Secular trends in physical fitness and obesity in Danish 9-year-old
girls and boys: Odense School Child Study and Danish substudy of
the European Youth Heart Study
N. Wedderkopp1, K. Froberg1, H. S. Hansen2, L. B. Andersen3
1Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense University,2Department of
Cardiology, Odense University Hospital,3Institute for Exercise and Sport Sciences, University of Copenhagen, Odense NV, Denmark
Corresponding author: Niels Wedderkopp, Østrupvej 18, 5210 Odense NV, Denmark. Tel: 65-9108016, E-mail:
Accepted for publication 10 September 2003
Introduction: Low physical fitness and obesity have been
shown to be associated with cardiovascular disease (CVD)
risk. Obesity is on the increase in many countries, but little
is known about physical fitness trends. Monitoring of
changes in fitness and obesity in the population is important
for preventive strategies, and the aim of this study was to
analyse the secular trends in fitness and body composition in
Danish children. Materials and methods: Two representa-
tive population studies were conducted 12 years apart on
9-year-old children in the same location: the Odense School
Child Study in 1985–86 and the European Youth Heart
Study in 1997–98. In both studies, physical fitness was
determined by a maximal cycle ergometer test, and obesity
was assessed by skinfolds. Results: Boys had a lower
physical fitness and were fatter in 1997–98 than in 1985–
86. In addition, an increased polarization is emerging, with
the difference between the fit and the unfit and the difference
between the lean and the fat being greater in 1997–98 than
in 1985–86. In girls, a similar polarization was found, but
no overall change in fitness or obesity. Conclusion: The
negative trend and increased polarization for physical
fitness and obesity in Danish children suggest a future
generation with a higher degree of CVD risk.
Risk factors for cardiovascular disease (CVD) are
manifest already in childhood, with studies in the USA
(Berenson et al., 1980), England (Armstrong et al.,
1991) and Northern Ireland (Boreham et al., 1993)
indicating that approximately 70% of 12-year-old
children have at least one modifiable CVD risk factor.
In epidemiologic studies, high levels of physical
fitness and obesity have been associated with a lower
prevalence of CVD risk factors and a lower CVD
mortality (Blair et al., 1996; Nielsen & Andersen,
2003). In addition, we have earlier shown that phy-
sical fitness is associated with CVD risk factors and
risk factor clustering in children (Wedderkopp, 2000).
Fitness, obesity and other CVD risk factors track
from childhood into adulthood (Andersen & Har-
aldsdottir, 1993). There is therefore a rationale for
monitoring trends in these traits in the young popu-
lation in order to create early preventive strategies
Secular trends towards an increase in obesity have
been shown in many countries, but the knowledge is
mainly based on measurements of height and weight
(Kikuchi et al., 1992). Little is known about secular
trends in fitness, and no other studies using maximal
testing have reported changes in a population in
fitness. The aim of the study was to analyse the
secular trends in fitness and obesity in children.
Materials and methods
Secular trends were analysed through two cross-sectional studies
performed 12 years apart on representative samples of 9-year-
old children from the Danish city of Odense: the Odense School
Child Study 1986 (Hansen et al., 1990) and the European Youth
Heart Study 1997–98 (Wedderkopp, 2000). Odense is the third
largest city of Denmark. The total population of third grade
children in Odense were invited to participate in the first study,
of which 1369 children (85%) (670 girls and 699 boys)
participated in the study in 1985–86. In 1997–98, 693 children
were randomly selected from the population of Odense and
invited to participate, of which 589 children (85%) (310 girls and
279 boys) participated in the study.
Body height to the nearest millimetre and body mass to the
nearest 100g were determined by standard anthropometric
methods (Council of Europe, 1988) using a stadiometer and a
beam-scale weight. Body height was measured with shoes on
in the first study, and without in the second study.
Fat percentage was assessed by using the triceps and the
subscapular skinfolds as described by Slaughter et al. (1988).
Skinfolds were measured with Harpenden callipers over the
m. triceps brachii, m. biceps brachii, subscapularly. The jaws
Scand J Med Sci Sports 2004: 14: 1–6
Printed in Denmark.All rights reserved
COPYRIGHT & BLACKWELL MUNKSGAARD 2004
of the callipers were placed around the skinfolds 1cm below
where it was held by the thumb and first finger. The observer
waited for 2–3s before taking the reading and kept hold of the
skinfold while making the measurement. Measurements were
performed on the left side of the body with the child standing.
Two measurements were taken on each position. If there was
a difference of more than 2mm, a third measurement was
taken, and the mean of the two closest measurements was then
used. Measurement was performed over the centre of the
muscle on a line drawn between olecranon and acromion at
the midpoint between olecranon and acromion. Subscapu-
larly, the skinfold was measured under the angulus inf. on a
line with a 451 downward tilt compared with the vertical line.
The following equations were used to calculate the fat
percentage for boys and girls:
boys: fat percent51.21 ? (triceps skinfold1subscapular
skinfold)?0.008 ? (triceps skinfold1subscapular skinfold)2
girls: fat percent51.33 ? (triceps skinfold1subscapular
skinfold)?0.013 ? (triceps skinfold1subscapular skinfold)2
Changes in the proportions of children exceeding inter-
nationally accepted cut-points of BMI were analysed using the
cut-points published by Cole et al. (2000).
In both studies, physical fitness was determined by a
maximal work test, the watt-max test.
When performing the watt-max test, the children started at
20W, when their body mass was less than 30kg with a rise in
workload of 20W for each 3min, and 25W with a body mass
of 30kg or more with an increment in workload of 25W for
The cycle ergometers were a Monark 839 Ergomedic in
1997–98 and a Meditronic 40-3 in 1985–86. The bikes were
pre-programmed to increase the workload every third minute
with 20 or 25W. The workload was increased until exhaus-
tion, and the time and heart rate (HR) were registered.
Criteria for exhaustion were HRs above 185bpm and a
levelling off of HR, that the child could not keep a pedalling
frequency of 30rpm or more, and a subjective judgement of
the observer that the child could no longer keep up, even after
vocal encouragement. All tests were performed by only two
researchers, one in each cross-sectional study. The maximal
power output (Wattmax) was calculated as the watts in the
last completed workload (Wl), plus the increment in watts
(Wi) of the last step divided by 180s multiplied by the number
of seconds completed of the last step (tls).
Wattmax ¼ W1þ ðWi? t1s=180sÞ:
Physical fitness was assessed as the volume of oxygen
extracted at exhaustion in mL O2 per kg body mass per
minute (VO2). This was calculated in both studies through
separate equations derived from separate validation studies
using the maximal power output. The test has been validated
in several studies, and has been found to have high correlation
(r540.9)todirectlymeasuredVO2max(mL ? kg?1? min?1),
and a high reproducibility (r40.9) (Hansen et al., 1989;
HR was measured using Polar heart rate monitors: Polar
3000 in the first study and Polar Vantage NV in the second
study. The Polar 3000 used integration of the heart rate for
every 5s for calculating the reported heart rate per minute,
whereas the Polar Vantage NV used beat by beat integration
for the heart rate per minute measurement.
The Odense School Child Study
This study has been described in detail earlier (Hansen et al.,
1989). In all, 1369 children participated: 1284 children
performed the watt-max test, while 85 children (6.2%) were
excluded for not meeting the criteria of exhaustion.
The Danish substudy of the European Youth Heart Study
A total of 539 children performed the test, while 50 children
(8.5%) were excluded for not meeting the criteria of
Validity of the maximal work test, the watt-max test
A validation study was performed for each of the two cross-
sectional studies on a sub-sample of the participating children
to test the validity of the watt-max test and to create the
algorithms for calculating VO2max from the maximal watt
performed by the children.
Thirty-nine and 22 subjects, respectively, pedalled the same
cycle ergometer as used in the Odense School Child Study and
in the Danish part of the European Youth Heart Study. They
performed the test twice 2 days apart and the subjects were
randomized to direct measurement of VO2maxat either the
first or the second test round. In both validation studies,
multiple linear regression was used after subtraction of resting
metabolic rate, which was defined in mL O2as five times body
mass in kg, for constructing the formulae. In both cases, the
calculated maximal oxygen uptake provided an accurate and
valid estimate of actual oxygen uptake, with a correlation
between predicted and measured VO2max higher than 0.9
(Hansen et al., 1989).
In the Odense School Child Study, VO2max5(12.00*Watt-
max15*body mass), and in the Danish part of the Euro-
pean Youth Heart Study: VO2max5(13.16*Wattmax15*body
Separate validation studies were used because different
cycle ergometers were available in the two studies.
Comparisons of physical fitness were made between
the calculated VO2maxusing the algorithms obtained
in the individual validation studies. A comparison of
obesity was performed by comparing fat percentages
calculated from skinfolds according to Slaughter
et al. (1988).
For each study, the children were split into deciles
by their fat percentage and their fitness level. The
10th to 90th percentiles of fat percentage and fitness
level were plotted with 95% CI. We had several
reasons for looking at the populations in this way: (i)
It would show if only part of the populations were
changing. (ii) Polarization of data is possible without
the mean or median changing. (iii) Systematic error
in assessment might be found. The rationale for this
is that the upper level of fitness a subject can obtain
primarily is genetically determined, but the lower
level may be determined by inactivity, obesity and
disease. We therefore did not expect a major change
in fitness among the fit subjects.
ANOVA was used to test for differences between
deciles in 1985–86 and 1997–98 in fitness, and the
Mann–Whitney U-test was used to test between
Wedderkopp et al.
deciles of obesity, because the upper and lower
deciles of obesity were skewed. Linear regression was
used to test for trend in the differences between the
1985–86 and 1997–98 values over the ten deciles to
test for polarization.
All statistical analyses were performed on a
personal computer using STATA 7.
Key variables are described in Table 1. The boys in
1997–98 had a lower fitness level (Po0.001) and a
higher fat percentage than those in 1985–86 (Po
0.001), whereas no overall differences in fitness (P5
0.63) and fat percentage (P50.14) were found
between girls in 1997–98 and 1985–86 (Table 1).
The median values of the first to tenth deciles of
fat percentage are illustrated in Fig. 1 for girls and in
Fig. 2 for boys. The difference between the fat and
the lean increased over time in girls, with a difference
between the upper 10% and the lower 10% of 15.5%
fat in 1985–86 and 17.9% fat in 1997–98 (Po0.001
for trend in difference). No change was found in the
mean values for girls. For boys, the fat boys are
fatter today than in 1985–86, with no difference in
the fat percentage in the slim boys. Polarization was
found in boys (Po0.001 for trend in difference), and
an overall increase in obesity level was found in boys.
In all, 2.3% of the children in 1985 exceeded the
internationally accepted BMI cut-points of obesity,
whereas the corresponding proportion in 1997 was
The first to tenth deciles of fitness are illustrated in
Fig. 3 for girls and in Fig. 4 for boys. In 1997–98, the
most fit boys have the same level of fitness as in
1985–86, and the most fit girls have a significantly
higher level of fitness in 1997–98 than in 1985–86,
whereas both the girls and boys with the poorest
fitness level in 1997–98 have a significantly lower
level of fitness than the poorest fitness levels of girls
and boys from 1985–86, respectively. A polarization
was found in both sexes. The difference between the
least fit and the most fit increased over time in boys,
with a difference between the top 10% and the lowest
10% of 38% in 1985–86 and 45% in 1997–98
(Po0.001 for trend in difference). The same polari-
zation was found in girls, with a difference between
the top 10% and the lowest 10% of 37% in 1985–86
and 44% in 1997–98 (P50.001 for trend in
Figure 5 plots the differences in absolute VO2max
and in body weight between 1985–86 and 1997–98
values for each decentile of fitness (mLmin?1kg?1).
The decrease in fitness level (mLkg?1min?1) from
1985–86 to 1997–98 in the least fit is partly explained
by a higher body weight and partly by lower VO2max
Table 1. Descriptives of the two populations. Mean (SD) except for fat percent, where median with 95% CI are described
Girls Boys GirlsBoys
Fat percent (median with 95% CI)
Medians 1985 Medians 1997
Fig. 1. Body composition by decentiles in girls 1985 and 1997 in medians with 95% CI. A significant polarization has occurred
from 1985 to 1997; the difference between the fat and the lean has increased (P50.0003).
Secular trends in physical fitness and obesity
The mean maximal HR was 203 in 1985 and 200 in
1997 (Po0.01). If a linear relationship between HR
and VO2is assumed, three beats will correspond to
The main findings of this study were a polarization
in fitness and obesity over a 12-year period, and
1985 Median1997 Median
Fig. 2. Body composition by decentiles in boys 1985 and 1997 in medians with 95% CI. A significant overall increase in
obesity has occurred (P50.01), and a significant polarization has occurred from 1985 to 1997; the difference between the fat
and the lean has increased (P50.001).
Fig3. Fitness of girls by decentiles in mL O2min?1kg?1with 95% CI. A significant polarization in fitness has occurred from
1985 to 1997; the difference between the girls with the highest and the lowest fitness has increased (P50.0002).
Fig. 4. Fitness of boys by decentiles in mL O2 min?1kg?1with 95% CI. A significant polarization in fitness has occurred
from 1985 to 1997; the difference between the boys with the highest and the lowest fitness has increased (Po0.0000).
Wedderkopp et al.
children with low fitness were less fit than their
counterparts 12 years previously. The fat children
were generally fatter at the second time point.
Importance of obesity and low physical fitness as risk
factors for CVD
Obesity and low physical fitness are important risk
factors for CVD. Physical fitness has earlier been
used as a proxy of physical activity (Hansen et al.,
1990), but physical fitness might by itself be an
important independent factor associated with CVD
risk factors in both children and adults (Blair et al.,
1996; Rowlands et al., 1999). This relationship has
been shown in adults, where there is a clear negative
relationship between physical fitness and CVD
regardless of body composition. It has even been
suggested that being fit, per se, may reduce the
hazards of obesity (Lee et al., 1999). In children, an
increased level of CVD risk factors and a 6–7 times
increased risk of clustering of high levels of risk fac-
tors have been found to be associated with low fitness
The high correlation of obesity with CVD risk
(Kikuchi et al., 1992; Boreham et al., 2001) and the
clustering of risk factors in the obese (Voors et al.,
1982; Smoak et al., 1987; Chu et al., 1998) makes it
imperative to follow trends in the population in an
attempt to control and lower the number of obese
and the degree of obesity. This is especially pertinent
because of the increase in the degree of obesity and
the proportion of obese in both Denmark and other
countries (Aristimuno et al., 1984; Sorensen, 1988;
Heitmann, 1999; Thomsen et al., 1999).
Secular trends in fitness
The strength of the present study is the use of a
reliable fitness test in the comparison of two large
cohorts. To our knowledge, a secular decline in
fitness has previously been described only by Doll-
man et al. (1999), but they used different and less
reliable tests, the 1 mile run/walk test, the 50m sprint
and broad jump as fitness variables. In a validation
study, Rowland et al. (1999) found that the ‘‘one
mile run performance in children may not serve
as a strong indicator of cardiovascular fitness’’.
With this in mind, Dollman et al. (1999) still
found a lower fitness in 10–11-year-old children of
today of the same magnitude as we found in the
8–10-year-old children and also a trend towards a
greater difference between the low-fitness and high-
The difference in maximal HR of three beats was
constant across the deciles of fitness. The difference
could be explained by the use of two different heart
rate monitors using two different methods for
calculating the HR. Even if subjects were not fully
exhausted at the second test, the difference is too
small to explain the large difference in fitness among
the least fit.
Secular trends in body composition
Skinfolds were chosen as measures of obesity
because it is one of the few good validated measures
of fat percentage in children (Slaughter et al., 1988).
The increases in obesity have been found in many
other countries (Troiano et al., 1995; Dollman et al.,
1999), and obesity has been suggested to be a
growing problem (Seidell, 1999).
The negative trend inand polarization of, physical
fitness and obesity in Danish children suggests a
future generation with a higher degree of CVD risk.
Relative difference in weight and absolute VO2max (ml. oxygen per min).
Males absolute VO2 diff. Females absolute VO2 diff.Males Weight diff. Females weight diff.
Fig. 5. Relative difference in absolute VO2maxand weight; the difference in fitness (mL O2kg?1min?1) in the girls and boys
with the lowest fitness level is partly explained by the decrease in VO2maxand partly by the increase in weight.
Secular trends in physical fitness and obesity
To change this trend, it is imperative to encourage
more physical activity. One way of doing this could
be an increase in the number of compulsory physical
education lessons in school. Parents also have a
great responsibility, to encourage and support active
living with high levels of physical activity in
Key words: physical fitness, obesity, secular trend,
Andersen LB. A maximal cycle exercise
protocol to predict maximal oxygen
uptake. Scand J Med Sci Sports 1995:
Andersen LB, Haraldsdottir J. Tracking
of cardiovascular disease risk factors
including maximal oxygen uptake and
physical activity from late teenage to
adulthood. An 8-year follow-up study.
J Intern Med 1993: 234: 309–315.
Aristimuno GG, Foster TA, Voors AW,
Srinivasan SR, Berenson GS. Influence
of persistent obesity in children on
cardiovascular risk factors: the
Bogalusa Heart Study. Circulation
1984: 69: 895–904.
Armstrong N, Williams J, Balding J,
Gentle P, Kirby B. Cardiopulmonary
fitness physical activity. Patterns and
selected coronary risk factor variables
in 11 to 16 year olds. Ped Exerc Sci
1991: 3: 219–228.
Berenson GS, McMahan CA, Voors AW,
Webber LS, et al. Cardiovascular Risk
Factors in Children. The Bogalusa
Heart Study. Oxford: Oxford
University Press, 1980.
Blair SN, Kampert JB, Kohl HW III,
Barlow CE, Macera CA, Paffenbarger
RS Jr, Gibbons LW. Influences of
cardiorespiratory fitness and other
precursors on cardiovascular disease
and all-cause mortality in men and
women. JAMA 1996: 276: 205–210.
Boreham C, Savage JM, Primrose D,
Cran G, Strain J. Coronary risk factors
in schoolchildren. Arch Dis Child 1993:
Boreham C, Twisk J, Murray L, Savage
M, Strain JJ, Cran G. Fitness, obesity,
and coronary heart disease risk in
adolescents: the Northern Ireland
Young Hearts Project. Med Sci Sports
Exerc 2001: 33: 270–274.
Chu NF, Rimm EB, Wang DJ, Liou HS,
Shieh SM. Clustering of cardiovascular
disease risk factors among obese
schoolchildren: the Taipei Children
Heart Study. Am J Clin Nutr 1998: 67:
Cole TJ, Bellizzi MC, Flegal KM, Dietz
WH. Establishing a standard definition
for child overweight and obesity
worldwide: international survey. BMJ
2000: 320: 1–6.
Council of Europe. Description of
anthropometric measurements. In:
Rasmussen B ed. The Eurofit Test
Battery, Danish Edition. Strasbourg:
Council of Europe, 1988: pp. 67–70.
Dollman J, Olds T, Norton K, Stuart D.
The evolution of fitness and obesity
in 10–11-year-old Australian
schoolchildren: changes in
distributional characteristics between
1985 and 1997. Ped Exerc Sci 1999: 11:
Hansen HS, Hyldebrandt N, Froberg K,
Nielsen JR. Blood pressure and
physical fitness in a population of
children – the Odense Schoolchild
Study. J Hum Hypertens 1990: 4:
Hansen HS, Hyldebrandt N, Froberg K,
Rokkedal NJ. Blood pressure and
physical fitness in school children.
Scand J Clin Lab Invest Suppl 1989:
Hansen HS, Froberg K, Nielsen JR,
Hyldebrandt N. A new approach to
assessing maximal aerobic power in
children: the Odense School Child
Study. Eur J Appl Physiol 1989: 58:
Heitmann BL. Occurrence and
development of overweight and obesity
among adult Danes aged 30–60 years.
Ugeskr Laeger 1999: 161: 4380–4384.
Kikuchi DA, Srinivasan SR, Harsha DW,
Webber LS, Sellers TA, Berenson GS.
Relation of serum lipoprotein lipids
and apolipoproteins to obesity in
children: the Bogalusa Heart Study.
Prev Med 1992: 21: 177–190.
Lee CD, Blair SN, Jackson AS.
Cardiorespiratory fitness, body
composition, and all-cause and
cardiovascular disease mortality in
men. Am J Clin Nutr 1999: 69:
Nielsen GA, Andersen LB. The
association between high blood
pressure, physical fitness, and body
mass index in adolescents. Prev Med
2003: 36: 229–234.
Rowland T, Kline G, Goff D, Martel L,
Ferrone L. One-mile run performance
and cardiovascular fitness in children.
Arch Pediatr Adolesc Med 1999: 153:
Rowlands AV, Eston RG, Ingledew DK.
Relationship between activity levels,
aerobic fitness, and body fat in 8- to
10-yr-old children. J Appl Physiol 1999:
Seidell JC. Obesity: a growing problem.
Acta Paediatr Suppl 1999: 428: 46–50.
Slaughter MH, Lohman TG, Boileau RA,
Horswill CA, Stillman RJ, Van Loan
MD, Bemben DA. Skinfold equations
for estimation of body obesity in
children and youth. Hum Biol 1988: 60:
Smoak CG, Burke GL, Webber LS,
Harsha DW, Srinivasan SR, Berenson
GS. Relation of obesity to clustering of
cardiovascular disease risk factors in
children and young adults. The
Bogalusa Heart Study. Am J Epidemiol
1987: 125: 364–372.
Sorensen TI. Obesity in the Scandinavian
countries: prevalence and
developmental trends. Acta Med Scand
Suppl 1988: 723: 11–16.
Thomsen BL, Ekstrom CT, Sorensen TI.
Development of the obesity epidemic in
Denmark: cohort, time and age effects
among boys born 1930–1975. Int J
Obes Relat Metab Disord 1999: 23:
Troiano RP, Flegal KM, Kuczmarski RJ,
Campell SM, Johnson CL. Overweight
prevalence and trends for children and
adolescents. The National Health and
Nutrition Examination Surveys. Arch
Pediatr Adolesc 1995: 149: 1085–1091.
Voors AW, Harsha DW, Webber LS,
Radhakrishnamurthy B, Srinivasan
SR, Berenson GS. Clustering of
anthropometric parameters, glucose
tolerance, and serum lipids in children
with high and low beta- and pre-beta-
lipoproteins. Bogalusa Heart Study.
Arteriosclerosis 1982: 2: 346–355.
Wedderkopp N. Cardiovascular Risk
Factors in Danish Children and
Adolescents. A Community based
approach with a special reference to
Physical Fitness and Obesity. 2000.
Institute of Sport Science and Clinical
Biomechanics, Faculty of Health
Sciences, University of Southern
Denmark, Main campus: Odense
Wedderkopp et al.