Validity and reliability of the Ekblom-Bak test in children 10 to 14 years of age

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Validity and reliability of the Ekblom-Bak

test in children 10 to 14 years of age

Andrea Eggers

THE SWEDISH SCHOOL OF SPORT

AND HEALTH SCIENCES

Master Degree Project 26:2016

Master Education Program in Sport Science: 2015-2016

Supervisor: Björn Ekblom

Examiner: Mikael Mattson

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Abstract

Aim

The aim of this study was to evaluate the validity and reliability of the Ekblom-Bak test in children. Specific questions were:

- Is the Ekblom-Bak test valid in children 10-14 years of age?

o Does the validity vary in prepubescent and pubertal children? - How reliable is the Ekblom-Bak test in this age group?

Method

54 children (27 boys and 27 girls) aged 10-14 years were included in the study. The setup of the study consisted of three parts, 1. Validity, consisting of physical tests including the Ekblom-Bak test and an incremental VO2max treadmill test, 2. Test-retest reliability of the physical tests, 3.

Confounding factors, consisting of a physical examination conducted by either a female nurse or

a male intern including measurements of resting blood pressure, weight, height and maturity level determination using Tanner stages.

Results

54 children (mean age 11 years, mean height 153.3 cm, mean weight 40.8 kg, mean VO2max 2.22

l·min-1 corresponding to 54.9 ml·kg-1·min-1) conducted the physical tests, 16 of them also

conducted the retests and 40 participants’ the physical exam. No significant differences were seen between girls and boys for any of the subject characteristic measures. The Ekblom-Bak test overestimates VO2max with 0.16 l·min-1 (Standard Deviation 0.41 and 95 % Confidence Interval

0.04-0.27), or when expressed in relative values 3.88 ml·kg-1·min-1, compared to measured VO2max. A significant correlation between estimated and measured VO2max could be found, r =

0.603 and R2=0.364. Standard Error of Estimate between estimated and measured VO2max is

0.275 l·min-1. Further investigations show that prepubescent, early pubertal children and boys are mainly overestimated. Test-retest correlation of the Ekblom-Bak test is r = 0.84.

Conclusion

The Ekblom-Bak test is highly reliable in children 10-14 years of age and valid for pubertal girls, but the prediction equations derived from the test should not be used in prepubescent children or in boys.

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Sammanfattning

Syfte och frågeställningar

Syftet med denna studie var att studera validitet och reliabilitet för Ekblom-Bak testet hos barn. Specifika frågeställningar var:

- Är Ekblom-Bak testet valitt för barn i åldrarna 10-14 år?

o Skiljer sig validiteten mellan prepubertala och pubertala barn? - Hur reliabelt är Ekblom-Bak testet i denna åldersgrupp?

Metod

54 barn (27 pojkar, 27 flickor) i åldrarna 10-14 år inkluderades i studien. Upplägget bestod av tre delar, 1. Validitiet, innehållande fystesterna Ekblom-Bak test och ett stegrande VO2max

löpbandstest, 2. Test-retest reliabilitet av fystesterna, 3. Andra påverkande faktorer, bestående av en hälsokontroll utförd av antingen en kvinnlig sjuksyster eller manlig AT-läkare bestående av mätningar av vilo-blodtryck, längd, vikt och bestämning av mognadsgrad med hjälp av Tanner-skalan.

Resultat

54 barn (medelålder 11 år, medellängd 153,3 cm, medelvikt 40,8 kg, medel VO2max 2,22 l·min-1

motsvararande 54,9 ml·kg-1·min-1) utförde fystesterna, 16 av dem utförde även återtesterna och 40 deltagare genomgick hälsokontrollen. Ingen signifikant skillnad kunde ses mellan flickor och pojkar i någon av bas-egenskaperna. Ekblom-Bak testet överskattar VO2max med 0.16 l·min-1

(Standard Deviation 0,41 och 95 % Konfidensintervall = 0,04-0,27), eller i relativa värden 3,88 ml·kg-1·min-1, i jämförelse med uppmätt VO2max. En signifikant korrelation mellan uppskattat och

uppmätt VO2max hittades, r = 0,603 och R2 = 0,364. Standard Error of Estimate mellan uppskattat

och uppmätt VO2max är 0,275 l·min-1. Vidare utredning visar att prepubertala, tidigt pubertala

barn och pojkar huvudsakligen överskattas. Test-retest korrelation av Ekblom-Bak testet är r = 0,84.

Konklusion

Ekblom-Bak testet är i hög grad reliabelt hos barn i åldrarna 10-14 år och valitt för pubertala flickor, men prediktionsekvationen som härrör från testet bör inte användas för prepubertala barn eller för pojkar.

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1. Introduction

Maximal oxygen uptake (VO2max) is the maximal rate for oxygen consumption during exercise.

VO2max is not just a measurement of your aerobic fitness, but also a strong health predictor in

both children and adults. For example, a low VO2max correlates strongly with all cause mortality.

To measure VO2max, expensive lab equipment is needed and the participant has to perform a

maximal effort. Therefore, submaximal tests that do not require expensive equipment, and where the participant does not have to perform at maximal level, have been developed. The Ekblom-Bak test is a relatively new submaximal ergometer cycle test that predicts VO2max with high

accuracy in adults. However, the validity and the reliability for the Ekblom-Bak test in children are unknown.

1.1 Aerobic fitness in children and adolescents

VO2max is extensively recognized as the best measure there is for aerobic fitness (Armstrong

2013; Howley, Bassett & Welch 1995) and is also a strong health predictor in both children and adults. For example there is strong evidence for substantially decreased risk of all cause mortality and cardiovascular events in adults with high VO2max (Kodama et al. 2009). In children, a high

VO2max is also strongly correlated with decreased risk of cardiovascular diseases (Andersen et al.

2008; Hurtig-Wennlöf et al. 2007), but also other health parameters such as decreased risk of depression (Esmaeilzadeh 2015), decreased metabolic syndrome markers (Brage et al. 2004; Rizzo et al. 2007), increased insulin sensitivity (Henderson et al. 2012) and increased cognitive performance (Hillman et al. 2014).

Absolute aerobic capacity (VO2max expressed in l·min-1) seems to increase relatively linear with

age in boys, and a similar trend can be seen in girls according to cross-sectional data (Armstrong 2013). 15-year old boys have around the double absolute VO2max value compared to 9-year old

boys, while girls improve their absolute VO2max with around 60 % between the ages 9 to 15

(Kolle et al. 2010). In regard to the relative VO2max value (expressed as ml·kg-1·min-1), it is

usually increased in boys with age and decreased in girls with age, mostly explained by changes in body mass with an increased amount of fat mass in adolescent girls (Kolle et al. 2010; Pate et al. 2006). Normally, boys and girls with normal body mass have a higher relative VO2max

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compared to their overweight peers, and just as in the adult population, sedentary behaviors and non-active commuting correlates negatively with VO2max in children (Pate et al. 2006).

Both absolute and relative aerobic capacity have decreased in 16-year olds in Sweden from 1987 to 2007 (Ekblom, Ö., Ekblom-Bak & Ekblom, B. 2011) while Armstrong (2013) reports a small mean change of around 0.1 % decrease in VO2max per decade in children and adolescents from

five different countries.

1.2 VO

2

measurements

It is very important to be able to measure VO2max, since it is such a good marker for health and

since the values of VO2max have decreased in the past decades. Whole-body oxygen consumption

(VO2) is calculated as the product of cardiac output (Q), which gives stroke volume (SV) x heart

rate (HR), and arteriovenousoxygen difference (a-vO2 diff). This principle is called the Fick

equation, named after cardiovascular physiologist Adolph Fick. VO2 = Q x (a-v)O2 diff

Or

VO2 = HR x SV x (a-v)O2 diff

The Douglas bag method is considered to be golden standard in measuring VO2 and calculates

VO2 via the difference in inspired oxygen (O2i) and expired oxygen (O2e) times ventilation (VE).

The equation being used to calculate VO2 in l·min-1 is:

VO2 = (O2i – O2e)VE

The Douglas bag method is the most accurate method up to date, but is a relatively slow method whereas more modern and rapid electronic computer systems have been developed (Kenney, Wilmore & Costill 2015, s. 120ff).

1.3 VO

2max

testing

In order to reach VO2max, a whole-body exercise (e.g. uphill running or arm + leg cycling) with

maximum effort has to be performed. One way to reach VO2max is to perform an incremental

VO2max test on a treadmill (Howley, Bassett & Welch 1995). If VO2max has been reached or not is

usually determined with some varying criteria’s, where the presence of a plateau in VO2, i.e.

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in adults. This is despite the fact that the criterion varies between studies and that not all people are capable of reaching a plateau when performing a VO2max test (Howley, Bassett & Welch

1995). Children are even less capable of reaching a plateau than adults (Armstrong et al. 1995). Since a plateau criterion is not 100 % accurate, secondary criteria are also often used. For example are a Respiratory Exchange Ratio (RER) > 1.1-1.15, blood Lactic Acid (LA)>5.5-8.8 mM (Howley, Bassett & Welch 1995) and/or a maximum HR criterion often used in adults.

The most common equations to calculate maximal HR are 220-age and 208-07*age. Both

equations are based on the premise that maximal HR decreases with age, and are derived from an adult population. Furthermore, the above-mentioned equations also tend to misclassify maximum HR. Hence, the equation 208-07*age seems to be more accurate than 220-age, in children and adolescents (Machado & Denadai 2011; Mahon et al. 2010). Since maximum HR is often misclassified, many researchers’ state that predicted maximum HR should not be used as a criterion of achieved VO2max (Howley, Bassett & Welch 1995). When it comes to LA, children

do not have the same capability to produce LA as adults (Armstrong et al. 1995), which might be one of the reasons that children less frequently achieve a plateau in an incremental VO2max test.

Also, since children have a harder time producing LA they are also less capable of reaching a RER as high as adults, therefore the criterion of a RER >1.0 is often used in children (Armstrong 2013). Anyhow, there is still no consensus in the question of what criteria that should be used to verify VO2max, neither in adults nor in children.

1.4 Tests for estimation of VO

2max

1.4.1 Field tests

Aforementioned, a direct measurement of VO2max requires a lab with a lot of expensive

equipment. Therefore, different field tests have been developed in order to estimate VO2max. The

1-mile test, the Cooper test, the 20 meter shuttle run test (20 MSR) and the Yo-Yo Intermittent test are four commonly used field tests that are easy to set up and administer for a large group of people at the same time. The tests ability to predict actual VO2max have varying accuracy

(Bandyopadhyay 2015; Póvas et al. 2015; Weiglein et al. 2011) and are, as previously mentioned, easy to perform on larger groups with good efficiency. On the other hand, these field tests require the subjects to run or walk at their maximum level (Council of Europe 1988, s. 24), which for a

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variety of reasons could be harmful or not suitable for everyone. Another aspect is that field tests do not measure VO2max directly, but is more of a performance test where VO2max only is one of

the components along with psychological and other physiological mechanisms. As a result, submaximal tests have been developed, that do not require the subject to exercise at their

maximal level. Many of these tests are based on the subjects’ HR response to a given submaximal work load, with prediction models for calculation of VO2max (Åstrand 1960; Åstrand & Ryhming

1954; Ekblom-Bak et al. 2014; Council of Europe 1988, s. 30ff).

1.4.2 Submaximal cycle ergometer tests

The most common cycle ergometer submaximal test is the Åstrand test. The Åstrand test is being used all over the world and is widely validated in different gender- and age groups (Washburn & Montoye 1984; Ekblom 2014). The Åstrand test consists of 6 minutes of cycling on one

submaximal work rate to generate a steady state HR. Results are derived from the Åstrand-Rhyming Nomogram, via extrapolation from the HR at a submaximal workload to a theoretical maximal HR and are the base for the estimation of VO2max. This prediction is based on the theory

that the relationship between HR and power output is relatively linear. VO2max can therefore be

predicted from power output with a somewhat good accuracy (Åstrand & Ryhming 1954).

Stewart and Gutin (1975) evaluated the Åstrand-Rhyming method in boys 10-12 years and showed a correlation of r = 0.60 (p <0.05) between measured and estimated VO2max when

expressed in l·min-1_{and r = 0.55 (p <0.05) when expressed in ml·kg}-1_·min-1_{. Wells et al. (1973)}

found r to be 0.76 (p <0.01) and 0.28 (not significant) in teenaged well-trained boys and girls respectively between measured and estimated VO2max from the Åstrand-Rhyming nomogram.

Åstrand & Rhyming (1954) discusses that the test is only valid in healthy 18-30 year olds and other researchers that have validated the test in children have therefore also suggested a revised regression model.

Woynorowska (1980) suggested a regression model for girls and boys where correlations between estimated VO2max and measured VO2max was as high as 0.82 in girls, but only 0.52 for

boys. Binyildiz (1980) tested ninety-six 11-13 year old boys and found a significant correlation (r = 70, p < 0.001) between steady state HR at submaximal work rate and measured VO2max. When

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including height and age in the analysis r was improved to 0.80, with a standard error (SD) of ±18 % of the mean (Binyildiz 1980).

Ekblom (2014) evaluated the Åstrand-Rhyming nomogram and the two above mentioned regression models in 62 children aged 11-12 years. VO2max was measured with a VO2max

treadmill test and estimated with all three methods. Low mean misclassification was found in the Woynarowska and Åstrand-Rhyming nomogram (0.14 l·min-1 and 0.35 l·min-1, respectively), high correlations (r = 0.81 and 0.73, respectively), but high standard error of estimate (SEE) (0.398 l·min-1_{and 0.340 l·min}-1_{, respectively). The Binyildiz method on the other hand gave a}

high correlation (r = 0.87), low SEE (0.298 l·min-1_{) but a large mean underestimation (0.66 l·min} -1_{). All methods underestimated predicted VO}

2max in well-trained children (Ekblom 2014).

The physical working capacity (PWC) test is another submaximal cycle ergometer test that do not require maximal exertion from the subject. PWC is based on the premise that the relationship between HR and oxygen consumption is linear at submaximal, steady state workloads. The PWC test comes in different variants, PWC170 or PWC190 with 2-min stages, 3-min stages or 6-min

stages. PWC170 represents the workload where a HR of 170 beats/min is achieved. The goal is

therefore to, after a certain number of increasingly intense stages, reach a HR of 170 beats/min (Council of Europe 1988). The PWC tests predict VO2max with varying accuracy (r = 0.46-0.87)

in children, depending on sex, age, amount of minutes cycled at each stage, and if VO2max is

predicted with absolute or relative values (Bland, Pfeiffer & Eisenmann 2012; Mahoney 1992; McMurray et al. 1998; Rowland et al. 1993; Wells et al. 1973). For example Rowland et al. (1993) found best correlations for the PWC170 test when VO2 was expressed in absolute values (r

= 0.71 and 0.70 for girls and boys, respectively) while Wells et al. (1973) found a correlation of 0.87 (p <0.01) and 0.56 (p < 0.05) in teenaged well-trained boys and girls respectively between measured and estimated VO2max.

1.4.3 The Ekblom-Bak test

The Ekblom-Bak test is a relatively new test that is based on the theory that at a fixed increased workload the HR increment will be steeper for people with a low VO2max compared with those

with a high VO2max. The test uses delta HR (ΔHR) response between a lower fixed workload

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To perform the test you need a HR monitor, an ergometer cycle, a Borg RPE scale and a stopwatch. The pedaling rate is 60 RPM throughout the whole test and the test starts with the standard workload 0.5 kp for 4 minutes. At minute 3.15, 3.30, 3.45, and 4.00, HR is recorded and averaged. After 4 minutes, the workload increases to a predetermined chosen workload

depending on the subject’s physical activity level. After one minute at the high workload the subject will be asked for perceived exertion using the Borg RPE scale. If RPE is <10 or 10-11 the workload will be increased with 1 kp and 0.5 kp respectively. If RPE is 12-16 the test will

continue, and if RPE is ≥17 the test will be stopped in order to let the subject rest for 20 minutes before starting the test again and performing the high workload at a lower workload. HR will be recorded at minute 3.15, 3.30, 3.45 and 4.00 at the high workload and once again averaged. When the subject have cycled four minutes at the high workload the test is completed (Ekblom-Bak et al. 2014).

To estimate VO2max from the test, ΔHR and delta power output (ΔPO) have to be calculated. ΔHR

is calculated as the difference between HR at the high workload and HR at standard workload. PO is calculated from the workload from the cycle ergometer in W multiplied with 1.08 to correct for frictional loss in chain and drive train. ΔPO is therefore the difference between PO at the high workload and PO at the standard workload. The ΔPO is used as a proxy for ΔVO2 (Ekblom-Bak

et al. 2014).

The first model got published 2014 and includes ΔHR/ΔPO, sex and age in the final regression model (Ekblom-Bak et al. 2014). Anyhow, Björkman and coworkers have revised the model, with better accuracy and for a larger age-span (21-86 years), but the model is yet not published. Still, the new model will be used in this essay. The final new regression model is sex-specific and includes age, (ΔHR/ΔPO), ΔPO and HR at standard work rate.

Men: VO2max = 2.04900 – 0.00858(age) – 0.90742(ΔHR/ΔPO) + 0.00178(ΔPO) – 0.00290(HR at

standard work rate)

Women: VO2max = 1.84390 – 0.00673(age) – 0.62578(ΔHR/ΔPO) + 0.00175(ΔPO) –

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Some of the negative aspects of the Åstrand and the PWC test are that, as previously mentioned, the equations predicting maximal HR are not 100 % accurate and that HR can be affected by stress and nervousness which then affects the outcome of the predicted VO2max. The Ekblom-Bak

test on the other hand is not based on a theoretical maximal HR, but on ΔHR between two workloads. The Ekblom-Bak equation will therefore not be as affected by stress or nervousness since HR will be affected in the same extent at the standard and the high workload. The Ekblom-Bak test also has a very good test-retest correlation of 0.96 (Ekblom-Ekblom-Bak et al. 2014).

Anyhow the Ekblom-Bak test has never been validity and reliability tested in children 10-14 years of age.

2. Aim of the study

The aim of this study was to evaluate the validity and reliability of the Ekblom-Bak test in children 10-14 years of age.

- Is the Ekblom-Bak test valid in children 10-14 years of age?

o Does the validity vary in prepubescent and pubertal children? - How reliable is the Ekblom-Bak test in this age group?

3. Method

3.1 Participants

Statistical power was calculated based on previous research performed on children where SD on the difference between measured and predicted VO2max have been around 0.2 l·min-1 (Ekblom

2014). To be able to find a mean difference of 0.15 l·min-1 population had to be 34 individuals when Power = 0.8 and level of significance = 0.05. 54 healthy children (27 girls and 27 boys) were included in the study. Descriptive characteristics of the subjects’ are presented in the results section in Table 1. The children were recruited via colleagues, peers, friends, schools and/or sport clubs. Included in the study were healthy boys and girls aged 10-14 years. Excluded from the study were children treated with asthma medicine or other medicine influencing HR, children unable to perform the Ekblom-Bak test or an incremental maximal test.

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3.2 Procedures

The study was separated in three parts:

1. Validity consisting of physical tests including the Ekblom-Bak test (see below, page 9) and an incremental VO2max treadmill test (see below, page 9)

2. Test-retest reliability of the physical tests (Ekblom-Bak test and incremental VO2max

treadmill test)

3. Confounding factors consisting of a physical examination (see below, page 8) including measurements of resting BP, weight, height and maturity level determination using Tanner stages

Test-retests were executed 4-14 days after the first occasion. The physical examination was performed within ±30 days as the physical tests.

3.3 Physical examination

The physical examination was executed by a female nurse or a male intern and consisted of measure of height to the nearest 0.1cm, weight with light clothing to the nearest 0.1 kg (Lindells Jönköping, Sweden, type S992, Nr 21997), blood pressure (with Blood Pressure Monitor size 9*28 cm; AB Henry Eriksson, Stockholm, Sweden) and maturity level determination using Tanner stages. Tanner stages determine maturity level by a scale ranging from 1-5, or with other words from prepubescent (=1), to adult (=5). The scale consists of three parameters pubic hair (PH) (both boys and girls) (Marshall & Tanner 1969; 1970), breast size (only girls) (Marshall & Tanner 1969) and testicular volume (only boys) (Marshall & Tanner 1970). In this essay, only the PH parameter will be used.

3.4 Physical tests

3.4.1 Preparations

During the submaximal and maximal test (see below) gas exchange was measured using a computerized metabolic system with the breath-by-breath method (Oxycon Pro; Erich Jaeger GmbH, Hoechberg, Germany). Oxycon Pro has been validity- (Rietjens et al. 2001; Carter & Jeukendrup 2002) and reliability tested (Carter & Jeukendrup 2002) with very good accuracy compared to the Douglas bag method. The Oxycon Pro was started 30 minutes before arrival of the subject in order for the machine to warm up properly. The Oxycon Pro was thereafter calibrated in ambient conditions, volume and gas. Ambient conditions was calibrated with

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HygroPalm 0 (Rotronic, Crawley, West Sussex, UK) as reference and gas was calibrated via an automatic procedure using a certified calibration mixture gas of 15.00 % O2, 5.999 % CO2 (Air

Liquide, Paris, France). Calibration for volume and gas were repeated until variations between two calibrations were < 1 %. HR was recorded throughout the tests using a Polar RS400 watch, with recreation time 5 seconds and a Polar soft strap (Polar Electro Oy, Kempele, Finland).

Before the physical tests all subjects were asked to not do any vigorous physical activity 24 hours prior to the visit and to eat their last heavy meal at least 3 hours before the visit. When the subject arrived the Polar soft strap was applied over the chest and the watch was started to record HR. The visit then started with a 10 minutes rest in a supine position, while information about the procedure and Borg RPE scale was given. Borg RPE is a scale is used to determine perceived exertion and ranges from 6 (no exertion) to 20 (maximum exertion) (Borg 1998). After 10 minutes, the lowest HR during the 10 minutes rest was recorded and BP was measured twice. Weight was measured with light clothing and height to the closest 0.1 cm. The cycle ergometer (model 828E, Monark, Varberg, Sweden) was calibrated and seat and handlebar were

individually adjusted for each participant. The subject subsequently cycled 1-3 minutes on 0.5kp and 60 RPM to get used to the rhythm in the submaximal Ekblom-Bak test (for details, see below, page 9). Thereafter a Hans Rudolph nasal mask in size XS or petite (model 7450) with corresponding head gear (Hans Rudolph inc, Kansas, USA), connected with the Oxycon Pro, was put on the subject and checked if leaking air. If the mask was leaking, it was adjusted or re-applied until completely tight.

3.4.2 Submaximal Ekblom-Bak test

The subject cycled 4 minutes at 0.5 kp and with a pedal frequency of 60 RPM. At minute 3.15, 3.30, 3.45 and 4.00 HR was recorded and averaged, followed by an increase in workload to 1.0 kp. Since this was the first time children performed the test the high workload was always set to 1.0 kp at first, compared to when adults perform the test, the high workload depends on physical activity level (Ekblom-Bak et al. 2014). After one minute at 1.0 kp the subject was asked about their degree of exertion on the Borg RPE scale. For five subjects workload was further increased to 1.5 kp after one minute (for more information, see discussion, page 20). At last, HR was recorded and averaged at minute 3.15, 3.30, 3.45 and 4.00 at the high work rate. Oxygen consumption was measured throughout the test.

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3.4.3 Incremental maximal test

An incremental maximal treadmill test was performed in order to establish VO2max. The

Ekblom-Bak test was followed by a 5-10 minute warm up on the treadmill (RL2500 E, Rodby Innovation AB, Hagby, Sweden) to familiarize the subject with the treadmill. For a lot of the subjects it was the first or second time running on a treadmill, therefore the warm-up started at 5.0 km/h

followed by an increase of velocity to 7-8.5 km/h after 30-60 sec. After a two-minute jog the subject practiced jumping off the treadmill in order to continue the warm-up. After a five-minute warm-up, velocity was increased to 10.0-13.0 km/h for 10-30 seconds in order for the subject to practice to jump off the treadmill at a high speed. Lastly, velocity was decreased to jogging pace again and elevation increased to 3-6° in order for the subject to simulate jumping off the treadmill at high elevation.

The warm up session was followed by a VO2max test that started with a 1° incline and the same

speed as the warm-up-speed, 7-8.5 km/h. Prior to the incremental test the stopwatch was once again started to record HR. Speed and/or incline increased every minute until volitional exhaustion. The test was aimed to last between 5-10 minutes and the subjects were given extensive verbal encouragement in order to achieve their VO2max. When the subject had

completed the test maximal HR was noted and perceived exertion for breathing, legs and whole-body was recorded using the Borg RPE scale. After an eight minute rest the subject performed a supramaximal test on the treadmill in order to verify the incremental VO2max. Data from the

supramaximal test will not be discussed in this essay, but published elsewhere.

For the subjects that performed a test-retest the procedures of the physical tests were as identical as possible, with the same assessor and the same protocols for both the Ekblom-Bak test and the incremental VO2max treadmill test.

3.5 Data exclusion

The incremental VO2max test was accepted if: maximal HR was >190, RER was >0.95, Borg was

>17 in at least one variable and the test lasted >300 sec. The test was only accepted if all four variables were fulfilled. VO2max was referred to as the highest 30 seconds and RER was referred

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In two cases the values from the supramaximal test substituted the values from the incremental test since the subjects failed to fulfill all four criteria in the incremental test, but attained an approved supramaximal test.

3.6 Ethical considerations

Prior to the first visit the subjects’ parents or legal guardians signed a written consent. The subject and their parent(s) or legal guardian(s) got written and verbal information about the study and information about that at any given point, without reason, they could end the participation. The subjects got the opportunity to test their VO2max and previous studies have not reported any

problems with testing VO2max in children or adolescents (Ekblom 2014; Woynarowska 1980;

Binyildiz 1980). The female nurse and male intern that performed the physical exam and maturity level determination were recruited with caution and after ethical approval. The study was

approved by the Local Ethical Committee in Stockholm (dnr 2016/175-31/2).

3.7 Statistics

Continuous descriptive characteristics were summarized as means with SD and min-max. Correlation (r) between estimated and measured VO2max was calculated with spearman

correlation test with corresponding 95 % confidence interval (95% CI). Results in term of absolute validity are presented with standard error of estimate (SEE) and calculated via a

regression. Kendalls Tau correlation was used to investigate whether the data was homoscedastic or heteroscedastic and a Bland-Altman plot was thereafter produced to visualize mean values between estimated and measured values, individual misclassifications (LoA) and 95 % CI. LoA was calculated as 2SD. ANOVA backwards regression was used to determine which parameters that correlated the strongest with misclassification between estimated and measured values. Test-retest reliability of the Ekblom-Bak test and the incremental VO2max test was calculated with

spearman correlation test. Statistical significance was set to P < 0.05 for all analyses.

4. Results

4.1 Participants

54 children (27 girls and 27 boys) conducted the physical tests, 16 of them also performed the retests and 40 participants’ the physical exam. Subject characteristics are given in Table 1. No significant differences were seen between girls and boys, for any of the measures, respectively.

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Maturity level determination is presented in Table 2.

Table 2. Maturity level determination, amount of children in each Tanner stage group

PH1 PH2 PH3 PH4 Total Tanner, girls 5 4 7 5 21 (52.5 %) Tanner, boys 5 11 3 - 19 (47.5 %) Tanner, all 10 (25 %) 15 (37.5 %) 10 (25 %) 5 (12.5 %) 40 (100 %) PH = Pubic Hair

4.2 Validity

The Ekblom-Bak test overestimates VO2max with 0.16 l·min-1 (SD 0.41 and 95 % CI 0.04-0.27) or

when expressed in relative values, 3.88 ml·kg-1·min-1 (SD 9.06 and 95 % CI = 1.41-6.36)

compared to measured VO2max. A significant correlation between estimated and measured VO2max

could be found, both in absolute and relative values, r =0.603, R2 = 0.364 (Figure 1) and r = 0.633, R2=0.401 (Figure 2), respectively. Lastly, SEE between estimated and measured VO2max is

0.275 l·min-1_.

Table 1. Subject characteristics

All (n=54) Mean (min-max) SD Girls (n=27) Mean (min-max) SD Boys (n=27) Mean (min-max) SD Age (year) 11 (10-14) 0.9 12 (10-14) 1.1 11 (10-13) 0.7 Height (cm) 153.3 (134.0-167.2) 8.0 154.1 (134.0-167.2) 8.5 152.4 (136.7-164.6) 7.5 Weight (kg) 40.8 (30.1-57.8) 6.6 40.4 (30.2-50.0) 6.3 41.2 (30.1-57.8) 7.0 Resting syst. BP (mm Hg) 108 (95-125) 6.8 108 (95-125) 7.5 108 (98-120) 6.2 Resting dias. BP (mm Hg) 68 (50-88) 9.4 71 (60-88) 8.2 65 (50-80) 9.8 Resting HR (BPM) 72 (54-117) 10.9 75 (61-117) 12.2 69 (54-84) 8.8 Maximal HR (BPM) 204 (189-218) 6.6 205 (193-217) 5.7 202 (189-218) 7.3

Mean Aerobic capacity

Est. VO2max (l·min-1) 2.38 (1.17-3.37) 0.51 2.09 (1.39-2.91) 0.36 2.66 (1.17-3.37) 0.49

Est. VO2max (ml·kg-1·min-1) 58.8 (33.4-79.4) 11.6 51.6 (38.7-66.0) 7.8 66.0 (33.4-79.4) 10.3

Meas. VO2max (l·min-1) 2.22 (1.56-2.94) 0.34 2.11 (1.56-2.64) 0.32 2.34 (1.65-2.94) 0.33

Meas. VO2max (ml·kg-1·min-1) 54.9 (42.3-66.6) 6.2 52.6 (42.3-63.4) 5.8 57.2 (46.2-66.6) 5.8

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Figure 1. Correlation between measured and estimated VO2max, absolute values

Figure 2. Correlation between measured and estimated VO2max, relative values

1 1.5 2 2.5 3 3.5 1.0 1.5 2.0 2.5 3.0 3.5 M ea su re d V O 2m ax ( L /m in )

Estimated VO2max (L/min)

r = 0.603 (p<0.01) R2_{= 0.364} 30 40 50 60 70 80 30 40 50 60 70 80 M ea su re d V o2 m ax ( m l/ kg /m in )

Estimated VO2max - EB-test (ml/kg/min)

r = 0.633 (p<0.01) R2 = 0.401

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Kendalls Tau correlation was used to investigate whether the data was homoscedastic or heteroscedastic (τ = 0.06, p<0.5, i.e homoscedastic). A Bland-Altman plot (Figure 3) was thereafter produced to show mean values between estimated and measured values, individual misclassifications (LoA) and 95 % CI.

Figure 3. Bland-Altman plot. Full line represents mean bias, the dashed lines closest to the full

line represents 95 % Confidence Interval and the dashed lines furthest away from the full line represent Limits of Agreement.

When performing a regression with ANOVA with backwards method, systolic BP, PH-stages and sex were significant variables for explanation of the estimation error. When investigating mean difference between estimated and measured values in each PH-group and controlling for systolic BP, the results show that group 1 and 2 contributes to the mean overestimation and that PH-group 3 and 4 actually underestimates VO2max (Figure 4 & 5).

Figure 3. Bland-Altman plot. Full line represents mean bias and dashed lines represents 95 % CI

and LoA

0.96 0.27 0.16 0.04 - 0.64 14

and LoA

0.96

0.27 0.16 0.04

- 0.64

and LoA

0.96 0.27 0.16 0.04 - 0.64 1.0 0.5 0.0 -0.5 -1.0 1.5 2.0 2.5 3.0

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Figure 5. Mean differences between estimated and measured values in each PH stage group,

relative values

absolute values. * = Significant values.

relative values 0.27 0.23 -0.14 -0.07 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 M ea n d if fe re n ce e st im at ed m ea su re d (L /m in )

Pubic Hair stages

* *

PH1 PH2 PH3 PH4 7.48 3.51 -1.03 -1.71 -4 -2 0 2 4 6 8 M ea n d if fe re n ce e st im at ed -m ea su re d (m l/ m in /k g)

PH1 PH2 PH3 PH4 7.48 3.51 -1.03 -1.71 -4 -2 0 2 4 6 8 M ean d if fe re n ce e st im at ed -m eas u re d (m l/ m in /k g)

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When also splitting each PH-group by gender, and still controlling for systolic BP, the results show that the values from the boys is the main explanation for the large overestimation in groups 1-3, both in absolute and relative values. Girls also contribute to the overestimation in PH-group 1 and 2, but not in PH-group 3 and 4, when expressed in absolute values (Figure 6 & 7).

Figure 6. Mean difference between estimated and measured values in each PH stage group

divided in sex, absolute values.

Figure 7. Mean difference between estimated and measured values in each PH stage group

divided in sex, relative values. 0.35 0.20 0.26 0.19 0.30 -0.31 -0.07 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 M ea n d if fe re nc e es ti m at ed m ea su re d (L /m in )

Boys Girls PH1 PH2 PH3 PH4 11.7 5.5 4.6 2.9 -1.8 -3.4 -1.4 -6 -4 -2 0 2 4 6 8 10 12 14 M ea n d if fe re n ce e st im at ed m ea su re d (m l/ kg /m in )

Boys Girls

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4.3 Reliability

In total 16 participants performed test-retest of the Ekblom-Bak test and 13 out of the 16

performed test-retest of the incremental VO2max test. Results show that 1. Test-retest reliability in

the Ekblom-Bak test was good, with a significant correlation of r = 0.84. 2. Test-retest reliability of the incremental VO2max test was very good, r = 0.95.

5. Discussion

5.1 Results discussion

The aim of this study was to investigate the validity and the reliability of the Ekblom-Bak test in children, 10-14 years of age. The results show that the Ekblom-Bak test overestimates VO2max

with 0.16 l·min-1_{(SD 0.41 and 95 % CI 0.04-0.27) or when expressed in relative values, 3.88}

ml·kg-1·min-1 (SD 9.06 and 95 % CI = 1.41-6.36) compared to measured VO2max. Furthermore,

prepubescent children and early pubertal children (PH-group 1 and 2) and boys in all PH-groups stand for most of the overestimation, which gives the implication that the prediction equations derived from the test should not be used in prepubescent children or in boys. Further the test has a very good reliability of r = 0.84 in this age group.

As previously mentioned other studies that have investigated the validity in different submaximal cycle ergometer tests in children have shown a correlation ranging from r = 0.55 to r = 0.87, depending on the method and equation being used. In one study, the Binyildiz method had the strongest correlation with measured VO2max (r = 0.87). On the other hand, the Binyildiz method

resulted in a high mean underestimation of 0.66 l·min-1 (Ekblom 2014), compared to the Ekblom-Bak test that overestimated VO2max with 0.16 l·min-1.

Furthermore, Ekblom (2014) found that the age-adjusted Åstrand-Rhyming and the

Woynarowska methods were statistically free from mean misclassification. On the other hand, it was also shown that the higher aerobic fitness in the tested children, the more the tests

underestimated VO2max. These findings could not be seen in this study, where measured VO2max

was not a significant variable. Lastly, SEE was 0.275 l·min-1 in this study, which is lower than in the methods used in Ekblom’s (2014) study, where SEE ranged between 0.298-0.398 l·min-1_.

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Further, the difference between these three methods and the Ekblom-Bak test is that the other equations are already adjusted for children.

From the results in this study it is also possible to derive in which groups the misclassification is the largest. The results show that the Ekblom-Bak test on average overestimates VO2max with 0.16

l·min-1 (or 3.88 ml·kg-1·min-1), but when splitting sexes and dividing the group in Tanner Stages, the results show that 1) prepubescent children and early pubertal children (PH-group 1 and 2) get overestimated more than mid- and late pubertal children (PH-group 3 and 4) and 2) boys get overestimated more than girls, especially when expressed in relative values. From these findings two questions occur: 1. Which difference in the circulatory system between prepubescent and pubertal children may be the explanation to this divergence in the estimation error between Tanner Stages? 2. How come that the equation fits better for girls than for boys?

5.1.1 Work physiology

On average, men have 70-75 % higher VO2max compared to women. The main reason for this is

that women in general are smaller than men and therefore have smaller hearts and smaller blood volume (Wilmore & Costill 2015, s. 481ff). When men and women are cycling at 60 % of their VO2max women have a significantly higher HR, significantly lower SV and in total a lower Q (not

significant) than men (Wilmore et al. 2001). Anyhow, the difference in VO2max between men and

women disappears when VO2max is expressed relatively to fat free mass or active muscle mass

(Kenney, Wilmore & Costill 2015, s. 481ff). In prepubescent children, VO2max doesn’t vary

between girls and boys as much as in adults, but prepubescent boys still tend to have a higher VO2max than girls (Armstrong et al. 1995). Maximal HR is reported to be the same between boys

and girls (Armstrong et al. 1995; Turley 1997) while SV tend to be higher and Q tends to be the same between sexes (Turley 1997). Further no significant differences have been found in peak RER or peak LA in prepubescent children between sexes (Armstrong et al. 1995).

When comparing children to adults a higher maximal HR and a smaller SV can be seen in children, at both submaximal and maximal levels (Turley & Wilmore 1997; Vinet et al. 2002). When scaling SV to body surface area the differences in SV between children and adults are reduced and the higher maximal HR only compensates partially for the lower SV and

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children have a lower Q than adults at both submaximal and maximal work rates (Turley & Wilmore 1997; Vinet et al. 2002) and when separating all the parameters in gender the results are consistent (Turley & Wilmore 1997).

The physiological differences between boys and girls are not as distinct as the differences between men and women. The equation being used to estimate VO2max in girls has better

accuracy than the equation that estimates VO2max in boys. Since the physiological differences

between the sexes are not as distinct in children, it gives the implication that the female equation might be a better fit even for prepubescent and early pubertal boys.

5.1.2 Reliability

Only 16 out of 54 participants performed a retest, but showed a significant correlation of 0.84. In adults the test-retest correlation is higher (r = 0.96) (Ekblom-Bak et al. 2014) than correlation showed in this study. Mocellin et al (1971) found a test-retest correlation of 0.94 of the PWC170

test in children aged 13-14 years. No other studies, in the knowledge of the author, have investigated test-retest reliability in submaximal cycle ergometer tests and children.

5.2 Strengths and limitations

A limitation of this study is the fact that the participants in general had a very high aerobic fitness. Mean values were 52.6 and 57.2 ml·kg-1_·min-1_{for girls and boys respectively, while for}

example Pate et al. (2006) reports mean values of 39.7 and 44.6 ml·kg-1_·min-1_{in 12-13 year old}

girls and boys, respectively. We can therefore just say something about the Ekblom-Bak test from this exact sample and unfortunately we do not know anything about how valid or reliable the test is in children with an inferior aerobic fitness. A probability is that less fit children will be unable to cycle at 1.0 kp. Anyhow, when marketing the study mostly schools and sport clubs were targeted and only already very physical active children (and parents) would respond with interest. This could have been something to predict and maybe better-targeted marketing could have been done.

In the study, children down to 10 years were recruited. Younger children were not included in the study since children <10 most likely would be too short and/or too light to perform the cycle ergometer test. With other words, they would not reach down to the pedals and have a hard time

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cycling at 1.0 kp. Some of the children in the study reached a RER of >1.0 when cycling on 1.0 kp, which indicates that test is not feasible for very light children, even though they have a high aerobic fitness. One of the main purposes of the Ekblom-Bak test is that it is a submaximal test, for children that reach a RER >1.0, that is no longer the case. Many the children also had a hard time keeping pace when cycling and this is very important in order for the workload to be constant throughout the test. When workload changes, HR changes and consequently influences estimated VO2max.

When cycling the Ekblom-Bak test, the subjects get asked for their perceived exertion after one minute at the high work rate. In adults, the work rate increases 0.5 kp or 1.0 kp if they estimate their exertion to 10-11 or <10, respectively. For the first child in the study that estimated his exertion to be <11, work rate was increased to 1.5 kp. After four minutes of cycling at 1.5 kp his HR was 191 and had trouble keeping the rhythm. For all next-coming subjects an individual assessment was made if they estimated their exertion to <11. The assessment included body mass, the difference between HR at standard and high work rate and RER. In total five of the subjects cycled at 1.5 kp. Further investigations have to be done in order to control if the Borg RPE scale should be used at all in children performing the Ekblom-Bak test, or to regulate when to increase the high work rate and when to not, in children. Anyhow, a recommendation for right now is to let the child continue cycle at 1.0 kp, despite a perceived exertion of <11.

Strengths about the study are the large amount of participants that were included and that most of them (40 out of 54) were maturity level determined. In this age group the children vary in many aspects such as height and weight but also whether they have entered puberty or not. Since for example 11-year-olds are not a homogenous group, the maturity level determination is a very important factor to consider, in order to eliminate possible confounding effects that differs

between prepubescent and pubertal children. There are no studies, in the knowledge of the author, that have considered maturity status when examining validity and reliability in submaximal cycle ergometer tests or other tests predicting VO2max.

5.3 Future research

When it comes to the Ekblom-Bak test and children it would be interesting to further investigate whether boys should also use the female equations and to establish the validity in only pubertal

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children. Further research should focus on what physiological differences in prepubescent children, compared to pubertal adolescents, which makes the Ekblom-Bak test unusable. Also it would be interesting to investigate why prepubescent boys have a higher VO2max than

prepubescent girls, what differs physiologically between girls and boys already in a prepubescent stage?

6. Conclusion

The Ekblom-Bak test is highly reliable in children 10-14 years of age and valid for pubertal girls, but the prediction equations derived from the test should not be used in prepubescent children or in boys.

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Validity and reliability of the Ekblom-Bak test in children 10 to 14 years of age