Res Exerc Epidemiol 2013; 15(1): 1-6
c 2013 by the Japanese Association of Exercise Epidemiology
Copyright ○【Review Article】

Reducing Sedentary Time or Increasing Moderate and Vigorous Intensity
Physical Activity in Youth? Associations with Health Outcomes
Ulf Ekelund1,2)1）Department of Sport Medicine, Norwegian School of Sport Sciences, Oslo, Norway
2）Medical Research Council Epidemiology Unit, University of Cambridge, Cambridge, UK

ABSTRACT Current recommendations for physical activity in young people state that those between 5 and 18
years of age should accumulate at least 60 minutes of moderate and vigorous intensity physical activity per day.
Emerging evidence suggest that the majority of young people do not achieve the recommended levels of activity
which may influence on their health. On the opposite end of the energy expenditure spectrum, sedentar y time has
been recognised as a potential risk factor for chronic disease in adults.
Recent systematic reviews have suggested there is little evidence for a prospective association between baseline
time spent sedentary and later health outcomes. There is a paucity of data examining the prospective associations
between objectively measures physical activity and health outcomes. However, the cross-sectional evidence
linking time in moderate and vigorous intensity physical activity with cardio-metabolic health outcomes is
consistent with potentially greater magnitude of association for more vigorous intensity activity. Further, timespent sedentary appears unrelated to these outcomes following appropriate adjustments for time in moderate andvigorous intensity physical activity. The appropriate amount of time in moderate and vigorous intensity physicalactivity needed to prevent cardiovascular dysfunction in young people needs to be determined and the potential
benefits of more vigorous intensity physical activity established. Additional large scale, well-designed prospective
studies and randomised controlled trials are warranted to address these uncertainties.

Introduction

Nutrition Examination Survey (NHANES) in the US
showed that those aged 12 to 19 years spent between
50% and 60% of day time sedentary, an amount oftime comparable with those aged > 60 years 7). Data
from the Helena study comprising 9 European
countries (n = 2,200) have suggested that 71% of themeasured time was spent sedentary in 12 to 18 year
old boys and girls8). Further, time spent sedentary
increases by approximately 1.5 hours between 12 and16 years of age in a large sample of UK children 9).
Taken together, these data obtained by objective
measures of sedentary time suggest that; 1) contemporary young people spend a significant amount oftime sedentary; 2) sedentary time appears to increaseby age throughout adolescence and; 3) the sex
differences both in total sedentary time and the rate ofincrease in sedentary time are small.
Global estimates of young peoples’ self-reported
time spent in PA suggest that 80% of 13-15 year oldboys and girls do not achieve the recommended 60

Most public health authorities 1-6) agree that young
people, broadly defined as 5 to 18 years of age, should
accumulate at least 60 min of moderate-and-vigorousintensity physical activity (MVPA) on at least 5 days
per week. In addition, the benefits of vigorousintensity activity (VPA) and strength conditioning
exercises are acknowledged. And some of theseauthorities also specifically provide recommendations
for limiting the amount of time spent sedentary5).
Although the underlying evidence for these recommendations maybe weak, they provide a framework
when evaluating population levels of sedentary timeand physical activity (PA) in youth.
Recent objective data from the National Health andAddress for correspondence: Ulf Ekelund; Department ofSport Medicine, Norwegian School of Sport Science, PO
Box 4014, Ullevål Stadion, 0806 Oslo, Norway;
ulf.ekelund@nih.no1

Res Exerc Epidemiol 2013; 15(1): 1-6.

minutes of MVPA per day10). In opposite to the fairly
consistent data on time spent sedentary, data on
objectively measured time spent in MVPA varyconsiderably between studies due to the different
definitions of moderate intensity activity. Summary
data on the percentages of youth accumulating > 60
minutes of MVPA per day from studies including
more than 1,000 participants in which activity wasmeasured by accelerometry varied between 0% and100%11). To overcome this discrepancy attempts havebeen made to pool and reanalyse accelerometry
data 12). Unpublished data in more than 32,000 youth
from the International Accelerometer database (ICAD)
suggest that approximately 37% of children and 27%
of adolescents accumulated at least 60 minutes of
MVPA per day (Ekelund, unpublished observations).
In summary, the majority of youth do not accumulate
the recommended levels of physical activity every
day; boys are consistently more active than girls when
activity is defined as time spent in MVPA; and,activity levels decline by age. Given the large amountof time spent sedentary and the apparently low
prevalence of youth being physically active according
to PA recommendations for health it is pertinent to ask
whether sedentary time and PA are associated withhealth outcomes in youth and whether theseassociations are independent of each other and otherconfounding factors.

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outcomes in children including 232 studies with a
total of more than 983,000 participants. The authors
concluded there is a large body of evidence suggeststhat decreasing any type of sedentary behavior isassociated with lower health risks, particularly lower
obesity risk, in children aged 5 to 17 years. In contrast,
another systematic review examining the prospective
associations between sedentary behavior and healthoutcomes in youth concluded there is insufficientevidence for a longitudinal positive relationship between sedentary behaviors and body mass index (BMI),
more specific indicators of fat mass and other healthindicators such as blood pressure, blood lipids andbone health 18). This conclusion was partly confirmed
in another systematic review examining the prospective associations between sedentary behavior andweight gain during the last two years which suggested
weak evidence for a longitudinal association between
self-reported and objectively measured sedentary time
and weight gain 19). The authors also highlighted
the failure to appropriately adjust the longitudinal
results for baseline measures of the outcome 19).
In longitudinal studies trying to decide temporalsequence and the direction of causality, one of the key
components are that the cause precedes the effect.
When baseline measures are not accounted for,causality cannot be determined, suggesting that instudies that did not adjust their results for the baseline
measure of the outcome (e.g. BMI) there are not clear
evidences that sedentary behavior leads to weight gain.
The contra- dictory conclusions from these systematic
reviews are likely explained by different inclusion
criteria (i.e. all study designs vs. prospective cohortand intervention studies) and a more rigorous
assessment of study quality.
The vast majority of studies used self-reported
TV-viewing as an indicator of sedentary behavior andshould be interpreted cautiously. First, self-reported
TV-viewing is prone to recall bias. Second, TVviewing may be a proxy of a generally unhealthy
lifestyle, associations may therefore be explained byresidual confounding. Finally, total time spent sedentary is the reciprocal of overall physical activity
energy expenditure (PAEE). Studies that have claimed
the association between sedentary time and any healthoutcome is independent of PA have adjusted their

Is Sedentary Time Associated with Health
Outcomes in Youth?
Sedentary behavior has been defined as activities
characterized by sitting or reclining position andrequiring an energy expenditure < 1.5 METs (Metabolic Equivalent Tasks; multiples of the basal
metabolic rate). This behavior has gained tremendous interest from researchers during the lastyears due to its potentially hazardous health effects.
Longer hours of total sitting time has been linked withall-cause mortality13) and recent meta-analyses havesuggested that TV viewing is associated with increased risk of type 2 diabetes, cardiovascular disease
and all-cause mortality in adults14-16).
Tremblay et al.17) reviewed the literature on theassociations between sedentary behavior and health2Res Exerc Epidemiol 2013; 15(1): 1-6.

analyses for a sub-component of PA, e.g. time spent in
MVPA or self-reported leisure time activity. It could
therefore be hypothesized that the observed associations between sedentary time and health outcomes
would be similar but in opposite direction (i.e.
inverse) when employing a precise measure of overallPA.
Few studies have examined the prospective association between objectively measured sedentary time andhealth outcomes in young people. These studies areless prone to biases compared with self-report, andreduce the potential for differential measurement
errors. One of these studies did observe a positive
association between sedentary time and increases in
BMI above the 50th percentile on girls between ages 9
years and 15 years20) suggesting the magnitude of
association is more pronounced in those whom arecategorized as overweight or obese at the initialassessment. In contrast, no association was reported
between sedentary behavior and increased BMI
z-score from ages 7 to 9 years in UK children 21).
Others have suggested that the association between
sedentary time and gain in adiposity may be reverse.
That is, baseline sedentary time is unrelated to gain in
adiposity whereas greater adiposity level at baseline
predicts higher amounts of sedentary time at followup as observed in both children 22) and adults23).
However, even if both the latter studies assessed
sedentary time objectively differences in measurements precision between the exposure and outcome
should be considered when interpreting these results.
When the more imprecise variable is used as theoutcome, the magnitude of effect is estimated accurately, but with error. When the more imprecise
variable is used as the exposure, the measure of effectis attenuated. Because PA and sedentary time aremeasured much less precisely than is body weight, it
is not surprising that baseline body weight predicts
follow-up PA, whereas, because of measurement error,
the reverse may not be observed.

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well-designed, large-scale observational studies inchildren. However, most of the evidence has emerged
from cross-sectional association studies. For example,
Andersen et al.24) showed a graded inverse relationship between the total volume of PA measured byaccelerometry and clustered metabolic risk. The
highest risk was observed in the three lowest quintilesof PA. Interestingly, the results from this study alsosuggested that the current recommendation of 60
minutes of MVPA may be too low for reducing
cardiovascular risk factors in youth. However, due tothe cross-sectional nature of the study and the marked
variability in intensity thresholds used to define
MVPA, additional studies are required to confirm or
refute whether the current recommendations of 60
minutes of MVPA per day should be refined. Additional research has suggested that total PA assessedby accelerometry appears to be inversely associatedwith cardiovascular metabolic risk factors independent of both body fatness and aerobic fitness25).
This observation is important for public health as itsuggests that increasing overall activity levels in
populations of young people may have important
health implications regardless of whether there are anychanges in aerobic fitness or body fatness.
Another important question for preventive purposes
and public health policy relates to whether the magnitude of associations differ between different intensity
levels of PA (e.g. light, moderate and vigorous
intensity) and cardio-metabolic health outcomes inyoung people. Some recent studies have reported agreater magnitude of association between morevigorous intensity of PA and adiposity indicators inyoung people. Steele et al.26) examined the associations between various sub-components of objectively
measured sedentary time, and PA in a large sample (n= 1,862) 9-10 year old British children and observedthat the strongest and most consistent associations
between activity and adiposity indicators such as BMI,
waist circumference and fat mass was observed for
VPA. Another study examined these associations in 4
year old British pre-school children (n = 398) in which
PA was assessed by accelerometry and body composition by DXA measurements27). Time spent invigorous intensity activity was the only variableconsistently associated with all indicators of adiposity

Is Physical Activity Associated with Health
Outcomes?
Total amount of PA and time spent in MVPA areboth associated with cardio-metabolic risk factors in3Res Exerc Epidemiol 2013; 15(1): 1-6.

(BMI, waist circumference, fat mass, and trunk fatmass) after controlling for a number of confounding
factors, including time spent sedentary. This observation is indirectly supported by data suggesting that
objectively measured sedentary time is unrelated toadiposity in pre-schoolers28). Taken together, the results from these studies indicate that efforts to challenge paediatric obesity may benefit from prioritizing
vigorous intensity physical activity rather than focusing on sedentary time. If the results from the study
by Collins et al.27) are replicated the current physicalactivity recommendations for pre-schoolers 5,6) which
specify the amount ( > 180 min of activity per day)without considering the intensity of physical activity
may need to be reconsidered.
The focus on MVPA rather than sedentary time inrelation to health indicators in youth was further
acknowledged in a large pooled analysis comprisingmore than 20,000 young people between 4 and 18
years of age22). The authors examined the independentassociations between sedentary time and MVPA withcardio-metabolic risk factors including waist circumference, blood pressure, triglycerides, HDLCholesterol and insulin sensitivity. Time in MVPA
was associated with all metabolic outcomes independent of sex, age, monitor wear time, time spentsedentary and waist circumference (when this variable
was not the outcome). Time spent sedentary was not
associated with any of the metabolic outcomes after
adjusting for time in MVPA. The differences inoutcomes between higher and lower MVPA weregreater with lower sedentary time. Mean differences inwaist circumference between the bottom and top
tertiles of MVPA were 5.6 cm (95%CI, 4.8-6.4 cm) for
high sedentary time and 3.6 cm (95%CI, 2.8-4.3 cm)
for low sedentary time. Mean differences in systolicblood pressure for high and low sedentary time were
0.7 mmHg (95%CI, −0.07 to 1.6) and 2.5 mmHg
(95% CI, 1.7-3.3), and for high-density lipoproteincholesterol, differences were −2.6 mg/dL (95%CI,−1.4 to −3.9) and −4.5 mg/dL (95%CI, −3.3 to −5.6),
respectively. These differences between tertiles for
MVPA are substantial and similar to what have been
observed following an extensive lifestyle interventionin adults with type 2 diabetes29). It therefore appears
appropriate to encourage higher amounts of physical

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activity of at least moderate intensity in youth anddownplay the importance of reducing sedentary time.

Perspectives and Future Directions
The use of accelerometry for assessing time spentsedentary and at various intensity levels of PA has
increased our knowledge of population levels ofsedentary time and activity. Further, consistentassociations with health outcomes have been
established. However, there is still a paucity of data
examining the prospective associations between
sedentary time, light, moderate, and vigorous intensity
physical activity with cardio-metabolic and other
health outcomes in youth. The appropriate amount oftime in MVPA needed to prevent cardiovascular
dysfunction in young people needs to be determined
and the potential benefits of more vigorous intensity
PA established.

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