Maximum oxygen uptake and objectively measured
physical activity in Danish children 6–7 years of age: the
Copenhagen school child intervention study
S Eiberg, H Hasselstrom, V Grønfeldt, K Froberg, J Svensson, L B Andersen
See end of article for
Department of Health,
Norwegian University of
Sport and Physical
Education, Oslo 0806,
Accepted 29 March 2005
Br J Sports Med 2005;39:725–730. doi: 10.1136/bjsm.2004.015230
Objectives: To provide normative data on maximum oxygen uptake (V ˙O2MAX) and physical activity in
children 6–7 years of age and analyse the association between these variables.
Methods: V ˙O2MAX was measured in 366 boys (mean (SD) 6.8 (0.4) years of age) and 332 girls (6.7 (0.4)
years of age) from preschool classes in two suburban communities in Copenhagen, during a progressive
treadmill exercise. Habitual physical activity was measured with accelerometers.
Results: Boys had higher V ˙O2MAX both in absolute values (1.19 (0.18) v 1.06 (0.16) litres/min (+11%),
p,0.001) and relative to body weight (48.5 (6.0) v 44.8 (5.6) ml/kg/min (+8%); p,0.001) than girls.
The difference in V ˙O2MAX between boys and girls decreased to +2% when expressed relative to lean body
mass (LBM). Absolute V ˙O2MAX was related to LBM, body mass, and stature (all p,0.001). Boys were more
physically active than girls (mean counts +9.4%, p,0.001), and even when boys and girls with the same
V ˙O2MAX were compared, boys were more active. The difference in physical activity between the sexes was
higher when sustained activity of higher intensity was compared.
Conclusions: V ˙O2MAX is higher in boys than girls (+11%), even when related to body mass (+8%) and LBM
(+2%). Most of the difference in V ˙O2MAX relative to body mass was explained by the larger percentage
body fat in girls. When boys and girls with the same V ˙O2MAX were compared, boys engaged in more
minutes of exercise of at least moderate intensity.
recommended that they stay physically active and fit, because
a high physical activity (PA) level may prevent future illness.2
A high level of physical fitness has been associated with a
decreased risk of cardiovascular disease (CVD) in the adult
population.3Even in children, risk factors for CVD have
been identified,4and physical fitness also seems to have
an effect on the level of risk factors in children.5Even
though children do not suffer from CVD, it is recommended
that children are physically active and fit, as this may
help to prevent the disease in the future. Therefore it is
important to assess physical fitness at an early age.
Maximum oxygen uptake (V ˙O2MAX) is probably the best
index of physical fitness and has been studied intensively in
adults.6However, measurements of V ˙O2MAX in 6–7 year old
children are sparse, and representative data do not exist.6–11
Even in older children and adolescents, very few population
based studies exist.10 12 13The reasons for this include
difficulties in testing young children. Ethical considerations,
safety factors, and equipment made for adults render testing
with young children more challenging. Another health
related measure of children is their level of PA, as PA has a
positive effect on metabolism.14Most studies have assessed
PA by self report, but accelerometers provide a robust
measure of habitual PA.15No population study has analysed
the association between V ˙O2MAX and PA assessed by
V ˙O2MAX is consistently higher in boys than in girls even
before puberty.16This has been attributed to a different body
composition and a larger stroke volume of the heart in boys.
However, even though boys have been shown to be more
active than girls, the difference is too small to explain the
difference in V ˙O2MAX.8 17 18
he fitness level in children in the western world has
declined, but few data are available.1
children do not suffer from lifestyle diseases, it is
The aim of this study was to provide population data on
V ˙O2MAX and PA measured using accelerometry in 6–7 year old
children from Copenhagen, and also explore potential sex
differences and analyse the association between PA and
V ˙O2MAX. We hypothesised that boys have higher V ˙O2MAX than
girls, and that this difference may be the result of differences
in their levels of fat and PA.
Children from 46 preschool classes (6–7 years of age) in 18
schools in two suburban communities in the Copenhagen
area were invited to participate in the Copenhagen school
child intervention study. In 2000 the community of Ballerup
(10 schools, 27 classes) increased the number of PE lessons
from two to four a week for the first three years of school. The
community of Taarnby (eight schools, 19 classes) was chosen
as a control as it resembles Ballerup in sociodemographics. A
total of 706 children (69% of those eligible) volunteered for
the study, and written informed consent was obtained from
the parents/guardians. Of these 706 children, 415 from
Ballerup and 291 from Taarnby participated. The ethics
committee of Copenhagen county approved the study. The
tests were performed from December 2001 until May 2002 at
the 18 different schools involved. The exercise test was
performed using permanently installed equipment in a
camper trailer. All other physiological tests were performed
in a gym or a classroom. All tests were performed before noon
PA was generally measured one week after the other tests.
At schools with more than 50 participating pupils, two
weeks of PA measurements were necessary. The 31% not
Abbreviations: BMI, body mass index; CVD, cardiovascular disease;
LBM, lean body mass; PA, physical activity; SFS, sum of four skinfolds;
V ˙O2MAX, maximum oxygen uptake
participating were analysed about one year later. The first
systematic medical examination of children in Denmark
takes place one and a half years after they start school, so we
were unable to gather data from non-participants in their
first year. There were no significant differences between
groups with respect to age, height, weight, and body mass
index (BMI) for either sex. The analysis included 612
participants and 277 non-participants.
Height was measured by a Harpenden stadiometer to the
nearest 1 mm. Body weight was measured to the nearest
0.1 kg using a SECA electronic scale. Bicipital, tricipital,
subscapular, and suprailiac skinfolds were measured with a
Harpenden skinfold caliper according to criteria presented by
De Lorenzo et al.19The dominant side of the body was
determined by asking the child to take a pen and write his/
her name. The data shown in this study represent the mean
of three measurements taken on the non-dominant side of
the body. The sum of four skinfolds (SFS) was used as an
indicator of body fatness. Fat mass, fat percentage, and lean
body mass (LBM) were derived as:
Fat mass (kg) =0.38 6body weight + (0.30 6triceps) +
(0.87 6G) 2 9.42
where for boys G = 1 and for girls G = 2.20
Habitual PA was measured by the MTI 7164 activity
monitor (Manufactory Technology Inc, Fort Walton Beach,
Florida, USA). The monitor has been validated in several
studies and has shown both high mechanical reproducibil-
ity15and good validity with respect to free living conditions in
children.21The monitor samples acceleration at 10 Hz and
integrates counts over a time period (epoch) defined by the
user. In children, PA is characterised by short bursts of
activity. Therefore we chose an epoch of 10 seconds. The
choice of epoch limited the registration to three full days and
19 hours. To allow familiarisation, the children had the MTI
monitor put on one day before recording. It was secured
directly to the skin at the lower back using an elastic belt. The
children were instructed to wear the monitor continuously
except during water based activity or when sleeping. To
distinguish true zeros that arise as a result of below threshold
activity from zeros recorded when the MTI monitor was not
worn, the data were cleaned as follows: all MTI files were
screened for sustained periods of zero activity. Periods of
10 minutes or more with zero counts were interpreted as
‘‘MTI not worn’’ and removed from the file. In spite of
instructions, some children slept with the monitor on and
therefore had activity recorded even late at night. Therefore
we chose to control for nocturnal activity (2230 to 0600).
Given these criteria, the data were included if the child had
accumulated more than eight hours of activity a day for at
least three days. In the end, 466 children had four valid days
and 96 children had three valid days, of whom 82 had two
week days and one weekend day and 14 had one week day
and two weekend days. Fifty eight children had less than
three valid days. A mean count was calculated for each child.
Furthermore, the number of minutes in periods longer than
five or 10 minutes of sustained activity above 2000 counts/
min, which is about three METS, was calculated.
Measurement of V ˙ O2MAX
V ˙O2was determined with an AMIS 2001 Cardiopulmonary
FunctionTest System (Innovision,
Denmark). This system has been validated against the
Douglas bag system.22We used a Hans Rudolph mouth piece
with a volume of 15 ml especially designed for children.
Heart rate was measured continuously every fifth second
(Polar Sport Tester, Kempele, Finland).
We conducted a pilot study to establish a protocol that
enabled most of the children to reach exhaustion. We chose a
continuous walking and running protocol on a treadmill. The
velocity on the treadmill was initially set to 4 km/h without
inclination and kept there for the first three minutes to allow
familiarisation. At three minutes, the velocity was increased
to 8 km/h, and at five minutes the inclination was raised to
3%. At seven, nine, and 11 minutes, the inclination was
increased to 6%, 9%, and 11% respectively. If the child could
endure more, the velocity was increased to 9 km/h after
13 minutes and then 10 km/h at 15 minutes. No child
completed the last work load.
The children were instructed to run until exhaustion. One
subjective and three objective criteria were used to determine
if the test was maximal. Every child had to meet the
subjective criterion and at least one of the three objective
criteria. Criteria were chosen according to Rowland’s
recommendations.23The physiological criteria were: heart
rate .200 beats/min; respiratory exchange ratio >0.99; a
defined plateau of V ˙O2(an increase less than 2.1 ml/kg/min).
Subjective criteria were signs of intense effort such as
unsteady running pattern, sweating, facial flushing, and
clear unwillingness to continue running in spite of repeated
strong verbal encouragement.
Twenty 6–7 year old children outside the sample were
invited to do a test-retest on the peak V ˙O2 test. Written
informed consent was obtained from the parents/guardians.
The children were retested either five or seven days after the
first test. Four children did not meet the criteria on one or
both occasions, and their results were not used in the later
analysis, leaving 16 children with two valid tests. A Bland-
Altman plot showed no systematic difference between the
first and second test, suggesting no learning effect from the
first to the second test. The plot also showed that the size of
the difference was not dependent on the absolute level of
V ˙O2MAX. The typical error of measurement on the difference
between V ˙O2MAX between tests was 0.02 litre/kg.
The data were stored and analysed using SPSS 11.5.0. Data
that were not normally distributed were log transformed, and
the mean of transformed values was back transformed to
obtain the geometric mean as suggested by Altman.24V ˙O2MAX
relative to body mass was split into deciles by sex to compare
differences over the whole distribution, and V ˙O2MAX (ml/kg/
min) was calculated for each of the deciles. For a comparison
of differences between sexes, we adjusted for body size by
two methods of allometric scaling. Data from this study were
used to find the scaling factors. In the analyses, the scaling
exponent b was identified in the allometric equation Y = a1+
a2Xb, where Y is the physiological variable (V ˙O2MAX) and X is
the anthropometric scaling variable (weight or LBM). To
obtain b, both the Y and X were log transformed, and least
squares regression identified the b in the equation ln(Y) =
ln(a2) + bln(X).
Differences between sexes were tested using Student’s t
test. The relation between V ˙O2MAX and PA was assessed by
linear regression. A significance level of p,0.05 was chosen.
Table 1 shows the total number of boys and girls that entered
the study, the number of valid tests, the number of children
that stopped before exhaustion, the number of children not
fulfilling the criteria, and children who refused to do the
treadmill test. A valid V ˙O2MAX measurement was not assessed
in 114 children, because 37 failed to comply with the
approval criteria and 57 did not want to wear the V ˙O2
equipment or did not wear the equipment satisfactorily. Ten
children were unwilling to perform the test, and 10 children
were absent on all the test days. The final number of accepted
tests was 592. Analysis of children with non-valid tests was
performed for each sex separately. Time to exhaustion, heart
726 Eiberg, Hasselstrom, Grønfeldt, et al
rate, and V ˙O2MAX were lower for the 20 boys and 17 girls that
did not meet the criteria (p,0.001). The girls that did not
comply with the criteria were 2.6 cm shorter than girls who
did comply (p,0.05). Twenty eight boys and 29 girls had a
recorded time to exhaustion, but had no valid V ˙O2measure-
ments. The girls without a valid V ˙O2recording ran 42 seconds
less (p,0.05) than girls with a valid test. Differences between
all other age, anthropometric, and V ˙O2test data were non-
significant. Only data for the 592 children with a valid test
were included in the following analysis. Of the 592 children
that met the subjective criteria, 210 met the levelling off
criterion (35%), 283 (47%) met the pulse criterion, and 507
(86%) met the respiratory exchange ratio criterion.
Table 2 shows descriptive data for age, anthropometry, and
PA. Except for BMI, differences were found between boys
and girls for all variables. The girls had larger SFS (15%,
p,0.001) and fat percentage (23%, p,0.001). The boys were
older (2%, p,0.001), heavier (3%, p,0.05), taller (1%,
p,0.001), had a greater LBM (8%, p,0.001), a higher level
of PA (mean counts/min 8%, p,0.001), and more minutes in
periods of five or 10 minutes of sustained activity at a level of
2000 counts/min or greater (60% and 103% respectively,
p,0.001). It should be noted that all boys had five minute
periods of sustained activity, whereas three girls did not have
any. Further, 38 boys and 83 girls did not have any 10 minute
periods of sustained activity.
The absolute differences in the treadmill data between boys
and girls were moderate (table 3). The girls had higher
exchange ratio (5%, p,0.001), and the boys had higher
absolute V ˙O2MAX (11%, p,0.001), V ˙O2MAX relative to body
mass (8%, p,0.001), and V ˙O2MAX relative to LBM (2%,
p,0.05). V ˙O2MAX remained different (9%, p,0.001) after
p,0.001) and respiratory
allometric scaling using (body mass)0.712as scaling, but did
not differ when LBM1.105was used as the scaling factor.
Deciles in V ˙O2MAX (ml/kg/min) by sex were constructed
and then V ˙O2MAX was compared between boys and girls for
each decile (fig 1). This was done to see if the difference
between the sexes appeared over the whole distribution or if
only the lower or upper part of the distribution differed. The
whole distribution was shifted to the right in boys compared
with girls. Differences were found between all groups within
sexes using Bonferroni post hoc analysis of variance
Relation between PA and fitness
A scatterplot of V ˙O2MAX relative to body weight and the PA
variables showed a linear relation. A linear regression was
performed with mean count of PA, the number of minutes
above 2000 counts/min in five minute periods, and sex as
independent variables, and V ˙O2MAX relative to body weight as
a dependent variable.
The following equations where G = 1 for boys and G = 2
for girls were used:
Model (1) V ˙O2MAX (l/kg/min) = 49.9 – 3.8 6 G + 0.02 6
Model (2) V ˙O2MAX (l/kg/min) = 49.6 2 3.1 6G + 0.02 6
PAin 5 min periods(p,0.001).
Partial correlations for model (1) were 20.38 and 0.12 for sex
and PAmean countrespectively. Partial correlations for model
(2) were 20.25 and 0.24 for sex and PAin
To explore the relation V ˙O2MAX versus PA and fatness
further, all subjects were ranked into six fitness groups by
V ˙O2MAX (independent of sex) and the mean sums of PA level
(PAin 5 min periods) and SFS were calculated for each sex
5 min periods
Age, height, body weight, and maximal exercise data for groups with different test status for the maximal exercise test
Valid Non-validNo V ˙O2data?
Not willing to
be testedValid Non-valid No V ˙O2data?
Not willing to
Heart rate (beats/min)
Test time (min:s)
V ˙O2MAX (litres/min)
174 (8) *
179 (6) *
Values are mean (SD).
*Significant difference between valid test groups and other groups: p,0.05.
?Children performing a maximal test with no recording of V ˙O2.
`Mass was not a Gaussian distribution. Therefore data were log transformed and a mean was calculated and then back transformed to a geometric mean.
Anthropometric and physical activity data in 6–7 year old children by sex
p Value NoMean NoMean
PA (mean counts/min)
PA (minutes in 5 min bouts)
PA (minutes in 10 min bouts)
24.4 (19.8 to 34.9)
15.9 (13.5 to 20.2)
23.3 (15.3 to 51.2)
743 (452 to 1308)
122 (17 to 334)
53 (0 to 194)
23.7 (18.4 to 32.0)
15.9 (13.2 to 20.5)
27.5 (17.6 to 54.4)
679 (397 to 1062)
76 (0 to 194)
26 (0 to 94)
Values in parentheses are SD or 95% confidence interval. Mass, BMI, SFS, and PA were not normally distributed.
Therefore data were log transformed and a mean was calculated and then back transformed to a geometric mean
as described in the text. For these variables a 95% confidence interval is given.
BMI, Body mass index; SFS, sum of four skinfolds; LBM, lean body mass; PA, physical activity.
Aerobic fitness in 6–7 year old children727
within each fitness group. When PA was compared between
boys and girls with the same level of fitness, boys were more
active than girls except for the least fit decile (fig 2). In a
linear regression, the association between sex and PA level
was highly significant (r = 0.20, p,0.001) even after
adjustment for fitness (r = 0.26, p,0.001).
When fatness was compared between boys and girls with
the same level of fitness, girls had a larger SFS than boys
except for the least fit decile (fig 3). In a linear regression, the
association between sex and SFS was borderline significant
(r = 0.09, p = 0.05) after adjustment for fitness (r = 0.27,
p,0.001). After adjustment for both fitness and PA, there
was no difference between sexes with respect to SFS.
There were only two girls in the most fit group, and no
standard error was plotted for this group in fig 3. The
opposite analysis was also performed—that is, mean V ˙O2MAX
was calculated for boys and girls with the same activity level
and the same SFS respectively. The boys had a 3–5 ml/kg/min
higher fitness level in each category of activity (p,0.001) and
a 1–6 ml/kg/min higher fitness in each category of SFS
The main findings in this study are that, in a large sample of
young children, boys had a higher V ˙O2MAX and PA level than
girls. Most of the difference in fitness between the sexes
could be accounted for partly by body composition and partly
by PA. As no differences in haemoglobin or sex hormones
have been reported in this age group,17it is likely that the
difference in V ˙O2MAX relative to body weight is due to body
composition and PA. However, when children with the same
V ˙O2MAX were compared, boys were still more active, and in
boys and girls with the same PA level, boys were fitter.
Only a few studies in the literature report measured values
of V ˙O2MAX for children less than 8 years of age and only for a
small number of children.8 25–27Mean values of 39–53 ml/kg/
min have been reported. Differences in V ˙O2MAX between
studies seem to be due, at least partly, to differences in
protocol. The lowest mean value reported (39 ml/kg/min)
came from a field test.25The highest mean values found were
53 and 52 ml/kg/min for 5–7 year old boys and girls
respectively.26The highest values were calculated from the
original data of A˚strand.26Testing was carried out on a
treadmill with a continuous running protocol in a laboratory
setting, and subjects were highly selected. Compared with
earlier studies,8 26in which a difference in V ˙O2MAX relative to
body weight of less than 2% between the sexes in prepubertal
children was reported, our data show a considerable
difference (8%). Rowland23suggested that at least some of
the sex difference in children comes from the difference in fat
percentage. In our study, the difference was 23%. As the boys
were 3% heavier than the girls, this indicates that at least part
of the sex difference in V ˙O2MAX is related to difference in body
composition. The much smaller 2% difference we found in
V ˙O2MAX relative to LBM between boys and girls supports this.
Further, when allometric scaling was performed with LBM as
scaling factor, the difference was only 1% and statistically
insignificant. This implies that the sex difference in V ˙O2MAX is
due mainly to a difference in body composition.
Vinet et al28and Rowland et al29came to different
conclusions. Vinet et al found no difference in stroke volume
and V ˙O2MAX after allometric scaling and stated that body
composition alone (and not cardiac functional capacity)
could account for the sex difference. Rowland et al found a
difference between sexes even after allometric scaling and
Maximal exercise data on 6–7 year old children by sex
Boys (n=309) Girls (n=283)p Value
Heart rate (beats/min)
V ˙O2MAX (litres/min)
V ˙O2MAX relative to body mass (ml/kg/min)
V ˙O2MAX relative to LBM (ml/kg/min)
V ˙O2MAX relative to body mass (ml/kg0.71/min)
V ˙O2MAX relative to LBM (ml/kg1.105/min)
Values are mean (SD).
RER, Respiratory exchange ratio.
100 90 7080 60 4050
Deciles of fitness (ml/kg/min)
30 20 10
mean V ˙O2MAX was calculated for boys and girls separately in each
centile. Standard errors are so small that they are not visible.
Subjects were ranked into 10 centiles of V ˙O2MAX by sex. The
> 57.5 55 45 5040
Minutes > 2000 counts/min
relative to body weight. Physical activity level was plotted with SE for
boys and girls with the same fitness level. Numbers of girls and boys are
shown for each group.
All subjects together were ranked into six groups of V ˙O2MAX
728Eiberg, Hasselstrom, Grønfeldt, et al
stated that both body composition and cardiac functional
capacity accounted for the difference between boys and girls.
Further, Rowland et al found that anthropometric and aerobic
physiological factors cannot entirely explain the magnitude
of the sex differences.
We found a large sex difference (60%) in PA with regard to
sustained activity, and sustained periods of PA explained 9%
of the variance in V ˙O2MAX relative to body weight. It was
tempting to claim that habitual PA may be part of the
explanation for the difference in V ˙O2MAX between boys and
girls. This is also the case, but even within the same stratum
of V ˙O2MAX the boys had a higher PA level, so differences in PA
seem only partly to explain the differences in V ˙O2MAX.
Sundberg30compared V ˙O2MAX (ml/kg/min) in sighted chil-
dren and blind children, who are not able to participate in
vigorous play, and found 22% and 26% higher values in
sighted boys and girls respectively compared with blind
children. From Sundberg’s data, it could be expected that
much of the difference between fitness levels in prepubertal
children can be explained by differences in PA. It is therefore
surprising in the present study that boys were more active
than girls even within the same stratum of fitness. It is
perhaps less surprising that boys have smaller SFS compared
with girls in the same stratum of fitness. These two
observations support the view of Rowland et al29that the
higher level of fitness in boys is due to both the higher level
of PA and the lower level of fatness.
The number of children in this study is relatively high and
represents 69% of all children in preschool in two suburban
communities of Copenhagen. There were no significant
differences between participants and non-participants in
age, height, weight, and BMI in the study, so it seems safe to
conclude that the participant cohort is representative of the
two communities. Further, we have compared our sample
with a sample from nationwide Danish studies with
measurements of height and weight in children.31 32The
differences between these two studies and our study with
regard to mean height and BMI are less than 1%. It seems
reasonable to conclude that with regard to height and BMI
our cohort is representative of Danish children aged 6–7
years. The strengths of this study are that a large number of
subjects were examined using objective and direct measure-
ments and that the sample was representative.
The number of rejected tests in this study is relatively high.
Even though there were no differences in age and size
between subjects with valid tests and rejected tests, the
tendency was that younger and smaller children chose not to
run or stopped prematurely. We also chose to reject the data
for 57 children who actually ran a subjectively approved test,
but did not have a V ˙O2MAX recording. In the end, 84% of the
participating children completed a valid test, and we consider
this satisfactory, especially as some authors have claimed that
the reliability or validity of testing V ˙O2MAX in children aged
less than 8 years is questionable.33 34Our own validation of
the protocol showed good reliability, and it seems reasonable
to conduct reliable physiological testing on 6–7 year old
children with satisfying results. It should be noted that care
must be taken to carefully examine every child’s measure-
ments to ensure that the appropriate criteria have been met,
and to discard non-valid tests. In conclusion, the absolute
levels of V ˙O2MAX relative to body weight were 48.5 ml/kg/min
and 44.8 ml/kg/min in girls, V ˙O2MAX is larger in boys than
girls (+11%), also when related to body mass (+8%) and to
LBM (+2%). Most of the difference in V ˙O2MAX relative to body
mass was due to the greater percentage body fat in girls and
lower level of PA. When boys and girls with the same V ˙O2MAX
were compared, boys engaged in more minutes of at least
moderately intense activity and had a smaller SFS.
S Eiberg, H Hasselstrom, V Grønfeldt, K Froberg, J Svensson, Institute
for Exercise and Sport sciences, University of Copenhagen, Denmark
L B Andersen, Norwegian University of Sport and Physical Education,
Competing interests: none declared
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> 57.555 45 5040
Sum of 4 skinfolds (mm)
relative to body weight. Sum of four skinfolds was plotted with SE for
boys and girls with the same fitness level. Numbers of girls and boys are
shown for each group.
All subjects together were ranked into six groups of V ˙O2MAX
What is already known on this topic
No study has assessed population data in 6–7 year old
children on aerobic fitness and physical activity using direct
measurement of V ˙O2MAX and objective measurement of
What this study adds
N Even in 6–7 year old children, boys had 8% higher
fitness levels than girls, and the difference in fitness
may mainly be explained by differences in physical
activity level and body composition
N Moderate intensity physical activity may not influence
physical fitness but still change body composition
Aerobic fitness in 6–7 year old children 729
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