We are told that phys ed in schools is a good idea.
This blog shares some recent research showing that there are indeed benefits.
But it is pretty useless when it comes to treating childhood obesity. I intentionally posted the complete research. Read the summary if you only want the bottom-line.
Here are the facts:
Effectiveness of intervention on physical activity of
children: systematic review and meta-analysis of
controlled trials with objectively measured outcomes
Brad Metcalf research fellow and statistician 1, William Henley professor of medical statistics 2,
Terence Wilkin professor of endocrinology and metabolism 1
1Department of Endocrinology and Metabolism, Peninsula College of Medicine and Dentistry, Plymouth University Campus, Plymouth, UK; 2Institute
of Health Services Research, Peninsula College of Medicine and Dentistry, University of Exeter Campus, Exeter, UK
Objective To determine whether, and to what extent, physical activity
interventions affect the overall activity levels of children.
Design Systematic review and meta-analysis.
Data sources Electronic databases (Embase, Medline, PsycINFO,
SPORTDiscus) and reference lists of included studies and of relevant
Study selection Design: randomised controlled trials or controlled clinical
trials (cluster and individual) published in peer reviewed journals.
Intervention: incorporated a component designed to increase the physical
activity of children/adolescents and was at least four weeks in duration.
Outcomes: measured whole day physical activity objectively with
accelerometers either before or immediately after the end of the
Data analysis Intervention effects (standardised mean differences) were
calculated for total physical activity, time spent in moderate or vigorous
physical activity, or both for each study and pooled using a weighted
random effects model. Meta-regression explored the heterogeneity of
intervention effects in relation to study participants, design, intervention
type, and methodological quality.
Results Thirty studies (involving 14 326 participants; 6153 with
accelerometer measured physical activity) met the inclusion criteria and
all were eligible for meta-analysis/meta-regression. The pooled
intervention effect across all studies was small to negligible for total
physical activity (standardised mean difference 0.12, 95% confidence
interval 0.04 to 0.20; P<0.01) and small for moderate or vigorous activity
(0.16, 0.08 to 0.24; P<0.001). Meta-regression indicated that the pooled
intervention effect did not differ significantly between any of the
subgroups (for example, for total physical activity, standardised mean
differences were 0.07 for age <10 years and 0.16 for ≥10 years, P=0.19;
0.07 for body mass index across the entire range and 0.22 for exclusively
overweight/obese children, P=0.07; 0.12 for study duration ≤6 months
and 0.09 for >6 months, P=0.71; 0.15 for home/family based intervention
and 0.10 for school based intervention, P=0.53; and 0.09 for higher
quality studies and 0.14 for lower quality studies, P=0.52).
Conclusions This review provides strong evidence that physical activity
interventions have had only a small effect (approximately 4 minutes
more walking or running per day) on children’s overall activity levels.
This finding may explain, in part, why such interventions have had limited
success in reducing the body mass index or body fat of children.
Physical activity is associated with many health benefits,1-3 but
most children fail to meet national recommendations.4-6
Prevention of obesity in particular is thought to be one of the
benefits to being more active, and accordingly most
interventions aimed at reducing childhood obesity incorporate
a physical activity component. Observational studies consistently
show that greater activity is associated (r~−0.2) with lower
body mass index and girth,7-11 yet physical activity interventions
to date have been largely unsuccessful in improving the body
mass index or body composition of children,12-14 and
understanding the reasons for this is important. One explanation,
for which some evidence exists,15 is that more activity leads to
higher calorie consumption, offsetting any gains in energy
expenditure. Another, yet more fundamental, is that physical
activity interventions do not increase the activity of children
sufficiently for it to affect adiposity.
To date, all systematic reviews that have explored the
effectiveness of physical activity interventions on the activity
Correspondence to: B Metcalf email@example.com
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levels of children have had important limitations. For example,
the reviews did not confine their analyses to whole day activity
or to objectively measured outcomes,12 16 17 although both may
be vital to their interpretation. These reviews included a large
proportion of studies that relied on questionnaires, which offer
a poor surrogate for objectively measured physical activity
(r~0.1-0.3).18-20 More importantly, questionnaires are liable to
bias through expectancy, raised awareness, or both.21
Accelerometers, by contrast, are considered the criterion method
for measuring free living activity and correspond well to activity
related energy expenditure.22 23 The existing reviews also include
studies that measured activity only during intervention specific
periods of the day (for example, physical education classes,
break/recess times, school hours only), and the effects observed
during such periods may not be representative of the
intervention’s effect on whole day physical activity.
The primary aim of this review was to determine whether, and
to what extent, physical activity interventions increase the
overall activity of children, using a meta-analysis approach. The
second aim was to examine the effects of the intervention in
relation to potential study level covariates by using
meta-regression. The review follows the PRISMA guidelines
for the preferred conduct and reporting of systematic reviews
Studies had to meet all of the following criteria for inclusion in
Population—Participants had to be aged 16 years or younger.
Intervention—The intervention must have incorporated a
component that aimed to increase physical activity. The control
condition must not have incorporated an activity/exercise related
element of any kind. The intervention period needed to be at
least four weeks.
Outcomes—Physical activity must have been measured
objectively by using accelerometry in the same participants at
both baseline and follow-up. We included only studies that
measured “follow-up physical activity” before or immediately
after the end of the intervention period (within four weeks).
Whole day activity had to be reported as total physical activity
volume (usually expressed as “mean accelerometer
counts/minute”), time spent in moderate or vigorous intensity
activity (time spent in activities of at least the intensity of brisk
walking, usually expressed as “minutes/day” or “proportion of
day”), or both. We excluded studies that reported only “part
day” activity levels.
Study design—Studies had to be randomised controlled trials
or controlled clinical trials (cluster or individual).
We searched four electronic databases (Embase, Medline,
PsycINFO, and SPORTDiscus) for articles published in peer
reviewed journals from January 1990 to early March 2012. We
imposed no language restriction. We used search terms relevant
to childhood (child*, youth, adolescen*, juvenile, teen*, infant*,
boy*, girl*), an activity intervention (physical activity, activ*,
exercis*, training, sedentary, obesity, overweight, BMI), type
of trial (randomised, controlled, trial, RCT, intervention), and
an objective measure of activity (objective*, acceleromet*,
activity monitor, actigraph, MTI, CSA, actical, actiheart, tritrac,
unidimensional, triaxial, MVPA). Two reviewers independently
assessed the abstracts retrieved from this electronic search for
potentially eligible studies. When the abstract clearly showed
that the study was not suitable, the study was discarded. We
obtained full text for studies that seemed to be potentially
eligible and also for those that could not be excluded on the
basis of the abstract information alone. We compared the
abstracts selected for full text retrieval by each reviewer at this
stage and resolved any discrepancies by discussion. The same
two reviewers independently assessed the full text articles and
selected studies that met the inclusion criteria, again resolving
any discrepancies by discussion. We also searched the reference
lists of relevant review articles and of the articles that met the
inclusion criteria. Finally, we checked the articles that met the
inclusion criteria for duplicate reporting of the same data.
One reviewer extracted information on study level covariates
and the results of the trial, and a second reviewer checked it.
We extracted information on study participants (percentage of
girls, baseline age, body mass index, and activity level), size,
duration, setting of intervention (for example, home, school, or
community based), the physical activity component of the
intervention, the method for measuring activity (type and model
of accelerometer, requested number of days the accelerometers
were to be worn, and when follow-up measure was taken: during
or after the intervention), and dimensions of study quality/risk
of bias (randomisation, proportion lost to follow-up, analysed
with intention to treat approach) as potential effect modifiers.
We extracted result specific information for total physical
activity, moderate or vigorous physical activity, or both. The
type of data extracted differed according to the way in which
the results were reported. Where available, we extracted relevant
model statistics (F statistic, t statistic, or P value), “group ×
time” model coefficients, the between group mean difference
(difference in “activity change from baseline” or difference in
“follow-up activity” controlled for baseline activity), or the
within group means (mean change from baseline or raw means
at both baseline and follow-up). For model derived statistics or
coefficients, we extracted information on the covariates
wherever possible. We also extracted the corresponding
measures of precision (standard deviations, standard errors, or
95% confidence intervals) for all means and coefficients. Finally,
we extracted the total number of participants involved in each
study and the number who provided valid accelerometer data.
Calculating effect sizes
Not all studies used the same type of accelerometer. As a result,
the units for both outcomes (total physical activity and moderate
or vigorous physical activity) were not the same across all
studies, so we standardised them. As both outcome measures
were continuous variables, the intervention effects of each study
were represented by the standardised mean difference in
outcome, calculated for this review by dividing the between
group difference in mean activity change from baseline (or
follow-up activity controlled for baseline activity) by the pooled
standard deviation of activity change from baseline. We used
the “MAd” package for R statistical software (R.2.13.0) to
calculate the effect size (its variance and 95% confidence
interval) for each study by using Hedges’ g.25 When a trial
reported the within group means and standard deviations at
baseline and follow-up separately, we assumed a correlation of
r=0.54 26 27 between baseline and follow-up activity to estimate
the standard deviation for activity change from baseline. Where
studies reported only one of either total physical activity or
moderate or vigorous physical activity, we emailed the
corresponding author requesting the other. Where moderate
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intensity activity and vigorous intensity activity were reported
separately, we combined them to obtain measures of moderate
or vigorous activity.
Where studies reported multiple comparisons (such as for girls
and boys, for two time points, for groups receiving different
interventions, or for two different cohorts), we calculated a
standardised mean difference for each comparison separately.
Where a study did not report the combined effect and variance,
we calculated the weighted mean of the multiple effects as
described by Borenstein and colleagues.28 For comparisons that
were not independent of one another (for example, multiple
time points in the same cohort), we adjusted the combined
variance for the degree of correlation between the comparisons,
again assuming it to be r=0.5 if not stated.4 26 27 We extracted
and analysed only comparisons based on outcomes measured
during or immediately after the intervention; measures taken
more than one month after the end of the intervention period
were not included in the analysis.
Calculating an overall summary effect size
We combined the effect sizes of all the included studies to
estimate an overall summary effect size (and 95% confidence
interval) for both total physical activity and moderate or
vigorous physical activity by using a random effects
meta-analysis model weighted by the inverse of the effect
variances (MAd package in R.2.13.0). An overall standardised
mean difference of ~0.2 is considered small, ~0.5 is moderate,
and ~0.8 is large.29 We assessed heterogeneity among the study
effects by a visual inspection of the forest plots and by the I2
statistic. We used both to help to determine whether continuing
with our secondary aim—examining study level covariates as
possible sources of heterogeneity—was appropriate.
Effect sizes relative to study level covariates
For this analysis, the studies were not necessarily represented
by a single effect size. We did not pool multiple comparisons
within a study, considering them to be independent samples.
The notable exception was for multiple time points, for which
the two effects are clearly not independent of each other. We
pooled multiple time points for all analyses except that which
explored “duration” as a potential effect modifier.
We grouped all comparisons according to baseline
characteristics of the study population (sex: exclusively boys
and exclusively girls; age: <10 years and ≥10 years; body mass
index status: all categories and exclusively overweight/obese;
activity level: total physical activity <500 counts/minute and
≥500 counts/minute); study design (duration: ≤6 months and
>6 months; study size: n<200 and n≥200; methodological
quality: quality score ≤2/3 and quality score=3/3; timing of
follow-up: during intervention and after intervention); and the
type of intervention (setting: home/family based and school
based; activity sessions provided as part of intervention: yes/no).
We entered each of these dichotomous variables into a weighted
random effects meta-regression model separately (“metafor”
package in R.2.13.0). We then entered all variables into a
multivariate meta-regression model using a backward
elimination approach with a removal criterion of P>0.05. In
addition, we analysed the covariates that we extracted as
continuous variables to test whether the relation was linear and
consistent with the findings of the categorical analysis. We
calculated the proportion of total between study variance
explained by the model and reported it as R2.30
We used two recommended forms of sensitivity analysis to
verify the reliability of the meta-analysis results.31 Firstly, we
varied the assumed correlation of r=0.5 between baseline and
follow-up activity from r=0.3 to r=0.7 for all relevant studies
to see if this altered the overall summary estimates. Secondly,
we did quality specific subgroup analysis to assess whether the
overall intervention effect changed when studies deemed to be
of “lower” methodological quality were excluded.
We assessed publication bias by using two funnel plots, both
regarding total physical activity. The first plotted the
standardised mean differences, and the second plotted the
residuals produced from the meta-regression model containing
the associated study level covariates. We did the second one to
test whether any asymmetrical scatter of the mean differences,
indicative of publication bias, could alternatively be explained
by differences in studies’ characteristics.
The electronic search identified 344 potentially eligible reports
(fig 1⇓). We were able to exclude 286 of these on the basis of
the title or abstract alone. Of the 58 articles for which we
retrieved full text, a further 30 were excluded. We found two
additional eligible articles from searching the reference lists of
the 28 eligible articles and of relevant review articles, bringing
the total number of studies to 30.32-61
All eligible articles had been published between May 2003 and
December 2011. The studies varied in size, duration, and
intervention type (see web extra table). Study sizes ranged from
18 to 2840 participants (median 307, total 14 326), and the
number of children providing accelerometer based measures of
physical activity in each study ranged from 18 to 1138 (median
165, total 6153). The duration of follow-up ranged from 4 weeks
to 140 weeks (median 26 weeks); nine studies reported an
intervention effect at two time points (mid-intervention and end
of intervention). Seventeen of the trials were school based, 10
were home/family based, one was community centre based, one
was university gym based, and one was boy scout centre based.
Nineteen studies provided activity/exercise sessions as part of
the intervention, and 11 did not.
Of the 6153 children with appropriately measured activity, 3232
were girls and 2921 were boys. Two studies included girls
only,43 54 two included boys only,42 55 and 26 included both girls
and boys (the proportion of girls ranged from 27% to 64%,
median 51%), although only four of these reported sex specific
results.37 39 45 46 The mean age at baseline varied from 1.8 to 13.1
years (median 9.8 years). Eight studies involved exclusively
overweight/obese participants (n=691), and the rest involved
children recruited from all body mass index categories.
Types of outcome measures
Of the 30 eligible studies, 21 reported total physical activity,
23 reported moderate or vigorous physical activity, and 14
reported both. Where an article reported only one of these
outcomes, we emailed the corresponding author for the other.
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Six authors responded, increasing the number of studies for
which we could analyse total physical activity to 25 (n=4386)
and moderate or vigorous physical activity to 25 (n=5001).
Twenty-six studies used uni-axial accelerometers to measure
activity (23 with the CSA/MTI Actigraph, two with the IM
Systems BioTrainer, and one with the Mini-Mitter Actical
uni-directional model), one study used a bi-axial accelerometer
(BodyMedia SenseWear Pro2 Armband), and three studies used
tri-axial accelerometers (two with the Hemokinetics
TriTrac-R3D and one with the Mini-Mitter Actical
omni-directional model). The accelerometer data collection
period varied according to study from one to 21 days. Sixteen
studies stipulated at least six days, which is the minimum
required to achieve at least 80% reliability in the measure of
Methodological design and quality
We deemed 16 studies (n=3883) to be of “high” methodological
quality, as they scored positively on all three of the quality
criteria described above. Of the other 14 studies (n=2270), 12
scored positively on two of the quality criteria and two scored
positively on just one. Losses to follow-up ranged from 0% to
46% (median 11%); 20 studies reported less than 20% attrition.
All but three of the studies carried out intention to treat analysis.
Twenty-seven studies were randomised controlled trials; the
remaining three were controlled clinical trials. For two of the
controlled clinical trials, the intervention schools and control
schools were matched appropriately on age, ethnicity, and
socioeconomic status. The other trial selected schools from areas
that were “broadly comparable” on several socio-demographic
variables but did adjust for age, sex, and baseline activity in the
analyses. Fifteen of the included studies were cluster designs,
and all but two of these accounted for clustering in their
analyses. For these two studies, we multiplied the reported
variances by a “design factor” (1+((m−1)×ICC), where m is the
average number of participants in each cluster and ICC is the
intra-cluster correlation),63 which decreased their respective
weightings (towards the pooled effect) from 2.9% to 1.1% and
from 0.9% to 0.5%.
Overall summary estimate
The pooled analysis across all studies showed a statistically
significant effect in favour of the intervention group for both
total physical activity (standardised mean difference 0.12, 95%
confidence interval 0.04 to 0.20; P<0.01) and moderate or
vigorous physical activity (0.16, 0.08 to 0.24; P<0.001), although
both seemed to be of limited clinical significance according to
Cohen’s definitions.29 Hence, strong evidence exists from this
analysis to suggest that physical activity interventions have a
small to negligible effect on overall total physical activity and
a small effect on moderate or vigorous physical activity. Figure
2⇓ shows a forest plot of the effect sizes, confidence intervals,
and percentage weighting for both activity outcomes for the
individual studies and for all studies pooled. The largest
weighting of any individual study for total physical activity was
11.2%, and for moderate or vigorous physical activity it was
17.3%. A visual inspection of the forest plots suggests that a
degree of between study variation exists in terms of the effect
sizes. I2 values of 38% for total activity and 51% for moderate
or vigorous activity confirmed moderate levels of
Effect sizes relative to study level covariates
We used 31 intervention effects (standardised mean differences)
for total physical activity and 33 for moderate or vigorous
physical activity in the analyses of all but three of the study
level covariates. The three exceptions were for duration (38 for
total physical activity and 42 for moderate or vigorous activity),
sex (9 and 13), and baseline activity (20 and 18). For all but
two of the covariates, the degree of heterogeneity did not differ
considerably between the subgroups. For study size and duration,
the I2 values were very different between the subgroups (for
example, for total physical activity, I2=0% for larger studies and
58% for smaller studies, and I2=0% for longer term studies and
50% for shorter term studies). In general, the subgroup specific
I2 values were slightly lower for total physical activity than for
moderate or vigorous physical activity (median I2=22% and
33% with interquartile ranges of 0-40% and 12-48%). Figure
3⇓ shows the pooled intervention effects (with 95% confidence
intervals) for the subgroups of each of the 10 study level
covariates analysed. None of the differences reached statistical
significance, although one was borderline. For total physical
activity, the standardised mean difference was 0.15 greater for
studies of overweight/obese only populations than for studies
with populations recruited from all body mass index categories
(P=0.07). Multivariate meta-regression confirmed that even
when considered simultaneously, none of the study level
covariates reached statistical significance (all P>0.05).
Figure 4⇓ shows the strength of linear association between the
intervention effects on total physical activity and each of the
continuous study level covariates (mean age, mean body mass
index, study size, and study duration). Body mass index z score
explained 7% of the variation in effect sizes (P=0.04) for which,
on average, the standardised mean difference was 0.074 greater
for each 1.0 increase in z score in mean body mass index.
However, this association did not exist across the range of mean
z scores—when we repeated the analysis without studies of
overweight/obese only participants (mean z score ≥2.4), the
association disappeared completely (β=0.008, P=0.69, R2=0.00).
This observation confirms the appropriateness of analysing body
mass index status as a categorical variable. None of the other
continuous covariates was significantly associated with the size
of the intervention effects (all P>0.18).
The findings of this meta-analysis were robust to the
assumptions that we made about the correlation between baseline
and follow-up activity. When the correlation was assumed to
be lower, at r=0.3, the resulting summary effects were
standardised mean differences of 0.11 for total physical activity
and 0.15 for moderate or vigorous physical activity. When the
correlation was assumed to be higher, at r=0.70, the summary
effects were standardised mean differences of 0.12 for total
physical activity and 0.17 for moderate or vigorous physical
activity. The meta-analysis results were also robust to the
inclusion of studies of lower methodological quality, as the
summary effects differed little when we excluded such studies
(based only on studies with a quality score of 3/3, standardised
mean differences were 0.09 for total physical activity and 0.11
for moderate or vigorous physical activity; based only on
randomised controlled trials, standardised mean differences
were 0.11 for total physical activity and 0.13 for moderate or
vigorous physical activity; based only on studies with <20%
loss to follow-up, standardised mean differences were 0.11 for
total physical activity and 0.15 for moderate or vigorous physical
activity; based only on studies that carried out intention to treat
analysis, standardised mean differences were 0.11 for total
BMJ 2012;345:e5888 doi: 10.1136/bmj.e5888 (Published 27 September 2012) Page 4 of 11
physical activity and 0.13 for moderate or vigorous physical
The funnel plots of the standardised mean differences and of
the residuals (produced from the meta-regression model
containing body mass index status) both showed a slightly
asymmetric scatter consistent with publication bias (fig 5⇓).
However, we cannot rule out the possibility of the “small study
effect,”65 as the asymmetry was attributable to the absence of
just two or three small studies producing negative effects.
Whichever is the case, the absence of some small negative
studies is likely to have had little effect on our findings, given
that the removal of small studies producing large positive effects
reduced the summary effect only from a standardised mean
difference of 0.12 to 0.09 for total physical activity and from
0.16 to 0.13 for moderate or vigorous physical activity.
This systematic review found that physical activity interventions,
on average, achieved small to negligible increases in children’s
total activity volume, with small improvements in the time spent
in moderate or vigorous intensity activities (~4 minutes more
walking or running per day), the clinical effect of which is likely
to be minimal (for example, ~2 mm in waist circumference or
~0.06 mm Hg in systolic blood pressure).11 This review also
shows that the pooled intervention effect did not differ
significantly according to subgroups of study level
characteristics. Although trials of exclusively overweight/obese
participants tended to be slightly more effective at increasing
total activity than did those that recruited children from all body
mass index categories, the pooled intervention effect for this
group of studies was still small.
Understanding why past attempts to increase children’s activity
have largely proved unsuccessful might help to improve future
attempts. Where interventions have failed to increase activity
or reduce body fat, authors have speculated about poor delivery
or poor uptake of the activity sessions,46 or suggested that the
physical activity component of the intervention was not
sufficiently intense.34 However, although these reasons are
intuitive and plausible, they are difficult to test. An alternative
explanation could be that the intervention specific exercise
sessions may simply be replacing periods of equally intense
activity. For example, after school activity clubs may simply
replace a period of time that children usually spend playing
outdoors or replace a time later in the day/week when the child
would usually be active.
Strengths and limitations
This review, uniquely we believe, focuses exclusively on
objectively measured, whole day physical activity.
Accelerometers are considered the criterion method for
measuring free living physical activity,22 23 and the avoidance
of studies using self reported measures meant avoidance of the
bias associated with them.21 Taber and colleagues showed that
for a given level of accelerometry derived activity, self reported
levels of activity were significantly higher in the intervention
group than in the control group.21 Although accelerometers are
accepted as being limited in their ability to measure water based
activities and cycling, such limitations are unlikely to have
biased the results given that none of the interventions was
specific to promoting swimming or cycling.
Our review also excludes studies that reported only activity
differences during intervention specific parts of the day, as effect
sizes derived from such short periods risk an over-estimation
of whole day effects. This can be demonstrated with the results
of the KISS study, in which the school time intervention effect
was more than four times greater than the whole day intervention
effect (total physical activity z scores 0.92 v 0.21).32 The reviews
by van Sluijs et al and Salmon et al incorporated studies that
reported any component of activity or inactivity, including those
that reported only “sedentary time” as their outcome.16 17 Our
review was specific to total physical activity and time spent in
moderate or vigorous activity, as these are strongly correlated
with activity related energy expenditure.22 23 We did not analyse
sedentary time, as this seems to be a poor surrogate measure of
Inevitably, some studies failed on just one of the inclusion
criteria but otherwise might have made a useful contribution to
the aim of this review. We excluded the TAAG trial because
the baseline and follow-up measures were not paired but were
collected from essentially independent samples.67 Nevertheless,
its largest intervention effect (+1.6 minutes/day of moderate or
vigorous physical activity, standardised mean difference ~0.06)
was comparable to the findings of our review.67 Three studies
that had accelerometer data at follow-up were excluded because
baseline activity was measured by questionnaire only.68-70 All
three produced effects sizes that were similar to the pooled
estimates obtained from this meta-analysis (the STOPP study:
standardised mean difference 0.1168; the LEAP and LEAP2
studies (both exclusive to overweight/obese children):
standardised mean difference ~0.269 70). We note these data to
show that the findings of our review would not have differed
had they been incorporated.
Many of the potential effect modifiers that we chose to explore
were evidence generated. For example, sex, age, and body mass
index have all been shown to be associated with objectively
measured physical activity.10 71 Duration, study size, and
mid-intervention outcomes were also positively associated with
greater reductions in sedentary time according to a previous
meta-analysis.12 The reporting of information on the dose,
frequency, and content of activity/exercise sessions varied so
much between trials that we were obliged to dichotomise it
simply as “provision of designated activity/exercise sessions:
yes/no” for the purpose of analysis, being aware nevertheless
that resolution of the measure might be compromised in the
process. We acknowledge that within study subgroup effects
would have provided a more powerful method of examining
subgroup differences (that is, testing whether the intervention
effect differed according to categories of risk—for example, by
baseline activity or body mass index status). However, of the
studies we inspected for this review, none reported such
“intervention × risk” analyses.
Comparisons with other recent reviews
The findings of this review are consistent with those of a
previous meta-analysis by Kamath and colleagues.12 That review
was primarily concerned with the effect of behavioural
interventions on the prevention of childhood obesity. It also
explored the effect of physical activity interventions on activity
levels, reporting an overall effect identical to the summary effect
calculated in our meta-analysis (standardised mean difference
0.12), despite only three studies being common to both. The
studies included in the review by Kamath et al were not specific
to whole day activity, nor to accelerometry, and half the studies
relied on questionnaire based physical activity. The authors
excluded studies in overweight/obese only populations,12 and,
BMJ 2012;345:e5888 doi: 10.1136/bmj.e5888 (Published 27 September 2012) Page 5 of 11
when such studies are removed from our meta-analysis, the
overall summary effect for total activity falls to a standardised
mean difference of 0.07. Two other systematic reviews
summarising the effectiveness of interventions on physical
activity were published in 2007 and reported similar findings
to one another.16 17 Salmon et al reviewed 76 studies and van
Sluijs et al reviewed 57, of which 40-50% were deemed to have
had a positive and statistically significant effect on activity.
Both reviews reported a higher proportion of positive
intervention effects in studies that measured activity objectively,
compared with those that measured it by questionnaire.
However, both considered “direct observation” to be an
objective measure, and this may have inflated the number of
positive results, given that direct observation is used only to
measure short, intervention specific periods of the day (such as
physical education classes or break/recess times).
Context, implications for health policy, and
In the mind of the public, physical inactivity is a major cause
of childhood obesity, and the need to increase activity is
intuitive. However, the small increase in physical activity that
emerges from formal interventions seems insufficient to improve
the body mass/fat of children. Understanding why physical
activity interventions fail to increase activity sufficiently is
important, particularly as activity is associated with better
metabolic health independently of fatness.11 Accelerometers
relate minute by minute data to clock time, so future studies
could capture both whole day activity and the activity related
to intervention specific periods. Such an approach could not
only measure the real time activity response to an intervention
but could also establish whether the response was subsequently
subject to a degree of replacement or compensation.72 Also,
future physical activity intervention studies might usefully do
within study risk group analyses to examine whether the
intervention does indeed achieve the intended response in those
children who stand to benefit the most.
Systematic reviews and meta-analysis are not without their
critics, but they nevertheless represent the best available
evidence to government strategists. However counterintuitive
or discomforting it may be, strong evidence from this review
shows that physical activity interventions have little effect on
the overall activity of children. Organised physical activity may
nevertheless still offer benefits such as improved coordination
skills, greater self confidence, team participation, and social
Physical activity interventions have little effect on the overall
activity levels of children, which may explain, at least in part,
why such interventions have had a limited effect on body mass
index or body fat. The outcome of this meta-analysis questions
the contribution of physical activity to the prevention of
Contributors: BM was involved in the design, research, analyses, and
writing of this article. WH was involved in the analysis. TW was involved
in the design, research, and writing. TW is the guarantor.
Funding: EarlyBird (BM and TW) is currently supported by the Bright
Future Trust, the Kirby Laing Foundation, the Peninsula Foundation,
and the EarlyBird Diabetes Trust. These funding sources had no role
in the study design, data collection, analysis, interpretation of results,
or the writing of the report. WH was supported by the National Institute
for Health Research (NIHR) Collaborations for Leadership in Applied
Health Research and Care (CLAHRC). The views expressed in this
publication are those of the authors and not necessarily those of the
NHS, the NIHR, or the Department of Health.
Competing interests: All authors have completed the Unified Competing
Interest form at www.icmje.org/coi_disclosure.pdf (available on request
from the corresponding author) and declare: no support from any
organisation for the submitted work; no financial relationships with any
organisations that might have an interest in the submitted work in the
previous three years; no other relationships or activities that could appear
to have influenced the submitted work.
Ethical approval: Not needed.
Data sharing: No additional data available.
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What is already known on this topic
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outcomes and less obesity
Physical activity interventions nevertheless have little effect on the prevention of childhood obesity, and the reason for this has been
What this study adds
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have had limited success in preventing childhood obesity
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Accepted: 21 August 2012
Cite this as: BMJ 2012;345:e5888
This is an open-access article distributed under the terms of the Creative Commons
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BMJ 2012;345:e5888 doi: 10.1136/bmj.e5888 (Published 27 September 2012) Page 7 of 11
Fig 1 Summary of study selection process
BMJ 2012;345:e5888 doi: 10.1136/bmj.e5888 (Published 27 September 2012) Page 8 of 11
Fig 2 Forest plot showing standardised mean difference in change in physical activity between intervention and control
groups for each included study. NA=not available
BMJ 2012;345:e5888 doi: 10.1136/bmj.e5888 (Published 27 September 2012) Page 9 of 11
Fig 3 Standardised mean differences in changes in physical activity between intervention and control groups by subgroups
of studies. BMI=body mass index; cpm=counts per minute; PA=physical activity
Fig 4 Intervention effects in total physical activity in relation to continuous study level covariates: age at baseline (top left),
body mass index (BMI) at baseline (top right), study size (bottom left), and study duration (bottom right)
BMJ 2012;345:e5888 doi: 10.1136/bmj.e5888 (Published 27 September 2012) Page 10 of 11
Fig 5 Assessment of publication bias: funnel plots of observed effects in total physical activity for each study (top) and
meta-regression residuals of total physical activity for each comparison with body mass index status as a study level
BMJ 2012;345:e5888 doi: 10.1136/bmj.e5888 (Published 27 September 2012) Page 11 of 11