 they do not represent the total amount of everyday sedentary time.22 In this sense, similar to PA,23 objectively measured total sedentary time, using devices such as accelerometers, may offer particular advantages, since they do not rely on the subject recall and they may capture the entire daily pattern of both PA and sedentary behaviour.

    In children and adolescents, a recent review has shown that objectively measured PA is positively associated with CRF.23 On the other hand, the associations between CRF and sedentary behaviour have been reported mostly using self-reported leisure-time sedentary behaviour.24–31 A study by Martinez-Gomez et al32 examined the relationship between objectively measured sedentary behaviour and CRF, taking into consideration the PA levels and other important confounders such as body fat. This study  reported a significant negative association between excessive sedentary time (defined as more than 69% of waking hours) and CRF in adolescent girls who did not meet the PA guidelines (<60 min/day in MVPA) that was not present in those who met the recommendations for PA (≥60 min/day in MVPA), leading the authors to suggest that the adverse influence of sedentariness might be attenuated if adolescent girls meet the current PA guidelines.32

    In the light of these findings, and because both PA and sedentary behaviour seem to influence CRF, understanding the combined association of PA and sedentary behaviour on CRF is important from a public health prevention point of view. Indeed, in today's western societies children and adolescents seem to have fewer opportunities to be active and more to be sedentary than previous generations and this shifting in PA and sedentary patterns called ‘the PA transition’ is associated with the obesity epidemic and may have the potential to attenuate future gains in life expectancy.33 Moreover, PA, CRF and sedentary behaviour seem to have a clear dose–response relationship with all cause and CVD mortality.34 New evidence also suggests that the relation between sedentary behaviour and all-cause and CVD mortality is independent of PA levels.34 However, the question that remains to be answered is whether, during childhood and adolescence, PA and sedentary behaviour act independently in their relation with CRF.

    The aim of this study was to examine the combined association of objectively measured sedentary behaviour and MVPA on cardiorespiratory fitness in children and adolescents aged 10–18 years.

    Methods

    Study design and sampling

    Data were derived from a cross-sectional school-based study conducted in 2008. The study aimed to evaluate PA, physical fitness, overweight/obesity prevalence and related factors in Portuguese children and adolescents aged 10–18 years. Study design and sampling are reported elsewhere.35 Briefly, data were collected by means of proportional stratified random sampling taking into account the location (region), and the number of students by age and gender in each school, in all mainland Portuguese administrative regions (Alentejo, Algarve, Centro, Lisboa and Norte). Data were collected on 22 179 children and adolescents from whom written informed consent was obtained from the parents or guardians (89% response rate). In 3165 children and adolescents, PA and sedentary behaviour were objectively measured with accelerometers. Of those, 660 participants did not comply with the accelerometer criteria (see below for explanation) and were excluded from the analysis. Children and adolescents, whose main PA during leisure time involved some water activity, did not use the accelerometer, to avoid misclassification of these participants. The final sample for this study comprised 2506 children and adolescents aged 10–18 years (1326 girls and 1180 boys).

    Sedentary time and physical activity

    The accelerometer GT1M Actigraph (ActiGraph, Pensacola, Florida, USA) was used to obtain detailed and objective information about daily PA and sedentary behaviour over five consecutive days. This lightweight, biaxial monitor was the latest model available by the manufacturer at the time of data collection, and studies have demonstrated that it is a technically reliable instrument, both within and across monitors.36 The accelerometer was attached tightly in the hip, on the right side, with the notch faced upwards, and participants were instructed to use the accelerometer during waking hours and remove it during water-based activities, according to established procedures.37 The epoch length was set to 15 s to allow a more detailed estimate of PA intensity.37

    Accelerometer data were analysed by an automated data reduction program (MAHUffe,  www.mrc-epid.cam.ac.uk) that provided options for screening the data and computing outcomes. Data files from individual participants were screened by detecting blocks of consecutive zeros. Periods with 60 min of consecutive zeros were detected and flagged as times in which the monitor was not worn.38 Participants had to have at least 10 h of data to count as a valid day and to have at least three valid days to be included (two weekdays and one weekend day). The screening procedures were consistent with current accelerometry studies and also similar to the screening used in National Health and Nutrition Examination Survey (NHANES).38 ,39 After screening was completed, the raw activity ‘counts’ were processed for determining the time spent in the different PA intensities. Activity levels were expressed in mean counts/min and also in estimates of the time spent in MVPA. The established accelerometer cut-points proposed by Freedson and published by Trost et al40 were used to determine PA intensities. Sedentary behaviour was identified using a cut-point of <100 counts/min, as this cut-off was shown to have an excellent classification accuracy.41

    Participants were classified as meeting current PA guidelines for youth (≥60 min/day in MVPA) or not meeting PA guidelines (<60 min/day in MVPA).42 Participants were also classified in two groups according to the median value by age and gender of the time spent in sedentary behaviour. Participants were then grouped in PA/sedentary time groups as follows: low active/high sedentary; low active/low sedentary; high active/high sedentary; high active/low sedentary.

    Cardiorespiratory fitness

    CRF was measured using the 20 m shuttle run test as previously described by Léger et al.43 This test requires participants to run back and forth between two lines set 20 m apart. Running speed started at 8.5 km/h and increased by 0.5 km/h each minute, reaching 18.0 km/h at minute 20. Each level was announced on a tape player. The participants were told to keep up with the pace until exhausted. The test was finished when the participant failed to reach the end lines concurrent with the audio signals on two consecutive occasions. Otherwise, the test ended when the subject stopped because of fatigue. Participants were encouraged to keep running as long as possible throughout the course of the test. Number of shuttles performed by each participant was recorded. This test is valid,44 reliable45 and feasible46 in young people.

    Vo2max was calculated using Léger's equation.43 Adolescents were then classified in two groups according to the age-specific and sex-specific cut-off points of FITNESSGRAM criteria, as belonging to the healthy zone or above—‘fit’, or under the healthy zone—‘unfit’.47

    Statistics

    Descriptive data are presented as means and SD or percentages. Analysis of variance and analysis of covariance with Bonferroni post hoc comparisons were performed to compare gender or PA/sedentary behaviour group differences in continuous variables, and nominal data were analysed with χ2 tests. Linear regression models were performed to assess independent associations of MVPA and of sedentary time and cardiorespiratory fitness. The relationship between high CRF and the combined association of PA/sedentary behaviour was analysed using binary logistic regression. Because there was not a significant interaction between gender×PA/sedentary time groups, we performed the regression models with girls and boys together, controlling for age, gender, body mass index and accelerometer wear time. Data were analysed using the IBM SPSS Statistics V.19 (SPSS, Inc IBM, New York, USA). A p value less than 0.05 denoted statistical significance.

    Results

    Participants’ characteristics are shown in table 1. The overall prevalence of participants in the healthy zone or above on the CRF test was 77.6%. A higher percentage of fit participants was found in the high-active/low-sedentary group (89.1%) and the lowest percentage in the low-active/high-sedentary group (73.9%) (p<0.001).

    View this table:
    Table 1

    Descriptive characteristics of the participants

    The high-active/low-sedentary and the high-active/high-sedentary groups showed higher mean values of VO2max and MVPA when compared with participants belonging to the low-active/low-sedentary and low-active/high-sedentary groups (p<0.05). Significant differences were also found between all MVPA/sedentary time groups for sedentary time, with those in the high-active/low-sedentary group showing the lowest mean value (p<0.05).

    As reported in table 2, MVPA was positively associated with Vo2max independent of sedentary time and accelerometer wear time (model 2), and sedentary time was negatively associated with Vo2max, independent of MVPA and accelerometer wear time (model 4).

    View this table:
    Table 2

    Linear regression models predicting independent associations of moderate–to-vigorous physical activity and of sedentary time and cardiorespiratory fitness

    Results of the logistic regression analysis for CRF and MVPA/sedentary time groups, adjusting for age, gender, body mass index and accelerometer wear time, are shown in table 3. Participants classified as high active/low sedentary (OR=1.81; 95% CI 1.21 to 2.69), as well as those classified as low active/low sedentary (OR=1.27; 95% CI 1.01 to 1.61) were more likely to be fit, compared with those from the low-active/high-sedentary group.

    View this table:
    Table 3

    Logistic regression predicting belonging to the healthy zone or above for cardiorespiratory fitness by the physical activity/sedentary time group

    Discussion

    In this study, we assessed the combined association of MVPA and sedentary behaviour on CRF, in a sample of children and adolescents.

    Several studies have reported the prevalence of participants with adequate levels of CRF; however, the methodological differences in the CRF assessment make comparisons difficult. Compared with previous studies that assessed CRF with the 20 m shuttle-run test, the overall prevalence of fit participants found in this study (77.6%) is similar to that reported for Australian children and adolescents (70–75%)48 and higher than that reported for other European adolescents (59.4%).49 As expected, the results indicate a positive association of combining adequate levels of MVPA and low-sedentary behaviour time on CRF. Indeed, a higher percentage of fit participants was found in the high-active/low-sedentary group (89.1%) and the lowest percentage in the low-active/high-sedentary group (73.9%; p<0.001).

    The results show that participants classified as high active/low sedentary (OR=1.81; 95% CI 1.21 to 2.69), as well as those classified as low active/low sedentary (OR=1.27; 95% CI 1.01 to 1.61), were more likely to be in the healthy zone or above in the CRF test, compared with those from the low-active/high-sedentary group, independent of age, gender, body mass index and accelerometer wear time. Previous studies have addressed cross-sectional relationships between objectively measured sedentary behaviour and CRF. Ekelund et al50 found an inverse association between CRF and sedentary behaviour. However, it is important to note that this study did not control the analysis for PA and body fat, two important confounders. In the present study, when analysing the combined association of MVPA and sedentary behaviour, it was observed that children and adolescents with low-sedentary behaviour exhibited higher odds of being fit regardless of their PA level. Despite the fact that those classified as high active/low sedentary had the highest odds of being fit, these results suggest that PA and sedentary behaviour may act independently in their relation with CRF, and that MVPA levels may not overcome the deleterious influence of high levels of sedentary behaviour. In spite of the methodological differences between studies, the results of the present study seem to conflict with those found by Martinez-Gomez et al32 who suggested that the adverse influence of sedentariness on CRF might be attenuated if adolescent girls met the current PA guidelines.

    PA and sedentary behaviours are two different constructs with different effects on health in young people.4 It is known that VO2max is reduced in healthy individuals confined to bed rest and that the magnitude of its loss depends on the duration of bed rest and the initial level of VO2max, independent of gender and age.51 ,52 It seems that the small compensatory increase in maximal heart rate during bed rest does not compensate for the reduction in stroke volume, and because the arterial-venous oxygen difference does not change, the compromised stroke volume appears to be the primary mechanism for VO2max reductions induced by immobilisation.51–53 However, lying down and sitting are two different postures that may have different physiological implications. The decline in exercise performance (VO2max) with bed rest may be minimised just by regular exposure to orthostatic stress, such as intermittent sitting or standing during the immobilisation period.51–53 Unfortunately, the accelerometers used in this study were not able of distinguish between lying, sitting and standing still, as the new generation of accelerometers equipped with inclinometers may be able to do so. Also in this perspective, the findings of Healy et al54 ,55 on the beneficial associations of breaks in sedentary behaviour on cardiometabolic risk factors, even in individuals meeting PA guidelines may inspire future investigations on the associations between CRF and sedentary behaviour. Moreover, it is also known that the adverse health effect of TV viewing during childhood on CRF, fatness, smoking and total cholesterol may persist into adulthood and that these associations are not mediated by adult TV viewing and are independent of PA levels.56

    In the sample of this study, the average time spent in total sedentary behaviour was ∼9 h/day, a similar value to the one reported for European adolescents,32 but higher than that reported for youth from the USA.57 However, considering that Portuguese children and adolescents spend on average 6–7 h/day in school and that physical education classes only account for 2:15–3 h/week, these results may imply that a significant proportion of children and adolescents exceeded the recommended daily limit of total media time of less than 2 h/day,58 particularly on weekends. Certainly, not all sedentary activities are undesired or possible to modify, but these results suggest that strategies to promote PA and discourage sedentary behaviour in this population are necessary, given that the adverse effects of high sedentary behaviour and low PA levels have important health4 ,23 ,59 and behavioural60 implications that go beyond reducing CRF. In the light of this issue, another important finding worthy of discussion is that participants classified as highactive/low sedentary exhibit a higher mean value for MVPA than those classified as high active/high sedentary (76.9±0.8 vs 68±1.2 min/day, p<0.001); significant partial correlation were also found between sedentary time and MVPA (controlling for age, gender and accelerometer wear time; r=0.256; p<0.001, data not shown) which may provide some support for the idea of the ‘displacement theory’. These results allow us to speculate that a decrease in sedentary time could be accompanied by an increase in PA levels. Indeed, an intervention study, with 8-year-old to 12-year-old obese children reported that targeting decreased sedentary behaviour (watching TV, playing computer games, talking on the telephone or playing board games) led to increases in both PA and non-targeted sedentary behaviours, which, in turn, led to increases in CRF and decreases in body fat percentage.61

    A better understanding of how PA and sedentary behaviour combine to influence the risk of low CRF, an important health marker in children and adolescents seems to be of substantial public health importance.

    Strengths and limitations

    Strengths of this study include the novelty of the analyses of the combined association of PA levels and sedentary behaviour with CRF in a large sample of children and adolescents; the objective assessment of both total MVPA and total sedentary time, since most of the previous studies have limited their analysis to self-reported leisure-time sedentary behaviour and/or PA; the use of an accelerometer cut-point of <100 counts/min to identify sedentary behaviour, as this cut-off has been shown to have an excellent classification accuracy;41 and, the use of a valid field test for CRF assessment44 which can be administered in school settings where a large number of participants can be tested simultaneously, enhancing participant motivation and making it a valuable tool for routinely measuring CRF in youth.

    Despite these strengths, this study has some limitations, which must be considered when interpreting its results. First, around 20% of participants did not have adequate amounts of accelerometer data to be included in the analysis. Second, accelerometers do not identify PA or sedentary patterns or contexts, and the accelerometer used in this study do not allow one to distinguish the type of sedentary behaviour (ie,  lying, sitting or standing still). Third, it is not possible to draw cause–effect conclusions because of the cross-sectional nature of the data.

    Conclusions

    In conclusion, the results of this study indicate that children and adolescents classified as high active/low sedentary had the highest odds of being in the healthy zone or above in the CRF fitness test, and those with low-sedentary behaviour exhibit higher odds of being fit regardless of their MVPA level. These findings suggest that PA and sedentary behaviour may contribute independently to CRF, and that MVPA levels may not overcome the deleterious influence of high-sedentary time. These results extend previous research on the adverse effects of low PA levels and high-sedentary time on CRF, by showing their combined association on this important health outcome. The data stress the importance of promoting MVPA and discouraging sedentary behaviour.

    What are the new findings

    • Participants classified as high active/low sedentary were more likely to be fit.

    • Physical activity and sedentary behaviour may act independently in their relation with cardiorespiratory fitness, in children and adolescents.

    • Physical activity levels may not overcome the deleterious influence of high-sedentary time in maximising cardiorespiratory fitness, in children and adolescents.

    How might it impact on clinical practice in the near future

    During childhood and adolescents, for cardiorespiratory fitness, it seems to be important not only to promote PA but also to discourage sedentary behaviour.

    Acknowledgments

    Authors are grateful to Professor Robert Malina for his helpful comments on an earlier draft of this manuscript. Authors also thank all the teachers and technical staff who were involved in data-collection procedures.

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    View Abstract

    Footnotes

    • Contributors RS, JM, MJCS and LBS have contributed substantially to conception and design, acquisition of data, and analysis and interpretation of data. ADO, CM, SV and MP were involved in the analysis and interpretation of data. All authors were involved in the drafting of the article, revised the manuscript critically for important intellectual content, and gave final approval of the version to be published.

    • Funding This study was supported by FCT grants: BD/44422/2008; PTDC/DES/098309/2008; BPD/65180/2009; SAB/1025/2010.

    • Competing interests None.

    • Disclaimer The findings and conclusions in this report are those of the author(s) and do not necessarily represent the views of the Centers for Disease Control and Prevention.

    • Ethics approval Portuguese Institute of Sport.

    • Provenance and peer review Not commissioned, externally peer reviewed.