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Total body fat percentage and body mass index
and the association with lower extremity injuries
in children: a 2.5-year longitudinal study
Eva Jespersen,
1
Evert Verhagen,
2
René Holst,
3
Heidi Klakk,
1
Malene Heidemann,
4
Christina Trifonov Rexen,
1
Claudia Franz,
1
Niels Wedderkopp
1,5
1
Centre for Research in
Childhood Health, Institute of
Sports Science and Clinical
Biomechanics, University of
Southern Denmark, Odense,
Denmark
2
Department of Public and
Occupational Health, EMGO+
Institute for Health and Care
Research, VU University
Medical Centre, Amsterdam,
The Netherlands
3
Biostatistical Research Unit,
Institute of Regional Health
Research, University of
Southern Denmark, Odense,
Denmark
4
Hans Christian Andersen
Children’s Hospital, Odense
University Hospital, Odense,
Denmark
5
Department of Orthopaedic,
Hospital of Lillebaelt, Institute
of Regional Health Service
Research and Centre for
Research in Childhood Health,
IOB, University of Southern
Denmark, Odense, Denmark
Correspondence to
Eva Jespersen, Centre for
Research in Childhood Health,
Institute of Sports Science and
Clinical Biomechanics,
University of Southern
Denmark, Campusvej 55,
5230 Odense M, Denmark;
ejespersen@health.sdu.dk
Accepted 26 October 2013
To cite: Jespersen E,
Verhagen E, Holst R, et al.
Br J Sports Med Published
Online First: [please include
Day Month Year]
doi:10.1136/bjsports-2013-
092790
ABSTRACT
Background Overweight youths are generally
recognised as being at increased risk of sustaining lower
extremity injuries in sports. However, previous studies are
inconclusive and choices for measuring overweight are
manifold.
Objective To examine two different measures of
overweight, body mass index (BMI) and total body fat
percentage (TBF%), as risk factors for lower limb injuries
in a school-based cohort.
Study design A longitudinal cohort study.
Methods A total of 632 school children, baseline age
7.7–12.0 years, were investigated. Whole body dual
energy x-ray absorptiometry scans provided measures of
TBF%. Measures of BMI were obtained by standard
anthropometric methods. Musculoskeletal complaints
were reported by parents answering weekly mobile
phone text messages during 2.5 years. Injuries were
diagnosed by clinicians. Leisure time sports participation
was reported weekly using text messaging.
Results During 2.5 years of follow-up, 673 lower
extremity injuries were diagnosed. Children being
overweight by both BMI and TBF% showed the highest
risk of sustaining lower extremity injuries (IRR 1.38 (95%
CI 1.05 to 1.81)). Children who were overweight using
BMI and TBF% showed the highest risk of sustaining
lower extremity injuries (IRR 1.38 (95% CI 1.05 to
1.81)).
Conclusions The risk of lower extremity injuries
appeared to be increased for overweight children.
When comparing two different measures of overweight,
overweight by TBF% is a higher risk factor than
overweight by BMI. This suggests that a high
proportion of adiposity is more predictive of lower
extremity injuries, possibly due to a lower proportion of
lean muscle mass.
BACKGROUND
Injuries sustained in sports and leisure time activ-
ities have been established as a leading cause of the
paediatric injury burden in Western countries.
1–5
These induce high direct and indirect costs for chil-
dren and parents and may cause short-term disabil-
ity, absence from school and/or physical activity
(PA), lost enthusiasm for participating in PA and
long-term consequences such as osteoarthritis.
6–11
Both intrinsic and extrinsic risk factors have pre-
viously been investigated and some attention has
been shown to body weight and body composition
as a potentially modifiable risk factor on sport
injury risk.
12 13
The importance is emphasised as
overweight and obesity is affecting an increasing
proportion of children globally.
14
Hence the
paradox is that while PA is associated with numer-
ous health benefits, including lowering the levels of
overweight and obesity,
15
overweight might at the
same time cause a rise in injury rates as the preva-
lence of overweight and obesity increases.
Overweight youths are generally considered as
being at increased risk of sustaining lower extremity
injuries in sports, due to a corresponding increase
in the forces that joints, ligaments, tendons and
muscular structures must endure.
16–18
However,
findings in studies on the association between body
composition and injuries are inconclusive and
choices of measures of body composition have
been varied, such as height and weight, lean muscle
mass, body fat content and most commonly body
mass index (BMI).
18
Overweight and obesity should be defined as
excess body fat. The most widely used measure-
ment to define obesity is BMI. It is an indicator of
overweight and obesity from a population perspec-
tive, but has limitations on an individual level and
is only a proxy measurement of body fat.
19
The
association between BMI and total body fat per-
centage (TBF%), especially in athletes, has been
shown to be lower than in non-athlete controls.
20–23
Moreover, the common use of BMI as a criterion
measurement may be an issue when it involves
physically active children. A high BMI might in that
case be an expression of a high proportion of lean
muscle mass, rather than overweight or ‘unhealthy’
weight. TBF% is a measure of adiposity and in the
area of sports it has been shown to be a more
precise measure for classification of overweight.
21–24
Different mechanisms by which overweight
increases the risk of injuries have been proposed.
These include a relatively higher musculoskeletal
strain and impaired postural control when control-
ling for a disproportionately large body mass in
sport activities that require rapid alterations during
changes in direction.
16
Poor physical fitness and
low PA levels among overweight young people add
to this risk.
15 16
Based on this, overweight caused
by a high proportion of TBF% appears to be a
more obvious risk factor than overweight caused by
a high proportion of lean muscle mass.
Overweight, defined by measures of TBF%, pos-
sibly associates differently with PA-related injuries
than overweight defined by measures of BMI.
The objective of this study was to examine two
different measures of overweight, BMI and TBF%,
as risk factors for lower limb injuries in school chil-
dren, followed for 2.5 years in a longitudinal
Jespersen E, et al.Br J Sports Med 2013;0:1–6. doi:10.1136/bjsports-2013-092790 1
Original article
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setting, while considering the potential confounding effects of
gender, age, fitness levels and exposure times in physical educa-
tion (PE) and leisure time sports participation.
MATERIAL AND METHODS
Setting
Data from the Childhood Health, Activity, and Motor
Performance School Study Denmark (CHAMPS Study-DK)
August 2008 to July 2011 were used.
25
This investigation is a
large prospective controlled school-based study in Denmark
using the design of a natural experiment to evaluate the effect
of increased levels of PE on childhood health in general.
26
Participants
All boys and girls from preschool to fourth grade in 10 public
schools participating in the CHAMPS Study-DK also agreed to
participate in the registration of musculoskeletal pain and injur-
ies. The study was kept open, with the possibility for new chil-
dren to enter. Owing to the novel data collection method of
automated mobile phone text messaging, the schools were
included gradually in order to allow for a phasing-in process.
A subsample of children attending second to fourth grade
(age range 7.7–12.0 years) was invited to a dual energy X-ray
absorptiometry (DXA) scan providing TBF% in 2008. Children
were examined at baseline and at a 2-year follow-up. Height
and weight were assessed at the same time points. Data on injur-
ies and participation in organised sport were recorded from
November 2008 to June 2011.
Measurements
Musculoskeletal pain and injuries
Weekly information on musculoskeletal pain and injuries was
measured using mobile phone text messaging. Each week,
parents answered a text message asking questions on the pres-
ence or absence of musculoskeletal pain. A report of pain eli-
cited a telephone consultation, to distinguish children with
trivial complaints from those in need of clinical examination.
Physiotherapists, chiropractors or a medical doctor clinically
examined the children during the coming fortnight and a stand-
ard medical record was performed. Injuries were diagnosed
using the International Classification of Diseases (ICD-10),
WHO.
27
If needed, the child was referred for further examin-
ation, such as X-ray, ultrasound or MR scans, or to consulting
specialist doctors. Information on children being treated else-
where during the study period, for example, emergency depart-
ment, general practitioner, was collected concurrently to get a
complete data collection on injuries in this cohort.
Physical activity
Weekly amount of PE was 4.5 h for children in sport schools
and 1.5 h for children in normal schools, corresponding to
three and one double lesson per week, respectively. Pupils at
sport and normal schools were therefore assigned three and one
sport exposure units per week, respectively. Leisure time sport
was also assessed using text messaging, by parental reports on
how many times the child had participated in leisure time sport
activities during the past week.
Total body fat percentage
TBF% was measured by DXA (GE Lunar Prodigy, GE Medical
Systems, Madison, Wisconsin, USA), ENCORE software (V.12.3,
Prodigy; Lunar Corp, Madison, Wisconsin, USA).
TBF% was calculated for each participant from the equation:
((FM (g) × 100)/total body weight (g)). Cut-offs to classify
children as normal weight or overweight were defined using the
cardiovascular health-related and gender-related TBF% stan-
dards according to Williams et al.
28
The cut-off for overweight
boys was ≥25 TBF% and a similar cut-off for girls was ≥30
TBF%.
Body mass index
Weight was measured to the nearest 0.1 kg on an electronic
scale (Tanita BWB-800S, Tanita Corporation, Tokyo, Japan).
Height was measured to the nearest 0.5 cm using a portable sta-
diometer (SECA 214, Seca Corporation, Hanover, Maryland,
USA).
BMI was calculated as (weight (kg)/height
2
(m)). BMI classifi-
cations for normal weight, overweight and obese were defined
using age-specific and sex-specific cut-offs as recommended by
the International Obesity Taskforce recommendations.
29
Dichotomised categories were made for weight classes as
normal weight or overweight/obese (hereafter referred to as
overweight) for easier comparison with the dichotomous vari-
able of normal weight versus overweight as described above.
Fitness level
Fitness was assessed by the Andersen test. This is a 10 min inter-
mittent running test to estimate maximal oxygen uptake and
indicate aerobic fitness.
30
The test was carried out indoors in
20 m running lanes marked by cones. Children were urged to
run as quickly as possible for 15 s, then stopped for the next
15 s and this pattern was repeated for 10 min. The total distance
measured in metres was the test result. The validity and reliabil-
ity of this field test were tested and described thoroughly for the
age group of our cohort in a study by Ahler et al.
31
Statistical methods
Data from the text messaging system and data on diagnosed
injuries were analysed using STATA V.12.0 (StataCorp, Texas,
USA).
Excessive levels of pain and the number of injuries were
reported during the start-up-phase. This is possibly explained by
the novelty of the study and the method. Observations from the
first 4 weeks relative to the time of inclusion were therefore
excluded.
The risk of getting injured according to absolute levels of
baseline BMI and TBF% were explored. Furthermore, the
potential effect of children changing body composition through
the 2.5 years of injury surveillance was evaluated by a logistic
regression using variables with ‘no change’,‘change to elevated
BMI or TBF%’and ‘change to normal BMI or TBF% values’as
categories.
Concerns about children being underweight were addressed,
as injury patterns could possibly be different in this group.
20 32
For this reason, the prevalence of underweight was determined
in the baseline population, using the recommended
cut-offs.
33 34
An initial analysis excluding the group of under-
weight children did not change the estimates of risk of injury or
the estimated effect of other covariates. Underweight children
were therefore not considered different from normal-weight
children regarding the risk of injury. Hence, they were cate-
gorised as normal-weight children.
The calculation of incidence rates accounted for the total
exposure expressed in 1000 athletic exposure units. These com-
prised the PE exposures and the participations in leisure time
sport.
A multilevel mixed-effects Poisson regression was used to esti-
mate the incidence-rate ratios (IRR) with BMI and TBF% as the
2 Jespersen E, et al.Br J Sports Med 2013;0:1–6. doi:10.1136/bjsports-2013-092790
Original article
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primary risk factors. BMI and TBF% were used as dichotomised
variables (0=normal values, 1=elevated values) in separate
regression analyses. For identification of groups of potential
clinical interest, the four combinations of normal and elevated
BMI with normal and elevated TBF% were likewise tested in a
regression analysis, with normal BMI and normal TBF% being
the reference groups.
Finally, BMI and TBF% were tested as continuous variables
and used for illustrating the adjusted risk of lower extremity
injuries in relation to the two measures of body composition.
The explanatory variables included gender, age, PE/leisure
time sport and fitness levels. Classes and schools were used as
random effects. The multilevel random effects model reflects
the hierarchical sampling structure and was chosen to allow for
potential variation between schools and between classes within
schools and to ensure correct modelling of the variances. The
number of weeks each child participated was included as an
exposure term in the model.
Potential patterns for the missing values in injury data were
addressed by a logistic regression analysis controlling for gender,
age, school type and leisure time sports effects. Missing values
because of practicalities concerning changed or wrong mobile
numbers were dropped for analyses.
RESULTS
A total of 632 children, aged 7.7–12 years at baseline, partici-
pated at baseline and the follow-up DXA scan and in the regis-
tration of musculoskeletal injuries. The mean baseline BMI was
16.6 (±SD 2.1) and TBF% was 20.1% (±SD 8.0). A total
number of 673 lower extremity injuries were diagnosed during
the 2.5 years of follow-up. Some children experienced more
than one injury; the range was from zero and up to eight epi-
sodes of lower extremity injuries. The range of participation
time in injury registration was 1–113 weeks, with 98.1 weeks
being the mean value. Dropouts were due to children moving
away from the municipality or changing to a non-project school,
but were counterbalanced by new children moving to project
schools. Fifteen children dropped out because answering SMS
questions every week was too bothersome. An average weekly
response rate of 96% was recorded during the study period of
113 weeks. A total number of 62 001 observations were
recorded and 2502 (4%) were missing. The analysis of missing
data did not show any patterns when looking at gender, age,
school type and leisure time sports. The mean weekly sport
exposures units in PE and leisure time sport were 3.9 (±SD 1.3)
and fitness level at baseline had a mean of 930 8 m (±SD101.9).
Differences in gender are presented in table 1.
The injury rates per 1000 athletic exposures showed a trend,
albeit not significant, towards higher risk for children being
overweight, whether defined by BMI or by TBF%. Injury rates,
95% CI and gender differences are described in table 2.
28 29
The multivariate and multilevel adjusted IRR estimates by dif-
ferent measures of overweight are summarised in table 3.
Overweight children were generally at higher risk of sustaining
lower leg injuries, by BMI: 1.28 (95% CI 0.98 to 1.66) and by
TBF% 1.34 (95% CI 1.07 to 1.68), the latter being statistically
significant.
Looking at the four combined groups of body composition,
children with elevated BMI and TBF% showed the highest risk
of sustaining lower leg injuries: 1.38 (95% CI 1.05 to 1.81)
relative to children having a normal BMI and a normal TBF%
(figure 1).
The possible effect of children changing body composition
during the 2.5 years of injury surveillance was also accounted
for in the adjusted analysis; it did not explain the risk of lower
extremity injuries, and nor did it influence the estimated effects
of other covariates.
Gender and age did not influence the risk, whereas the time
participating in PE and leisure time sport and fitness level
explained some of the lower extremity injury risk. The risk of
injury significantly increased for each additional time a child
participated in PE and leisure time sport from 0 to 6.5 weekly
exposure units. For the 18 children with a mean of more than
6.5 exposures a week, the risk again decreased. A positive linear
relationship was found between risk of lower extremity injuries
and aerobic fitness.
The adjusted risk of lower extremity injuries in relation to the
two measures of body composition measured on a continuous scale
are illustrated in figure 2 for girls and boys. A positive linear rela-
tionship was found between risk of lower extremity injuries and the
continuous values of TBF% and BMI across the full range.
Table 1 Sample characteristics in numbers (%) and means (±SD)
measured by gender during 2.5 years of follow-up
Girls Boys
Numbers 321 (50.8%) 311 (49.2%)
Age at baseline 9.6 (0.9) 9.6 (0.9)
Range 7.9–11.6 7.7–12.0
Baseline BMI 16.7 (2.1) 16.6 (2.1)
Baseline TBF% 23.0 (7.4) 17.1 (7.4)
Weekly exposure in PE and sports 3.9 (1.3) 3.9 (1.4)
Fitness level at baseline (metre) 892.7 (89.2) 967.8 (99.8)
Lower extremity injuries
Number of lower extremity injuries 336 337
Number of children with lower extremity
injuries
178 (55.5%) 179 (57.6%)
BMI, body mass index; PE, physical education; TBF%, total body fat percentage.
Table 2 Lower extremity injury rates by BMI/TBF% groups, overall and by gender
Lower extremity injuries
Injury rate (CI)* BMI†TBF%‡
Normal BMI Overweight by BMI cut-offs Normal TBF% Overweight by TBF% cut-offs
Overall 4.4 (4.1 to 4.8) 5.3 (4.1 to 6.5) 4.4 (4.0 to 4.7) 5.2 (4.3 to 6.1)
Girls 4.4 (3.9 to 4.9) 5.1 (3.6 to 6.5) 4.3 (3.8 to 4.8) 5.2 (4.0 to 6.5)
Boys 4.4 (3.9 to 4.9) 5.8 (3.7 to 7.9) 4.4 (3.9 to 4.9) 5.1 (3.7 to 6.6)
*Injury rate is per 1000 athletic exposures; values in parentheses are 95% CI.
†Age-specific and gender-specific cut-offs according to IOTF/Cole et al.
29
‡Cut-offs boys ≥25%, girls ≥30% according to Williams et al.
28
BMI, body mass index; TBF%, total body fat percentage.
Jespersen E, et al.Br J Sports Med 2013;0:1–6. doi:10.1136/bjsports-2013-092790 3
Original article
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DISCUSSION
This study is the first to evaluate and compare two different
measures of overweight as risk factors for lower extremity injur-
ies in a school-based cohort of children. The risk of lower
extremity injuries was observed to increase in overweight chil-
dren. Being overweight measured by TBF% or a combination of
elevated TBF% and BMI was more predictive than being over-
weight measured by BMI. This suggests that a high proportion
of adiposity is more predictive of lower extremity injuries, pos-
sibly due to a lower proportion of lean muscle mass.
In contrast, Kaplan et al
35
found that body weight was a
more powerful injury risk factor than adiposity, with no differ-
ences in injury risk between linemen and non-linemen in
American football. This was shown in a study comparing differ-
ent measures of body composition (body fat, BMI, weight,
height) to injury risk in a group of 98 high school players with
28 injuries registered by trainers. This was reproduced in
another American football study reporting injury rates by body
fat, weight, BMI and lean body mass in high school football
linemen,
36
whereas adiposity expressed as TBF% was a stronger
predictor of the magnitude and type (overuse/traumatic) of mus-
culoskeletal injuries in army cadets than BMI.
37
Direct compari-
sons may not be relevant because of differences in techniques to
measure TBF%, injury registration methods, magnitudes of
studies, ages and sports specific versus more heterogenic set-
tings. Still, it is possible that in some sports, the effect of
increased mechanical loading during weight bearing or collisions
has a more pronounced effect than in other sports.
Injury patterns might also differ in relation to different injury
types. Traumatic injuries provoked and/or aggravated by greater
collision forces due to heavy weight could be argued to be inde-
pendent of the muscle/fat distribution to a greater extent than
overuse injuries, where the quality of tissue (eg, muscle strength
and endurance) is important. The effect of overweight in rela-
tion to different injury types (overuse/traumatic), different diag-
noses, different anatomical regions and different sports still
needs to be clarified.
In this study, injury risk increased with increased participation
in PE and leisure time sport. This is in accordance with the
common understanding of the need to consider exposure time
when estimating injury risk. Surprisingly, children with high
fitness levels had a higher risk of sustaining lower extremity
injuries. This is in contrast to earlier beliefs where lower fitness
levels have been associated with muscle fatigue and subsequent
injury.
38
A possible explanation could be that children with high
aerobic capacities are also the children with the largest amount
of exposure time. Even though analyses were carried out with
adjustment for exposure time in terms of PE and leisure time
sports, there may have been uncaptured exposure time in the
most aerobically fit, as unorganised leisure time activity was
unknown.
Figure 2 Adjusted risk of lower
extremity injuries by different measures
of body composition in girls and boys
(BMI, body mass index).
Figure 1 Incidence-rate ratio estimates (95% CI) by four groups of
body compositions, adjusted for age, gender, physical education/leisure
time sport and fitness level (BMI, body mass index; IRR, incidence-rate
ratio; TBF%, total body fat percentage).
Table 3 Incidence-rate ratio estimates by different body
composition measures, adjusted for age, gender, physical education/
leisure time sport and fitness level
BMI
IRR (95% CI)
TBF%
IRR (95% CI)
Normal weight 1.00 1.00
Overweight 1.28 (0.98 to 1.66) 1.34 (1.07 to 1.68)*
*Statistical significance based on p<0.05%.
BMI, body mass index; IRR, incidence-rate ratio; TBF%, total body fat percentage.
4 Jespersen E, et al.Br J Sports Med 2013;0:1–6. doi:10.1136/bjsports-2013-092790
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Cut-offs to classify children as normal weight or overweight
were defined using cardiovascular health-related and gender-
specific TBF% standards
28
and age-specific and gender-specific
centiles from a pooled international dataset, linked to adult
cut-offs for BMI classifications.
29
It can be questioned if these
criteria have the same relevance in injury risk research, but
they permit comparison across studies and contribute to a
general evaluation of health risk among overweight children.
The presentation of data in figure 2 does not suggest any
obvious cut-off for a significant increase in risk of lower
extremity injuries in relation to overweight. Specificover-
weight cut-offs for being at increased injury risk might be less
important in the context of injury prevention, especially at an
individual level where a more comprehensive screening of
body composition involving an expression of TBF% would be
more relevant.
CONCLUSION
The risk of lower extremity injuries in a heterogenic cohort of
schoolchildren was shown to increase in overweight children.
When comparing two different measures of overweight, a body
composition of proportionally high levels of TBF% is a higher
risk factor than overweight as measured by BMI. This suggests
that a high proportion of adiposity is more predictive of lower
extremity injuries, possibly due to a lower proportion of lean
muscle mass.
Increased levels of PE and leisure time sports participation
and fitness were also associated with increased risk of lower
extremity injuries.
What are the new findings?
▸This study is the first to evaluate and compare two different
measures of overweight as risk factors for lower extremity
injuries in a school-based cohort of children.
▸Overweight children have an increased risk of lower
extremity injuries.
▸Overweight by measures of total body fat percentage is
more predictive of lower leg injuries in children than
overweight by measures of body mass index.
How might it impact on clinical practice in the near
future?
▸Injury prevention in children and adolescents should involve
a screening of body composition involving an expression of
total body fat percentage. While dual energy X-ray
absorptiometry scans are expensive and not feasible in most
settings, a measurement method such as waist
circumference is cheap and easy to obtain.
▸Further research is needed into the proposed underlying
mechanisms for overweight children being at increased
injury risk. Previously suggested mechanisms have been poor
postural control—leading to problems with balance and
co-ordination, poor physical fitness—associated with muscle
fatigue, and subsequent injury and low preparticipation
physical activity levels—associated with impaired
neuromuscular and motor learning.
Acknowledgements The authors wish to acknowledge K Froberg and LB
Andersen, Centre for Research in Childhood Health, University of Southern Denmark.
The authors would like to thank the participants and their parents and the
participating schools, The Svendborg Project and the municipality of Svendborg.
Finally, the authors wish to acknowledge the members of the CHAMPS Study-DK
not listed as coauthors of this paper: T Junge and NC Møller.
Contributors NW was responsible for the overall study concept and design. HK
and MH were responsible for the acquisition of the body composition data. EJ, CTR,
CF and NW were responsible for the acquisition of injury data. EJ, EV, RH and NW
were responsible for the analysis and interpretation of data. EJ drafted the
manuscript. All authors took part in a critical revision of the manuscript. RH
provided statistical expertise. NW obtained the funding.
Funding This study was supported by grants from The IMK Foundation, The
Nordea Foundation, The TRYG Foundation—all private, non-profit organisations,
which support research in health prevention and treatment, and TEAM Denmark, the
elite sport organisation in Denmark, that provided the grant for the text messaging
system.
Competing interests None.
Ethics approval Written informed consent was obtained from the child’s parent,
including verbal acceptance from the parent and the child prior to every clinical
examination for diagnosing injuries and medical record keeping. All participation in
the data collection was voluntary with the option to withdraw at any time. The
study was approved by the Ethics Committee for the region of Southern Denmark
(ID S20080047)
Provenance and peer review Not commissioned; externally peer reviewed.
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doi: 10.1136/bjsports-2013-092790
published online November 22, 2013Br J Sports Med
Eva Jespersen, Evert Verhagen, René Holst, et al.
longitudinal study
extremity injuries in children: a 2.5-year
index and the association with lower
Total body fat percentage and body mass
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