Physical fitness of adolescents

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Physical Fitness Of Dutch Adolescents: Does Physical Fitness decline during adolescence? 1

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Chantal M Koolhaas1, Evert ALM Verhagen1, Saskia AM Boonzajer1 4

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1 Department of Public and Occupational Health, and EMGO+ Institute for Health and Care Research, 6

VU University Medical Center, Amsterdam, the Netherlands 7

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Corresponding author: E.A.L.M. Verhagen, VU University Medical Center Van der 9

Boechorststraat 7 - Room C-568 1081 BT Amsterdam, the Netherlands 10

Telephone: +31 20 4449691 11

Fax: +31 20 4448387 12

Mobile phone: +31 6 46630221 13

E-mail: e.verhagen@vumc.nl 14

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Running title: Physical fitness of adolescents 17

No funding receiverd 18

No conflict of interests 19

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Physical fitness of adolescents

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Abstract 1

Purpose Physical activity (PA) levels of adolescents are under par and decline with 2

increasing age. Since PA has effect on physical fitness (PF), this study aimed to investigate 3

the age-related changes in PF of 12-16-year-old Dutch boys and girls. In addition, the 4

progress in PF with age was compared with the progress in PF of boys and girls from 1987 5

Methods To measure PF, 1378 boys and 1257 girls participated in the Eurofit test. Multilevel 6

regression analyses were executed for all test items, for boys and girls separately. Difference-7

scores were calculated for every Eurofit test item by subtracting reference scores from Eurofit 8

test scores. Age-related changes in difference-scores were examined with multilevel 9

regression analyses for all Eurofit test items. 10

Results Multilevel regression analyses showed that sum of skinfolds significantly decreased 11

with age, BMI increased and all Eurofit test items, with the exception of SAR, significantly 12

improved with age for boys. With girls, BMI significantly increased, scores on SAR, PLT and 13

rHGR improved and scores on SBJ, SHR, SUP and ESR deteriorated. Multilevel regression 14

analyses of difference-scores of boys showed a significant effect of age on BAH, SAR and 15

SUP. Difference-scores of girls showed a significant effect of age on sum of skinfolds, SBJ, 16

SHR, PLT, rHGR and ESR. With these items, girls’ progress of PF lagged behind in 17

comparison with the reference scores. 18

Conclusion In general, boys’ PF improved with age and this was in accordance with the 19

progress of PF with age in boys from 1987. Girls’ PF remained relatively stable with 20

increasing age, however, the progress of PF with age lagged behind in comparison with the 21

progress of PF in girls from 1987. 22

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Keywords: Eurofit test battery, physical activity, health, exercise 24

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Physical fitness of adolescents

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Paragraph 1: INTRODUCTION 1

The benefits of physical activity (PA) on physical and mental well-being have been well 2

established (19). Despite the well-known benefits of PA, PA levels in general are under par. 3

For example, a recent study in the Netherlands showed that in 2005 less than 30% of children 4

between ages 12-17 years met 30 minutes of PA per day (23), while the guidelines state that a 5

daily minimum of 60 minutes of PA is recommended (10). What is of most concern about 6

these numbers is that only 4% of boys and 3% of girls in the ages 6-11 meet these 7

recommended PA guidelines (10). This indicates that children are insufficiently active to gain 8

both physical and mental health benefits (6) and are at increased risk for obesity (37). Low 9

levels of PA during childhood have additional serious health consequences in adulthood, as 10

osteoporosis (9) and cardiovascular disease (1,9). 11

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Paragraph 2 13

Many studies have shown an age-related decline in PA (2,13,16,30,32), most pronounced 14

during adolescence. This decline is more prominent in girls than in boys (30,32), and the 15

decline seems to be higher in adolescents of low socio-economic level (21,28). Since PA is a 16

main determinant of physical fitness (PF) (24), a decrease in PF can be expected in line with 17

a decrease in PA levels. PF is defined as an integrated measure of various bodily functions 18

(cardiorespiratory, skeletomuscular, psychoneurological, hematocirculatory and endocrine-19

metabolic) involved in the performance of daily PA (24). It has already been established that 20

the PF of adolescents has declined over the last few decades (27,35,36). A meta-analyses of 21

data in adolescents from 1961 to 2000 showed that aerobic fitness of youth has declined since 22

1970 (35). In addition, Tremblay et al. (36) and Runhaar et al (27) have reported a decline in 23

general PF of adolescents from 1981 to 2009 and from 1980 to 2006, respectively. However, 24

only few studies report age-related changes in PF levels of adolescents. One of these studies 25

is a recent study from Ortega et al, who aimed to report sex- and age specific PF levels of 26

Physical fitness of adolescents

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European adolescents (25). This study showed a trend for incrementally higher PF for boys, 1

whereas girls showed stability, or a slight increase, across the years. In contrast, Tremblay et 2

al. (36) reported a difference between children aged 6 through 19, with 6 year olds scoring 3

better on predicted maximal aerobic power (VO2max). This indicates that next to a reduction 4

in PF over time, there also is a change in PF throughout a child’s maturation. 5

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Paragraph 3 7

To increase the knowledge on contemporary age-related changes in PF levels, the first aim of 8

this study was to investigate the PF, as measured with the Eurofit test, of 12-16-year-old boys 9

and girls following preparatory secondary vocational education. The second aim of this study 10

was to examine the difference in progress of PF with age, as measured with the Eurofit test, 11

between boys and girls in the current study and boys and girls from 1987 (8). The first 12

hypothesis was that that PF would be stable with age in girls, but would improve with age in 13

boys. The second hypothesis was that the contemporary progress of PF with age would be 14

equal to the progress in PF of boys and girls from 1987. 15

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Paragraph 4: METHODS 17

Eight Dutch high schools took part in the study. All schools were VMBO directed, holding 18

that schools offer preparatory secondary vocational education. Two schools in Amersfoort, 19

three schools in Amsterdam, two in Delft and one school in Rotterdam participated in the 20

study. In deliberation with the schools, the number of classes participating in the study was 21

agreed on. The number of classes participating and the number of children per class differed 22

per school. Parents of participating children received a passive informed consent form that 23

explained the nature and procedures of the study. If parents or their child(ren) did not want to 24

participate, they could withdraw. The Medical Ethics Committee of VU University Medical 25

Physical fitness of adolescents

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Center approved the study design, protocols, and informed consent procedure. All measures 1

were executed in the school year 2012-2013. A total of 1378 boys and 1257 girls participated 2

in the study. 3

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Paragraph 5: Procedure 5

All data were collected in the gymnasium of the particular school, during one of the classes’ 6

regular physical education lessons. Data were collected by university students who were 7

trained for this purpose during a two-hour training session. Demographic and behavioural 8

data were derived from a shortened version of a questionnaire that was also used in the 9

previous Do-It study (31). The questionnaire included several questions regarding PA that 10

were of particular interest in this study (e.g. ‘Do you participate in organised sports?’). These 11

Dutch questions were adapted versions of the Adolescent Physical Activity Recall 12

Questionnaire (APARQ), of which the reliability and validity is said to be acceptable (kappa 13

coefficients ranged from 0.33 to 0.71, depending gender and school grade) (4). Children 14

completed the questionnaire after completion of the Eurofit test in the gymnasium or in class 15

later in the week. In the later scenario, mentors supervised the completion of the 16

questionnaire. 17

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Paragraph 6: Measurements 19

Body height and body weight of children was measured according to the Eurofit protocol (8). 20

Height was measured in centimetres to the nearest 1,0 cm with a portable stadiometer (Seca 21

206, Leicester Height Mesaure; Seca GmbH & CO., Hamburg Germany). For practical 22

reasons, body weight was taken with children wearing clothes, but without shoes and heavy 23

accessories. Body weight was measured in kilogram (kg), with a digital scale (SECA 877; 24

Seca GmbH & Co., Hamburg, Germany). Skinfold thickness (measured at the triceps, biceps, 25

Physical fitness of adolescents

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subscapular and suprailiac sites) was measured to the nearest 0.2 mm using a Harpenden 1

skinfold calliper, on the left side of the body. 2

PF was measured with the Eurofit test battery (8), which has been widely accepted 3

and is currently the most widely used in European children and adolescents (15). The test 4

battery was developed as a standardised European fitness battery used to assess the 5

effectiveness of physical education and to measure the PF of schoolchildren (8). The eight 6

test items cover cardiorespiratory endurance, muscular strength, muscular endurance, 7

flexibility and speed and agility. Children in this study completed all test items from the 8

Eurofit test battery, mostly in the following sequence: standing broad jump (SBJ), bent arm 9

hang (BAH), 10 x 5 m shuttle run (SHR), sit and reach (SAR), plate tapping (PLT), sit-ups 10

(SUP), hand grip (HGR). A brief description of all Eurofit test items is available in 11

Supplement S1. If a certain test item was already occupied, the group of children moved to 12

another test item and returned to the particular test item later. SBJ, SHR, SAR, PLT, and HGR 13

were completed twice, unless not enough time was provided. In this scenario these test items 14

were only completed once initially. SUP, ESR and BAH were always completed once. The 15

ESR was either completed before or after the previous measures or during another physical 16

education lesson, in which case the teachers recorded the results. 17

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Paragraph 7: Data scoring 19

In accordance with the Eurofit test protocol (8), the best score of every test item was used in 20

further analyses. Since the HGR score was dependent on body weight, HGR was converted to 21

relative hand grip (rHGR) with help of body weight. Since the length of the gymnasium 22

differed between schools, the ESR was completed over 18 or 20 meters, depending on the 23

length of the gymnasium of the particular school. To be able to compare the results of the 24

different schools, the running speed of the last completed stage was used in analyses of the 25

Physical fitness of adolescents

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ESR, as was also done in the study of Tomkinson et al (33). The running speed was 1

calculated with help of the length of the gymnasium and time needed to complete a particular 2

stage. 3

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Paragraph 8: Statistical analyses 5

Mean values and standard deviations were calculated for all variables and grouped according 6

to gender (girl, boy) and chronological age (12-16 years). Possible interaction effects between 7

gender and age were explored with linear regression models for all test items and 8

anthropometric data. Significant interaction effects were found for sum of skinfolds and for 9

all test items, with the exception of SAR. For consistency, all analyses were run separately for 10

boys and girls. 11

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Paragraph 9: Statistical analyses of Eurofit test scores 13

Eurofit test item scores and anthropometric data with a skewed distribution were log-14

transformed to meet normality criteria (of SHR and SBJ, BMI and sum of skinfolds). BAH 15

also showed a skewed distribution, but this variable contained 257 zero’s (boys: 83 zero’s; 16

girls: 174 zero’s) that could not be log-transformed. Since analyses could not be executed 17

with skewed data, BAH was excluded from all analyses. 18

Children were clustered into eight different schools and therefore the influence of 19

school as a significant level was examined with multilevel analysis in MLwiN (MLwiN 20

2.22), for every Eurofit test item and BMI and sum of skinfolds. These analyses showed 21

significant level effect of school for BMI, SHR and ESR. Although school did not show a 22

significant level effect in the other variables, all variables were analysed with clustering in 23

MLwiN. 24

Since BMI of adolescents increases with increasing age (14) and several studies have 25

established a significant negative correlation between BMI and PF (7,34) and fat-percentage 26

Physical fitness of adolescents

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and PF among adolescents (18,40), BMI will probably be of influence in the PF results. 1

Therefore, to be able to find a true effect of age on PF, analyses were adjusted for BMI as 2

possible confounding variable. Furthermore, since PA has a great effect on PF (24), analyses 3

were also adjusted for organized sports participation as possible confounding variable. 4

To examine the relationship between age and Eurofit test item scores and 5

anthropometric data, multilevel regression analyses were executed for every test-item, 6

adjusting for BMI and organised sports participation as confounder when necessary: adjusted 7

for BMI with sum of skinfolds, SBJ, SHR, SAR, SUP, rHGR and ESR in boys and sum of 8

skinfolds, SBJ, SHR, PLT, SUP, rHGR and ESR in girls; adjusted for organized sports 9

participation with sum of skinfolds, SAR and ESR in boys and SBJ, SHR, SUP and ESR in 10

girls. 11

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Paragraph 10: Statistical analyses of difference-scores 13

To examine whether there was a difference in progress with age in contemporary Eurofit test 14

scores and the progress of PF in boys and girls from 1987 (8), difference-scores were 15

calculated by subtracting reference scores from original contemporary Eurofit test scores. 16

Thus also for variables that were log-transformed for prior analyses, difference-scores were 17

calculated from the non-log-transformed data. For the reference scores, the average reference 18

score, as indicated by the Eurofit test battery, was used. These reference scores come from 19

12-16-year-old boys and girls from 1987, of all levels of education, and indicate that 40% of 20

the scores fall below these values and 40% of the scores lie above these values. Since average 21

reference scores were often indicated by range, the mean of this range was taken as reference 22

score. A negative difference-score indicates a contemporary test score worse than the 23

reference score. Items for which a higher value indicated a worse score (sum of skinfolds, 24

BMI, PLT and SHR) were inverted (i.e. multiplied with -1) for consistency (in this case, a 25

Physical fitness of adolescents

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negative difference-scores always indicates a contemporary score worse than the reference 1

score). Visual analyses of the difference-scores showed that all difference-scores were 2

normally distributed. 3

The effect of school as a significant level in the analyses of the difference-scores was 4

examined with MLwiN software. These analyses showed a significant effect of school on the 5

difference-scores of SHR and ESR. Although the difference-scores of the other variables did 6

not show a significant level effect of school, all variables were analysed with MLwiN, with 7

clustering. This was done since it is likely that school had an effect on all variables, although 8

this was not visible in the analyses. 9

To examine the relationship between age and difference-scores, multilevel regression 10

analyses were executed, adjusted for the difference-score of BMI when necessary (SBJ, PLT 11

and ESR for boys and sum of skinfolds, SAR, SUP and ESR for girls). In addition, one-12

sample t-tests were executed to compare mean Eurofit test scores and anthropometric data 13

with reference scores of 12-16-year-old boys and girls. The alpha level of statistical 14

significance was set at p<0.05 for all analyses. Data are presented as means (standard 15

deviation) unless otherwise stated. 16

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Paragraph 11: RESULTS 18

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Data were only included if both Eurofit test data and questionnaire data were present and if 20

age was between 12 and 16 years. To be able to make maximum use of the collected data, all 21

valid data on Eurofit tests were included. In consequence, sample size vary for the different 22

Eurofit test items and sum of skinfolds and BMI (see Supplement S2). Of the initial 1378 23

boys and 1257 girls, complete data sets for 921 boys and 904 girls were available for further 24

analyses. Data sets were lost primarily due to missing Eurofit test data or missing data from 25

the questionnaire. 26

Physical fitness of adolescents

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Contemporary mean scores, reference scores and difference-scores for all Eurofit test 1

items and anthropometric data are reported in Figure 1A for boys and in Figure 1B for girls. 2

For all items, with the exception of BMI, sum of skinfolds, SHR and PLT, a higher value 3

indicates a better score. 4

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INSERT FIGURE 1. 6

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Paragraph 12: Eurofit test scores 8

With boys, after adjustment for BMI and organised sports participation for the necessary 9

variables, regression analyses showed significant effect of age for sum of skinfolds, BMI, 10

SBJ, SHR, PLT, SUP, rHGR and ESR. Sum of skinfolds decreased, BMI increased and scores 11

of Eurofit test items significantly improved with age. With girls, after adjustment for BMI 12

and organised sports participation for the necessary variables, regression analyses showed 13

significant effect of age on BMI, SBJ, SHR, SAR, PLT, SUP, rHGR and ESR. With 14

increasing age, BMI increased, scores on SAR, PLT and rHGR improved and scores on SBJ, 15

SHR, SUP and ESR deteriorated. All regression coefficients are reported in Table 1. 16

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INSERT TABLE 1. 18

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Paragraph 13. Difference-scores 20

Difference-scores are presented in Figure 1 as vertical lines between reference scores and 21

contemporary test scores. Multilevel regression analyses for boys showed a significant effect 22

of age on the difference-scores SAR and SUP. For SAR, the constant has a value of 17.07 and 23

a ß of -0.914. This indicates that boys 12- and 16-years of age have a difference-score of 6.03 24

and 2.38 respectively, indicating that the contemporary scores are better than the reference 25

Physical fitness of adolescents

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scores, but this difference gets smaller with age. For SUP, the constant has a value of -6.50 1

and a ß of 0.40. According to the regression, boys 12- and 16-years of age have a difference-2

score of -1.7 and -0.1 respectively. This indicates that although the contemporary scores are 3

worse than the reference scores, this difference gets significantly smaller with age. 4

With girls, after adjustment for the BMI difference-score when necessary, multilevel 5

regression analyses showed a significant effect of age on the difference scores of sum of 6

skinfolds, SBJ, SHR, PLT, rHGR and ESR. All these items have positive constants and 7

negative regression coefficients. Although the constants are positive, according to the 8

regression, difference-scores have a negative value with girls 12-years of age for sum of 9

skinfolds, SBJ, SHR, PLT and rHGR, indicating that contemporary scores are worse than the 10

reference scores and this difference gets significantly larger with age. For ESR, according to 11

the regression, the difference-score has a value of 0.18 and 0.03 at age 12 and 13 respectively. 12

From age 14, the difference-score is negative. This indicates that although the contemporary 13

scores are better than the reference-scores at age 12 and 13, the difference gets negative at 14

age 14 an grows larger till age 16. 15

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INSERT TABLE 2. 17

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Paragraph 14. Comparison of test scores with reference scores 19

One-sample t-tests were executed for all test items for 12-16-year-old boys and girls, with the 20

exception of BAH. Results showed significant differences for most test items in most age 21

groups in both boys and girls. All significant differences between reference scores and 22

contemporary scores are presented in Figure 1 with an asterisk. The following test items did 23

not differ significantly in the corresponding groups: PLT in 13-, 14- and 16-year-old boys, 24

Physical fitness of adolescents

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ESR in 13-, 14-, 15- and 16-year-old boys and 12-year-old girls, SUP in 16-year old boys and 1

SAR in 16-year-old girls. All the other scores differed significantly from the reference scores. 2

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Paragraph 15: DISCUSSION 4

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The first aim of this study was to investigate the PF, as measured with the Eurofit test, of 12-6

16-year-old boys and girls following preparatory secondary vocational education. The second 7

aim of this study was to examine the difference in the progress of PF with age, as measured 8

with the Eurofit test, between boys and girls in the current study and boys and girls from 9

1987 (8). This study showed that boys’ Eurofit test scores improved with age in 6 out of 7 test 10

items (BAH excluded) and remained stable in one, indicating that boys’ PF, as measured with 11

the Eurofit test, improved with age. In girls, Eurofit test scores improved in 3 out of 7 items 12

(BAH excluded) and deteriorated in 4 items. Levelling this out, it is concluded that PF of 13

girls remained relatively stable. Since multilevel regression analyses were adjusted for BMI 14

in most Eurofit test items and was adjusted for organised sports participation in several 15

others, it can be concluded this is a real effect of age. 16

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Paragraph 16 18

Unfortunately, BAH was not normally distributed and could not be log-transformed due to 19

the great amount of zero values, which resulted in the exclusion of this variable in the Eurofit 20

test analyses. In the current study, 9.24% of boys and 20.07% of girls scored 0 in the BAH. 21

Ortega et al (25) reported that in their study 28% of boys and 39% of girls scored 0 in the 22

test. Since children aged 6 to 18 participated in that study, this might explain the higher 23

percentage of children scoring 0. Woods et al (40) concluded that body fat percentage is the 24

main determinant of performance in BAH and therefore is not a valid test to assess muscular 25

endurance. This is supported by a review Castro-Piñero et al (5), who also concluded that the 26

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BAH is not appropriate to measure upper body endurance strength in adolescents. Taken 1

together, it seemed to be a good choice to exclude BAH from these analyses. 2

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Paragraph 17 4

Our findings are partially in accordance with our hypothesis and partially in agreement with 5

the findings of Ortega et al (25), who reported a trend towards increased PF in boys as their 6

age increased, whereas the PF in girls was more stable across ages. The main difference 7

between our findings and our hypothesis and Ortega’s findings is the fact that girls in the 8

current study also showed deterioration in scores with age in four test items. This finding 9

might be explained by the fact that solely girls following preparatory secondary vocational 10

education participated in the current study, whereas participating children in the study of 11

Ortega et al (25) represented an average education level. Recent studies have shown that a 12

positive link between PF and academic performance exists, and have shown this relationship 13

to be stronger in girls than in boys (19). This might explain the fact that girls in the current 14

study showed some deterioration in PF, while the girls in the study of Ortega et al did not, 15

and the fact that the findings regarding boys are similar between studies. 16

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Paragraph 18 18

The current study also examined the progress of PF with age by comparing current Eurofit 19

test scores with reference scores from 1987 (8). With this method, we were able to indicate 20

whether the progress of contemporary Eurofit test scores with age was in accordance with the 21

progress of the reference scores, or whether the progress in scores was better or worse. A 22

significant effect of age on difference-scores indicated that contemporary Eurofit test scores 23

of boys or girls did not show the same progress in scores as the reference scores. Since BMI 24

is a determinant of PF and contemporary BMI values were significantly higher than reference 25

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values in all age groups in boys and girls, this increase in BMI could partially explain the 1

difference in progress. Therefore, multilevel regression analyses were adjusted for the 2

difference-score of BMI, to be able to find a true difference in progress of PF with age 3

between contemporary Eurofit test scores and reference scores. 4

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Paragraph 19 6

This study found that in boys the difference-scores remained relatively stable across ages. 7

Only BAH, SAR and SUP revealed a significant effect of age on difference-scores and hence 8

a difference in progress. The difference-scores of BAH revealed that contemporary scores are 9

worse than reference scores and this difference got larger with age. The difference-scores of 10

SAR revealed that the contemporary scores are better than the reference scores, but this 11

difference gets smaller with age. Finally, the difference-scores of SUP revealed that the 12

contemporary scores are worse than the reference scores, but this difference gets smaller with 13

age. Taken together, it is concluded that the PF of boys in the current study mainly followed 14

the progress of PF of boys from 1987. 15

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Paragraph 20 17

In girls, the progress of PF of the contemporary scores differed more from the progress of PF 18

in girls from 1987. Age had a significant effect on the difference-scores of sum of skinfolds, 19

SBJ, SHR, PLT, rHGR and ESR. In all of these items, with the exception of ESR, 20

contemporary scores were worse than the reference scores in all age-groups and the 21

difference-scores got larger with age. In ESR, contemporary scores were better than 22

reference-scores in 12- and 13-year-old girls, but from age 14, scores got worse and the 23

difference grew larger with age. These findings indicate that the progress of PF of girls in the 24

current study lagged behind in comparison with the progress in PF of girls from 1987. 25

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Importantly, since analyses were adjusted for the difference-score of BMI, these results 1

indicate that girls in the current study performed worse independent of their higher BMI. 2

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Paragraph 21 4

Importantly, although Eurofit test scores of girls showed a significant improvement in PLT 5

and rHGR with age, the difference-scores of these variables indicated that the contemporary 6

progress of scores of these items was significantly worse than the progress of the reference 7

scores of these items. This stresses the fact that the improvement in Eurofit test scores lagged 8

behind in comparison with the reference scores. This is worrisome, since this indicates that 9

the discrepancy between the current Eurofit test scores and the reference scores increased 10

with age. In addition, it is important to acknowledge the effects of low PF levels on health. A 11

recent review (24) reported an association of both cardiorespiratory and muscular fitness with 12

established and emerging cardiovascular disease risk factors. Additionally, the report 13

concluded that improving cardiorespiratory fitness has positive effects on anxiety, depression, 14

mood status and self-esteem and improving muscular fitness seems to have a positive effect 15

on skeletal health. 16

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Paragraph 22 18

Two important factors influencing PF are BMI and the amount of time spent in PA 19

(7,24). In the analyses of the difference-scores, adjustments could be made for the difference 20

in BMI. However, since no numbers on the amount of time spent in PA are available of 21

adolescents from 1987, the effect of this difference could not be examined. However, the 22

study of Dollman et al (11) reported a general, secular decline in PA in many countries, 23

explained by the increasing availability of electronic and screen based entertainment and 24

sociocultural changes. This indicates that the time spent in PA might have differed between 25

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contemporary boys and girls and boys and girls from 1987. However, since no data on PA 1

exists of boys and girls from 1987, no definite statement can be made about the effect of PA 2

on PF in this regard. However, to stress the importance of PA on PF, secondary analyses were 3

executed to analyse the difference between boys and girls participating and not participating 4

in organised sports. Multilevel regression analyses, clustered for school for all items and 5

adjusted for BMI for all Eurofit test items and sum of skinfolds, revealed that boys 6

participating in organised sports scored significantly better than boys not participating in 7

organised sports on BMI, SHR, SAR, SUP and and ESR. Girls participating in organised 8

sports scored significantly better than girls not participating in organised sports on BMI, SBJ, 9

SHR, SAR SUP and ESR (p < 0,05 for all variables). These results underline the importance 10

of PA on PF. However, PA does not seem to effect PLT and rHGR, indicating that variables 11

other than PA effect the performance on these test items as well. 12

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Paragraph 23 14

Regardless of the progress of PF, contemporary mean test scores of both boys and girls were 15

significantly worse than reference scores in many test items in most age groups. This 16

indicates that contemporary PF, as measured with the Eurofit test, of both boys and girls is 17

worse than the PF of boys and girls in 1987. The only significant improvement in scores was 18

seen in SAR, in which boys and girls, with the exception of 16-year old girls, scored better 19

than boys and girls from 1987. This might be explained by the higher BMI values of 20

contemporary boys and girls. A recent study from Nikolaidis (22) also reported better scores 21

on the SAR-test for overweight girls and women, in comparison with normal weight girls and 22

women. However, another study found no (12) association between BMI and flexibility 23

performance, indicating that the relationship between sit-and-reach test performance and BMI 24

is unclear. One hypothesis (17) states that enhanced muscularity associated with higher levels 25

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of lower body explosive strength (which in turn is related to a lower risk of obesity (17)) has 1

a negative impact on flexibility. However, secondary analyses of data from the current study 2

does not support this finding. Pearson correlation analyses for both boys and girls show a 3

significant negative correlation between BMI and SBJ (a measure of lower body explosive 4

strength; boys r = -0.221 , p < 0.05; girls r = -0.337, p < 0.05), indicating that higher levels of 5

lower body explosive strength are related to lower levels of BMI; hence a lower risk of 6

obesity. However, Pearson Correlation analyses also show a significant positive correlation 7

between SAR and SBJ for boys (r = 0.232, p < 0.05) and girls (r = 0.261, p < 0.05) and a 8

weak but significant correlation between BMI and SAR for boys (r = 0.077, p < 0.05), but not 9

for girls. These results indicate that higher levels of lower explosive strength do not 10

negatively affect flexibility, thereby not supporting the above mentioned hypothesis. 11

Furthermore, higher BMI levels were associated with better SAR-scores in boys, but not in 12

girls. Therefore, the higher SAR-scores of boys and girls in the current study, in comparison 13

with scores from 1987, can not solely be explained by higher BMI-levels. Further research is 14

needed to better understand the association between BMI and SAR and to understand the 15

factors influencing SAR test scores. 16

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Paragraph 24 18

This study contains some major strengths; it is the first study to examine PF levels 19

systematically in a large population of children following preparatory secondary vocational 20

education, with the Eurofit test, which is a well appreciated test for PF (38). A recent 21

systematic review with regard to validity of the Eurofit test concluded that the ERS is the 22

most appropriate test to assess cardiorespiratory fitness (5,17) and that the HGR and SBJ are 23

valid tests to assess musculoskeletal fitness (5,26). For other test items there is limited 24

evidence about their validity due to a limited number of studies. Additionally, this study was 25

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the first study to calculate difference-scores and compare the contemporary progress of PF 1

with age in 12-16-year-old boys and girls with the progress of PF in boys and girls from 2

1987. Finally, all anthropometric measures were performed by the same observer, which 3

likely increased the reliability of measurements. 4

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Paragraph 25: Limitations 6

There were a few limitations to this study that need to be acknowledged. Since not all schools 7

offered the opportunity to administer the questionnaire ourselves, teachers often administered 8

the questionnaire to the students. This led to missing data from the questionnaire. By 9

administering the questionnaire ourselves, the loss of questionnaires could have been 10

diminished. Furthermore, different students accompanied the children during the tests. 11

Interrater reliability has not been examined, but it is imaginable that small methodological 12

differences exist between the students, which should be examined in future studies. Finally, 13

only Dutch boys and girls following preparatory secondary vocational education participated 14

in this study. Therefore, the results of this study cannot be generalised to the whole 15

population of 12-16-year-old adolescents. 16

17

Paragraph 26 18

For future studies it would be recommended to execute a longitudinal study instead of a cross 19

sectional study, to be able to report individual changes in PF levels. In addition, in the current 20

study, scores were categorised in age groups with help of the children’s chronological age, as 21

indicated by the Eurofit test battery, whereas biological age might be more accurate. It would 22

be interesting to examine the difference between categorising the scores with help of their 23

chronological age and categorising the scores with their biological age. Also, only children 24

following preparatory secondary vocational education participated in the current study. It is 25

Physical fitness of adolescents

19

possible that children of other levels of education do not show the same progress in PF as 1

these children. It is important to examine the differences between children following different 2

kinds of education, to be able to identify children at greater risk for lower PF. Finally, to 3

investigate the individual effects of BMI and PA on PF levels, it is suggested to perform a 4

longitudinal study in which either or both BMI and/or PA-levels are manipulated. However, 5

up to date, no specific intervention has been identified that has the ability to increase PA 6

levels effectively. Many studies show limitations in study design, lack of statistical power and 7

problems with implementation, which have likely hindered the effectiveness (3). Reviews 8

report the use of multi-component interventions and interventions that included both school 9

and community or family involvement to have potential to make a difference in PA levels 10

(20,39). Interventions in primary care settings and tailored advice counselling seems to be 11

effective as well (29). 12

13

Paragraph 27 14

This study showed that PF of 12-16-year old boys, as measured with the Eurofit test, 15

improved with age. In girls, PF remained relatively stable. Moreover, boys showed largely 16

the same progress in PF with age as boys from 1987, whereas the increase in PF of girls in 17

the current study lagged behind in comparison with girls from 1987. This lag is not due to an 18

increase in BMI, since the effect was apparent after adjusting for the difference-score of BMI, 19

indicating that the progress in PF of girls in the current study truly is worse than the progress 20

in PF of girls from 1987. In addition, mean test scores of both boys and girls were 21

significantly worse than reference scores in six out of eight test items in most groups. 22

Although the reference scores are no normative values, and thus it cannot be concluded that 23

PF of boys and girls in the current study is under par, it is important to acknowledge the 24

Physical fitness of adolescents

20

effects of low PF on health. Since PA and BMI are main determinants of PF, it is important to 1

improve PA levels and offer weight management to get PF to a higher level. 2

3

Paragraph 28: ACKNOWLEDGEMENTS 4

None 5

6

Paragraph 29: CONFLICT OF INTERESTS 7

All authors declare there is no conflict of interests. 8

9

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