227
ORIGINAL SCIENTIFIC ARTICLE
Developing endurance among children in preschool period
1 Department for Preschool 
Education, Motopedagogic, 
Faculty of Education, 
University of Ljubljana, 
Ljubljana, Slovenia
2 Faculty of Management, 
University of Primorska, 
Koper - Capodistria, 
Slovenia
3 Faculty of Organizational 
Sciences, University of 
Maribor, Kranj, Slovenia
4 Faculty of Sport, 
University of Ljubljana, 
Ljubljana, Slovenia
Correspondence/
Korespondenca:
Jera Gregorc, e: jera.
gregorc@pef.uni-lj.si
Key words:
preschool child; heart 
rate; types of exercise; 
cardiovascular system
Ključne besede:
predšolski otrok; frekvenca 
srca; vrste vadbe; srčno-
žilni sistem
Received: 25. 7. 2019
Accepted: 24. 2. 2021
eng slo element
en article-lang
10.6016/ZdravVestn.2977 doi
25.7.2019 date-received
24.2.2021 date-accepted
Cardiovascular system Srce in obtočila discipline
Original scientific article Izvirni znanstveni članek article-type
Developing endurance among children in 
preschool period Razvoj vzdržljivosti predšolskih otrok article-title
Developing endurance among children in 
preschool period Razvoj vzdržljivosti predšolskih otrok alt-title
preschool child, heart rate, types of exercise, 
cardiovascular system
predšolski otrok, frekvenca srca, vrste vadbe, 
srčno-žilni sistem kwd-group
The authors declare that there are no conflicts 
of interest present.
Avtorji so izjavili, da ne obstajajo nobeni 
konkurenčni interesi. conflict
year volume first month last month first page last page
2021 90 5 6 227 241
name surname aff email
Jera Gregorc 1 jera.gregorc@pef.uni-lj.si
name surname aff
Maja Meško 2,3
Mateja Videmšek 4
eng slo aff-id
Department for Preschool 
Education, Motopedagogic, 
Faculty of Education, University 
of Ljubljana, Ljubljana, Slovenia
Oddelek za predšolsko vzgojo, 
Motopedagogika, Pedagoška 
fakulteta, Univerza v Ljubljani, 
Ljubljana, Slovenija
1
Faculty of Management, 
University of Primorska, Koper - 
Capodistria, Slovenia
Fakulteta za management, 
Univerza na Primorskem, Koper, 
Slovenija
2
Faculty of Organizational 
Sciences, University of Maribor, 
Kranj, Slovenia
Fakulteta za organizacijske vede, 
Univerza v Mariboru, Kranj, 
Slovenija
3
Faculty of Sport, University of 
Ljubljana, Ljubljana, Slovenia
Fakulteta za šport, Univerza v 
Ljubljani, Ljubljana, Slovenija 4
Developing endurance among children in 
preschool period
Razvoj vzdržljivosti predšolskih otrok
Jera Gregorc,1 Maja Meško,2,3 Mateja Videmšek4
Abstract
Introduction: Prior research generally confirms the importance of endurance exercises in pre-
school period and suggests shorter and less intensive exercises. However, little research has 
been conducted to show and evaluate different types for developing endurance in preschool 
period. Our research aims to find out which type of exercise is more suitable for developing 
endurance for a preschool age child compared to an adult, taking into account developmental 
characteristics.
Material and methods: A total of 69 preschool children (40 children aged 3 years and 29 children 
aged 5 years) were included in the study. We measured their heart rates during two types of en-
durance exercises (uninterrupted walking vs “method of game”). We compared the average heart 
rate and the actual frequency curves. 
Results: Endurance exercise according to the “method of game” allows the child to adjust the 
effort more individually compared to uninterrupted walking when both exercises are performed 
with a group of pre-school children. A comparison of heart rate curves showed that when chil-
dren partook in the “ method of game”, they achieved different heart rate values at the same 
time. A further analysis of heart rate with 5 selected “time stamps” between the two exercises 
showed that children achieved statistically significant higher heart rates when performing the 
“method of game” (1’: Z = 2.20; p = 0.028, r = 0.19; 3 ‘: Z = 4.64; p = 0.00, r = 0.39; 5’: Z = 3.23; 
p = 0.001, r = 0.27; 8’: Z = 3.82 p = 0.00, r = 0.32; 13 ‘: Z = 3.33; p = 0.001, r = 0.28). The comparison of 
the mean heart rate values over the entire 13-minute time interval between the execution of the 
“method of game” (M = 143, SD = 14) and uninterrupted walking (M = 132, SD = 9) was statistically 
significantly higher in favour of the “method of game” (Wilcox test: Z = 5.83, p = 0.00, r = 0.24). The 
Mann-Whitney-Test showed that 3-year-old children achieved statistically significantly higher av-
erage heart rate during the “method of game” (Z = -2.34 p = 0.020, η² = 0.08) as well as during un-
interrupted walking (Z = 3.17 p = 0.002, η² = 0.15). No statistically significant differences between 
boys and girls were confirmed.
Conclusions: Based on the results obtained, we believe that endurance exercise according to the 
“method of game” is more suitable for a developing preschool child than uninterrupted walking. 
The “method of game” also allows the simultaneous implementation of endurance exercise in 
heterogeneous groups, since the children adapt to the effort themselves, regardless of age. We 
believe that other types of endurance exercises should be researched and all age groups of pre-
school children should be included in the research.
Izvleček
Uvod: Pomen vzdržljivostne vadbe v predšolskem obdobju so potrdile že mnoge dosedanje ra-
ziskave. Predlagale so predvsem krajšo in manj intenzivno vadbo, manj pa je bilo raziskanega 
o ustreznosti različnih vrst vadbe za razvoj vzdržljivosti v tem obdobju. Raziskava želi ugotoviti, 
Slovenian
Medical
Journal
228
CARDIOVASCULAR SYSTEM
Zdrav Vestn | May – June 2021 | Volume 90 | https://doi.org/10.6016/ZdravVestn.2977
1 Introduction
Endurance is a functional ability of 
the organism to perform a movement 
activity with moderate intensity over a 
longer period of time (1,2). The efficien-
cy of such exercise is limited by the re-
spiratory, cardiovascular, thermoregula-
tory, and other systems (3) and is closely 
related to a variety of healthy lifestyle 
factors (4). According to Degens (5) and 
his conclusions on the endurance train-
ing of children for healthy ageing, aero-
bic exercise is still an efficient means of 
slowing down the loss of muscle mass 
katera vrsta vadbe za razvoj vzdržljivosti je najprimernejša za predšolskega otroka, upoštevajoč 
razvojne značilnosti, v primerjavi z odraslo osebo. 
Material in metode: V raziskavo smo vključili skupaj 69 predšolskih otrok (40 otrok, starih 3 leta, 
in 29 otrok, starih 5 let). Med izvajanjem dveh vrst vzdržljivostne vadbe (enakomerne neprekinje-
ne hoje in vadbe po »metodi igre«) smo merili njihov srčni utrip. Primerjali smo srednje vrednosti 
srčnega utripa in krivulje dejanskih frekvenc. 
Rezultati: Vadba vzdržljivosti po »metodi igre« otroku omogoča več individualnega prilagajanja 
naporu v primerjavi z vadbo neprekinjene hoje, kadar obe vadbi izvajamo s skupino predšolskih 
otrok. Primerjava krivulj srčnega utripa je pokazala, da pri vadbi po »metodi igre« otroci dosegajo 
različne vrednosti srčnega utripa ob istem času. Nadaljnja analiza srčnih utripov ob 5 izbranih 
»časovnih štampiljkah« med obema vadbama je pokazala, da otroci med izvajanjem »metode ig-
re« dosegajo statistično značilno višje frekvence srca (1`: Z = 2,20; p = 0,028, r = 0,19; 3`: Z = 4,64; 
p = 0,00, r = 0,39; 5`: Z = 3,23; p = 0,001, r = 0,27; 8`: Z = 3,82; p = 0,00, r = 0,32; 13`: Z = 3,33; 
p = 0,001, r = 0,28). Povprečne vrednosti srčnega utripa v celotnem 13-minutnem časovnem in-
tervalu med izvajanjem »metode igre« (M = 143, SD = 14) in neprekinjene hoje (M = 132, SD = 9) 
je bila statistično značilno večja v prid »metodi igre« (Wilcox test: Z = 5,83, p = 0,00, r = 0,24). 
Mann Whitneyev test je pokazal, da so 3-letni otroci tako med “metodo igre” (Z= -2,34 p = 0,020, 
η² = 0,08) kot med neprekinjeno hojo (Z= 3,17 p = 0,002, η² = 0,15) dosegli statistično značilne viš-
je povprečne vrednosti srčnega utripa. Med spoloma statistično značilnih razlik nismo potrdili.
Zaključki: Glede na prej dobljene rezultate menimo, da je vadba vzdržljivosti po »metodi igre« 
bolj primerna za razvijajočega se predšolskega otroka kot neprekinjena hoja. »Metoda igre« omo-
goča tudi hkratno izvajanje vzdržljivostne vadbe v heterogenih oddelkih, saj si obremenitev otro-
ci prilagodijo sami, ne glede na starost. Menimo, da bi bilo treba raziskati tudi druge vrste vadbe 
za razvoj vzdržljivosti, v raziskavo pa vključiti tudi ostale starostne skupine predšolskih otrok.
Cite as/Citirajte kot: Gregorc J, Meško M, Videmšek M. Developing endurance among children in preschool 
period. Zdrav Vestn. 2021;90(5–6):227–41.
DOI: https://doi.org/10.6016/ZdravVestn.2977
Copyright (c) 2021 Slovenian Medical Journal. This work is licensed under a
Creative Commons Attribution-NonCommercial 4.0 International License.
and maintaining its function, thus im-
proving the quality of life. Therefore, dy-
namic aerobic endurance is the one that 
should be promoted in the preschool 
period (2,6). However, the way in which 
such training is carried out differs be-
tween adults and children. It is a long-
term, moderately intensive cyclical exer-
cise (e.g. running, walking, cycling, etc.) 
that adults perform continuously, while 
children perform it more intensively and 
with more frequent breaks (2,6). This is 
due to a more specific function of the 
229
ORIGINAL SCIENTIFIC ARTICLE
Developing endurance among children in preschool period
cardiovascular, respiratory, neuromus-
cular, and thermoregulatory systems of 
children.
Antipuesto (7) and Kliegman, Stanton, 
St. Geme, and Schor (8) analysed the 
differences between the cardiovascular 
and respiratory systems of children and 
adults. In their opinion, the tongue is one 
of the limiting factors in the respiratory 
system of a preschool child. Airflow is 
lower in children than in adults because 
of the different proportions between the 
airways and the tongue (7). For this rea-
son, breathing through the nose rather 
than through the mouth is more import-
ant for children (8).
Samuels and Wieteska (9) give guide 
values for differences in minute venti-
lation. It is known that minute ventila-
tion is the product of the breathing rate 
in one minute and a breathing volume. 
Samuels and Wieteska (9) give mean 
values for the respiratory rate of chil-
dren and adults. The respiratory rate per 
minute of a 3-year-old child is 25 to 30 
breaths per minute, while in 5-year-old 
children it is slowed to a range between 
20 and 30 breaths per minute, the respi-
ratory rate of adults at rest is about 12 
to 16 breaths per minute, which means 
that adults have more opportunities to 
increase the respiratory rate per min-
ute compared to children (9). They ar-
gue that greater minute ventilation at 
rest in children is also due to greater 
metabolic consumption. However, they 
believe that children still have an appar-
ently elastic chest. Therefore, increased 
breathing effort results in an obvious 
thoracic retraction, which reduces respi-
ratory efficiency. All these facts point to 
the importance of more frequent breaks 
for children compared to adults.
Dotan, Mitchell, Cohen, Klentrou, 
Gabriel, and Falk (10) analysed differ-
ent studies on the types of muscle fibres. 
They concluded that children have more 
type 1 muscle fibres. This makes them 
tired more quickly. However, their mus-
cle fibres also regenerate faster, which is 
an additional reason for more frequent 
breaks.
Krost, Mistovich and Limmer (11) 
explained differences in heart function 
in adults and children. The heart of an 
adult responds to increased stress by 
increasing both heart rate and stroke 
volume, while the heart of a child can 
only respond to increased stress by in-
creasing the number of heartbeats. This 
is the reason why a child has less ability 
to adapt compared to an adult. Fleming, 
Thompson, Stevens, Heneghan, 
Plueddemann, Maconochie, Tarassenko 
and Mant (12) and Agrawal (13) ana-
lysed the differences in heart rate be-
tween adults and children at rest and at 
the strain of 80%. They concluded that 
the heart rate of children who were 3 or 
4 years old was 3 to 4 beats higher than 
that of children who were 5 or 6 years 
old.
Different stress tests in laboratories 
are used to measure how an adult’s heart 
responds to a strain. There are not ma-
ny stress tests for healthy children. Even 
paediatricians conclude that it is neces-
sary to replace the stress testing with the 
measurement of heart rate in preschool 
children (8). The strain level is measured 
outdoors, usually with heart rate mon-
itors (14), especially because they are 
more cost-efficient, while stress tests in 
a laboratory are also followed by mea-
suring the oxygen saturation of blood, 
ECG, blood pressure, breathing (max-
imum oxygen uptake), etc. Regardless 
of the age, exercise intensity is usually 
defined by the percentage of maximum 
oxygen uptake (% VO2max), maximum 
heart rate (% FSmax), and reserve heart 
rate (% HRres), but also as metabolic 
230
CARDIOVASCULAR SYSTEM
Zdrav Vestn | May – June 2021 | Volume 90 | https://doi.org/10.6016/ZdravVestn.2977
equivalent (1 MET) (14). The same au-
thors (14) suggested that the measure-
ment of HRmax should be performed as 
an alternative to establish the submaxi-
mal intensity, as there is a linear relation 
between VO2max and HRmax. However, 
there are not many comparisons between 
maximum oxygen uptake and heart rate 
in the preschool period (15).
To ensure an adequate level of effort, 
we must first know at least the individu-
al HRmax, VO2max, and two ventilation 
thresholds (16). The ventilation thresh-
olds are most accurately determined 
by laboratory stress tests. The majority 
of the parameters can be obtained only 
in laboratories, and are not common in 
general practice (too expensive and too 
complicated) (17). Indirectly, however, 
some of the parameters can be calcu-
lated from a series of more or less reli-
able tests performed in the field, such 
as “UKK WALK TEST”, “CONCONI 
TEST”, “20mSRT-PREFIT”, etc., with 
simultaneous measurement of HR 
(18,19). By drawing a curve from the 
data obtained (x-axis time, y-axis HR) 
the two ventilation thresholds can be 
figured out and the intensity of exercis-
es can be divided into 3 zones (16). The 
endurance exercise is then performed 
with a combination of different “meth-
ods” (uninterrupted method, interval 
method, and Fartlek) at different levels 
of intensity oz. zones (20).
Uninterrupted exercise for adults 
is usually performed individually. 
Exercises are most often performed 
under the first ventilation threshold 
(app. 75-80% of HRmax). However, it 
can be performed at different inten-
sities. The exercise is mainly charac-
terized by cyclical, repetitive move-
ments, which usually last at least one 
hour (20). During the performance of 
such exercise, an attempt to maintain 
approximately the same effort is made.
We must ensure that the development 
of endurance in the preschool period is 
within the 80% HRmax range (21). In 
general practice, however, it is not pos-
sible to ensure individual uninterrupted 
movement, which is why the educators 
adjust the pace to the weakest during 
hikes, walks, mountaineering, and short 
runs (6). However, such an effort is not 
sufficiently intensive for the develop-
ment of other children’s endurance (17).
A “method of game” can be suitable 
for individual exercise with a group of 
children during developing endurance. 
This exercise is described under the con-
cept “Game - movement - development” 
(male). The comparison of heart rate 
curves conducted on a small sample (6) 
indicates that the heart of a 3- or 6-year 
old child responds in a similar way as the 
heart of an adult responds to a Fartlek 
(6). Fartlek represents a combination of 
low- and high-intensity exercise, as it can 
include both brisk walking and sprinting 
in one exercise and is an effective means 
of developing basic endurance (22). 
Because of all the content, length, and 
intensity combinations, it is often used 
as a universal tool for the development 
of endurance (20). It is essential that the 
range and length are not precisely deter-
mined and the intensity is determined 
spontaneously (22). The “method of 
game” differs from the Fartlek mainly in 
performance. The “method of game” is 
usually performed with a whole group of 
children while individual exercise is typ-
ical during Fartlek.
The “method of game” is a way of 
learning in the preschool period, and 
it is based on the concept of Game-
Movement-Development. According to 
this “method”, a child develops the mo-
tor skill that the educator wants to devel-
op in a child, which also implies obeying 
231
ORIGINAL SCIENTIFIC ARTICLE
Developing endurance among children in preschool period
the rules of the game. The child reacts 
in an unobtrusive way to the rules and 
environment, and the educator at the 
same time can take into account the in-
dividuality of each child. In the “method 
of game”, the educator must be includ-
ed in the game as a person in the game 
or must take over the role of a so-called 
environment. This enables him or her to 
change the level of difficulty or to adapt 
it to children at any time. In addition, 
the beforehand prepared environment 
facilitates children to search for creative 
motoric solutions by themselves within 
the game, to set pauses, their duration, 
frequency, and intensity. By correctly 
devised game rules, the educator boosts 
motivation, which makes children re-
peat the running to succeed in the game. 
For example, the child’s goal is to suc-
ceed in the game (to rescue the princess 
from the dragon’s castle), and the educa-
tor’s goal is that the child scuds or walks 
the distance as many times as possible, 
climb the hill or stairs to higher floors 
(23).
The main difference between uninter-
rupted walking and exercise according 
to the “method of game” lies in the indi-
vidual determination of the effort. With 
uninterrupted walking, the pace is usu-
ally determined by the educator, where-
by the weakest limb is usually taken into 
account. With the “method of game” the 
pace of walking as well as the length is 
determined by the children themselves, 
similar to Fartlek.
Due to anatomical and pathophysi-
ological differences between adults and 
children, as well as due to differences in 
physical fitness between children in the 
same group and the need to develop en-
durance within the entire group of chil-
dren at the same time, it seems that un-
interrupted walking may be less suitable 
for developing endurance in preschool. 
Considering that children themselves 
determine the intensity during exercise 
according to the “method of game”, we 
want to investigate whether they achieve 
higher or lower effort than with uninter-
rupted walking, where the intensity is 
usually determined by the educator.
2 Material and methods
2.1 Participants
The study included 69 preschool chil-
dren (40 3-year-old and 29 5-year-old 
children; 44.9% of girls and 55.1% of 
boys) from four different kindergartens 
in the central Slovenian region. All four 
kindergartens had approximately the 
same outdoor conditions (a small grassy 
hill with a lawn of at least 20 by 20 metres 
around it) to implement the “method of 
game” and 1 km of the straight sidewalk 
(mostly macadam) to perform unin-
terrupted walking. Both exercises were 
carried out as part of the regular pro-
gramme without any significant changes 
to their activities.
The Commission for Ethical Issues in 
the field of sport at the Faculty of Sport, 
University of Ljubljana, approves the 
conduct of research (No. 1466, dated 26 
September 2019).
2.2 Instruments
During the research, children wore 
heart rate sensors (H7, Polar), connect-
ed to fitness monitor tracker (Polar loop, 
Polar Electro Oy, Kempele, Finland). 
The heart rate (HR) was measured in 5-s 
periods. After finishing the individual 
exercise, we downloaded the data from 
the heart rate monitors to the comput-
er using the Polar FlowSync programme 
and classified the data according to re-
search codes.
232
CARDIOVASCULAR SYSTEM
Zdrav Vestn | May – June 2021 | Volume 90 | https://doi.org/10.6016/ZdravVestn.2977
2.3 Procedures
Children’s parents or caregivers were 
informed about the goals of the research. 
They completed a short survey question-
naire about the child’s health status and 
signed consent for testing. To ensure an-
onymity, a research code was assigned 
to each child. The code was linked to 
the heart rate monitor. The research was 
conducted in compliance with personal 
data protection.
The variable sample includes the 
heart rates of all children measured 
during two different types of endurance 
exercises: (1) the uninterrupted walk-
ing, which consisted of a 1 km of walk-
ing in 13 minutes, and (2) the “method 
of game” – regarding this method, only 
13 minutes of the main part of the hour 
were included in the research. With the 
first exercise (the uninterrupted walk-
ing), the strain was determined by time 
and distance whilst with the second ex-
ercise (the “method of game”), it was de-
fined by time and the rules of game. 
All children were monitored during 
both exercise models. They partook 
scheduled exercise sessions on two dif-
ferent Wednesdays of May 2016 at 10 
AM; the first week they partook in un-
interrupted walking; the following week 
they partook in “method of game”. The 
activities with younger and older chil-
dren were performed separately. Both 
exercises were performed by qualified 
students of Preschool education (UL, 
PeF), in the presence of the preschool 
educator in the kindergarten and under 
the supervision of the researcher.
To avoid any risk, all children had 
to wear sports shoes and sports clothes. 
The children knew both exercises, as 
they were often performed in all kin-
dergartens. To avoid injuries when im-
plementing the “method of game”, we 
have chosen the grassy lawn in front of 
the kindergartens and macadam side-
walk when implementing uninterrupted 
walking.
(1) The uninterrupted walking
The uninterrupted walking applied 
through a moderately intensive walk on 
a distance of 1 km for 13 minutes. It was 
indeed a modified walking test on 2 km 
“UKK WALK TEST” (18). We wanted to 
create artificial conditions by shorten-
ing the distance and increasing the time. 
Such conditions should be appropriate 
for preschoolers proportionally to the 
differences between metameres of chil-
dren and adults. The test was modified 
and it was conducted through walks and 
excursions. The student made sure that 
the children walked in a group at an av-
erage speed of 4.6 km/h and all did 1 km 
in 13 minutes at the same time.
(2) The “method of game”
The same content for all children 
was chosen when performing the exer-
cise according to the “method of game”, 
namely, “The delivery of postal items”. 
The student was a “postman”. The chil-
dren were “letter-carriers”. The postman 
always had enough different envelopes 
with different degrees of urgency. The 
letters had to be delivered to the right 
addresses. The titles were distributed 
in the outdoor playground of each kin-
dergarten. The children could choose to 
whom they brought which letter, how 
often they chose a letter, how quickly 
they wanted to pick up a new letter. So 
they could determine the intensity of 
the exercise, the dosage, and the type of 
movement.
2.4 Statistical analysis
To analyse the differences in heart 
rate (HR) responses to these two types of 
exercises, we transformed the data into 
233
ORIGINAL SCIENTIFIC ARTICLE
Developing endurance among children in preschool period
heart rate curves and compared them at 
the same points in time. The heart rate 
was measured in 5 seconds intervals 
during both exercises.
From a sample of 69 children we ran-
domly selected 10 3-year old children 
and 10 5-year old children. We formed 
10 pairs. Each pair had one 3-year-old 
and one 5-year-old child. We plotted 40 
graphs.
The first 10 graphs show the response 
of the hearts of two children (pair) 
during uninterrupted walking at the 
same time (simultaneously). The next 10 
graphs show the response of the hearts 
of the same pair of children during the 
“method of game”. Simultaneous repre-
sentations of the response of each child’s 
heart to both exercises are shown in the 
last 20 diagrams. (Due to limited article 
space, only 3 out of all will be presented 
for illustration in the Results.)
For statistical analyses, we used sta-
tistical package SPSS 20 (SPSS Inc., 
Chicago, IL). We obtained 312 (2 x 156) 
heart rate values for each child during 
both exercises.
To determine the individuality of the 
effort during exercises, we randomly se-
lected “time stamps”, namely at the 1st, 
3rd, 5th, 8th and 13th minute. We made a 
printout of all heart rates at the selected 
“time stamp” and calculate descriptive 
statistics (HR means ± SD). Normality 
assumptions for all data were tested us-
ing Shapiro Wilk (S-W) Test. Since the 
distribution was not normal, we run 
nonparametric tests. Wilcoxon Rank 
Sum Test was used to determine wheth-
er the difference in heart rates was sta-
tistically significant at the same “time 
stamp”. A p-value of 0.05 or lower was 
considered statistically significant. Effect 
size was tested with r square; 0.10 < 0.3 
(small effect), 0.30 < 0.5 (moderate ef-
fect) and >= 0.5 (large effect).
For all subsequent statistical treat-
ments, we took all data from both ex-
ercises. We calculated the average heart 
rate for each child individually, sepa-
rately according to the type of exercise. 
The data were summarised using routine 
descriptive statistics (HR means ± SD). 
Normality assumptions for all data were 
tested using Shapiro Wilk (S-W) test. 
Since the distribution was not normal, 
we run nonparametric tests.
To compare the differences in aver-
age heart rate during the two exercises, 
we used Wilcoxon Rank Sum. A p-value 
of 0.05 or lower was considered statis-
tically significant. Effect size was tested 
with r square; 0.10 < 0.3 (small effect), 
0.30 < 0.5 (moderate effect) and >= 0.5 
(large effect). To compare the differences 
(separately between boys and girls and 
separately between 3- and 5-year old 
children) in average heart rate during 
the two exercises, we used the Mann-
Whitney Test. A p-value of 0.05 or lower 
was considered statistically significant. 
To measure the effect size, Eta square 
(η²) was calculated.
3 Results
The results are presented in figures 
(graphs) and tables. First of all, due to 
limited article space, we show only three 
(3) graphs with comparisons of the heart 
reactions for a randomly selected pair 
of children (one pair). The first graph 
(Figure 1) shows the difference between 
the heart curves during uninterrupted 
walking, the second (Figure 2) during 
the “method of game”, and the third 
(Figure 3) shows the difference in heart 
rate response to the two types of exercise 
for the senior child from a pair.
Figure 1 shows the differences in 
heart response during the uninterrupt-
ed walking between two preschoolers. 
234
CARDIOVASCULAR SYSTEM
Zdrav Vestn | May – June 2021 | Volume 90 | https://doi.org/10.6016/ZdravVestn.2977
Figure 2 refers to the same couple of 
two preschoolers as with the uninter-
rupted walking (Figure 1). Again, we 
have a sample which indicates that also 
with this type of exercise the younger 
child performed a more intensive move-
ment activity. It shows that the young-
er child did not stop at a higher level of 
intensity or took the necessary rest at 
once.
After analysing all the other 9 graphs 
plotted from HR, obtained during 
“method of game”, we came to the con-
clusion that a child can adjust its own 
intensity of movement at any time and 
therefore the individual stress is guar-
anteed by the feeling of tension and the 
need for rest.
The third graph (Figure 3) shows the 
differences in the reaction of the heart of 
a 5-year-old child (from the previously 
selected and represented couple) to two 
different types of exercises.
According to the analysis of all 20 
individual graphs (for illustration, only 
one of them is presented in Figure 3), 
we concluded that the curves obtained 
by measuring the children during un-
interrupted walking is more linear than Figure 2: The presentation of heart response during the “method of game” from randomly chosen pairs (one 3- and one 5-year-old preschooler).
220
200
180
160
140
120
100
80
60
40
20
0
m
g
_
1
m
g
1
0
m
g
_
1
9
m
g
_
2
8
m
g
_
3
7
m
g
_
4
6
m
g
_
5
5
m
g
_
6
4
m
g
_
7
3
m
g
_
8
2
m
g
_
9
1
m
g
_
1
0
0
m
g
_
1
0
9
m
g
_
1
1
8
m
g
_
1
2
7
m
g
_
1
3
6
m
g
_
1
4
5
m
g
_
1
5
4
m
g
_
1
6
3
m
g
_
1
7
2
m
g
_
1
8
1
m
g
_
1
9
0
m
g
_
1
9
9
m
g
_
2
0
8
m
g
_
2
1
7
m
g
_
2
2
6
m
g
_
2
3
5
m
g
_
2
4
4
m
g
_
2
5
4
Time period in 5s intervals
H
e
a
rt
 r
a
te
3 years
5 years
According to this chart, the heart rate 
of the younger child varied more fre-
quently than the heart rate of the older 
child(ren). However, also in the older 
child, the frequency was not steady, as we 
could expect in adults. Based on a more 
accurate analysis of these two curves, the 
average value of the older child was for 3 
strokes per minute lower than the one of 
the younger child. The older child’s chart 
has only one emphasised and more du-
rable higher heart rate, while the chart 
of the younger child has a more frequent 
and shorter higher heart rate.
Figure 1: The presentation of heart response during the uninterrupted walking from a randomly chosen pair (one 3- and 
one 5-year-old preschooler).
200
180
160
140
120
100
80
60
40
20
0
w
_
k
m
_
5
w
_
k
m
_
4
5
w
_
k
m
_
8
5
w
_
k
m
_
1
2
5
w
_
k
m
_
1
6
5
w
_
k
m
_
2
0
5
w
_
k
m
_
2
4
5
w
_
k
m
_
2
8
5
w
_
k
m
_
3
2
5
w
_
k
m
_
3
6
5
w
_
k
m
_
4
0
5
w
_
k
m
_
4
4
5
w
_
k
m
_
4
8
5
w
_
k
m
_
5
2
5
w
_
k
m
_
5
6
5
w
_
k
m
_
6
0
5
w
_
k
m
_
6
4
5
w
_
k
m
_
6
8
5
w
_
k
m
_
7
2
5
w
_
k
m
_
7
6
5
w
_
k
m
_
8
0
5
w
_
k
m
_
8
4
5
w
_
k
m
_
8
8
5
w
_
k
m
_
9
2
5
w
_
k
m
_
9
6
5
w
_
k
m
_
1
0
0
5
w
_
k
m
_
1
0
4
5
w
_
k
m
_
1
0
8
5
w
_
k
m
_
1
1
2
5
w
_
k
m
_
1
1
6
5
w
_
k
m
_
1
2
0
5
w
_
k
m
_
1
2
4
5
Time period in 5s intervals
H
e
a
rt
 r
a
te
3 years
5 years
235
ORIGINAL SCIENTIFIC ARTICLE
Developing endurance among children in preschool period
Figure 2 refers to the same couple of 
two preschoolers as with the uninter-
rupted walking (Figure 1). Again, we 
have a sample which indicates that also 
with this type of exercise the younger 
child performed a more intensive move-
ment activity. It shows that the young-
er child did not stop at a higher level of 
intensity or took the necessary rest at 
once.
After analysing all the other 9 graphs 
plotted from HR, obtained during 
“method of game”, we came to the con-
clusion that a child can adjust its own 
intensity of movement at any time and 
therefore the individual stress is guar-
anteed by the feeling of tension and the 
need for rest.
The third graph (Figure 3) shows the 
differences in the reaction of the heart of 
a 5-year-old child (from the previously 
selected and represented couple) to two 
different types of exercises.
According to the analysis of all 20 
individual graphs (for illustration, only 
one of them is presented in Figure 3), 
we concluded that the curves obtained 
by measuring the children during un-
interrupted walking is more linear than Figure 2: The presentation of heart response during the “method of game” from randomly chosen pairs (one 3- and one 5-year-old preschooler).
220
200
180
160
140
120
100
80
60
40
20
0
m
g
_
1
m
g
1
0
m
g
_
1
9
m
g
_
2
8
m
g
_
3
7
m
g
_
4
6
m
g
_
5
5
m
g
_
6
4
m
g
_
7
3
m
g
_
8
2
m
g
_
9
1
m
g
_
1
0
0
m
g
_
1
0
9
m
g
_
1
1
8
m
g
_
1
2
7
m
g
_
1
3
6
m
g
_
1
4
5
m
g
_
1
5
4
m
g
_
1
6
3
m
g
_
1
7
2
m
g
_
1
8
1
m
g
_
1
9
0
m
g
_
1
9
9
m
g
_
2
0
8
m
g
_
2
1
7
m
g
_
2
2
6
m
g
_
2
3
5
m
g
_
2
4
4
m
g
_
2
5
4
Time period in 5s intervals
H
e
a
rt
 r
a
te
3 years
5 years
those obtained during the “method of 
game”. An increased heart rate during 
the “method of game” indicates that 
children were moving more often and 
with more intensity than during unin-
terrupted walking. However, they also 
rested more often during the “method 
of game” than during uninterrupted 
walking.
To support the above claims from the 
graphs that the “method of game” al-
lowed the child to individualise the ex-
ercise more than uninterrupted walking 
(more breaks, more different intensities 
at the same time) and still develop me-
dium to high endurance, we have creat-
ed five “time stamps” (at 1, 3, 5, 8 and 13 
minutes) and calculated average HR and 
SD at all selected points in time (Table 1).
When we observed the differences 
in the average of HR (but especially in 
SD) between all children at the same 
time point, we found out that heart 
rates during the “method of game” dif-
fered more than during the uninterrupt-
ed walking (at 1` SDmg-SDuw = 21-10; 
3`SDmg-SDuw = 23-9; 5` SDmg-SDuw = 
30-18; at 8` SDmg-SDuw= 26-10; at 5` 
SDmg-SDuw = 24-8).
Figure 3: The comparison of heart response with the uninterrupted walking and “method of game” in the same 5-year 
old child.
200
180
160
140
120
100
80
60
40
20
0 5
5
0
9
5
1
4
5
1
8
5
2
3
0
2
7
5
3
2
0
3
6
5
4
1
0
4
5
5
5
0
0
5
4
5
5
9
0
6
3
5
6
8
0
7
2
5
7
7
0
8
1
5
8
6
0
9
0
5
9
5
0
9
9
5
1
0
4
0
1
0
8
5
1
1
3
0
1
1
7
5
1
2
2
0
1
2
6
5
Time period in 5s intervals
H
e
a
rt
 r
a
te
Method of play
Unint. method
236
CARDIOVASCULAR SYSTEM
Zdrav Vestn | May – June 2021 | Volume 90 | https://doi.org/10.6016/ZdravVestn.2977
Wilcoxon Rank Sum test showed that 
differences at all points in time were 
statistically significant (at 1`: Z = 2.20; 
p  =  0.028, r = 0.19; 3`: Z = 4.64; 
p = 0.00, r = 0.39; 5`: Z = 3.23; p = 0.001, 
r = 0.27; 8`: Z = 3.82; p = 0.00, r = 0.32; 
13`: Z = 3.33; p = 0.001, r = 0.28).
Table 2 shows that the average heart 
rate of all participants during the perfor-
mance of the “method of game” was 143, 
with a standard deviation of 14. The av-
erage heart rate of all participants during 
uninterrupted walking was 132, with a 
Table 1: Average HR values for all children at selected “time stamps” (i.e. at the 1st, 3rd, 5th, 8th and 13th minute).
Legend: N – number of replies; Min – minimum value; Max – maximum value; Z – value of Wilcoxon Rank Sum; sig. - statistical 
significance – a p-value of 0.05 or lower was considered statistically significant.
Stamp time Exercise N Mean 
value
Std. 
deviation
Min Max Wilcoxon Rank Sum
Z sig.
1st min “method of game” 69 132 21 102 179
2.20 0.028
uninterrupted walking 69 125 10 102 158
3rd min “method of game” 69 135 23 107 181
4.64 0.000
uninterrupted walking 69 119 9 105 149
5th min “method of game” 69 140 30 95 195
3.23 0.001
uninterrupted walking 69 125 18 104 169
8th min “method of game” 69 145 26 110 193
3.82 0.000
uninterrupted walking 69 129 11 111 143
13th min “method of game” 69 145 26 105 186
3.33 0.001
uninterrupted walking 69 133 8 122 153
standard deviation of 5. This illustrates 
the fact that uninterrupted walking is 
less intense and results in a smaller range 
of heart rate. This is further illustrated by 
the minimum and maximum heart rate. 
The difference between the minimum 
and maximum heart rate during the 
“method of game” (HR max: 178 – HR 
min: 110) was 68 beats, while during the 
uninterrupted walking it was 28 beats 
(HR max: 146 – HR min: 118).
The distribution was not normal, so 
we used the Wilcoxon Rank Sum test to 
Table 2: Average heart rate during 13 minutes of uninterrupted walking and “method of game”.
Legend: N – number of replies; Min – minimum value; Max – maximum value; Z – value of Wilcoxon 
Rank Sum; sig. - statistical significance – a p-value of 0.05 or lower was considered statistically 
significant.
N Mean 
value
Std. 
deviation
Min Max Wilcoxon Rank Sum
Z sig.
“Method of game” 69 143 14 110 178
-5.83 0.000
Uninterrupted walking 69 132 5 118 146
237
ORIGINAL SCIENTIFIC ARTICLE
Developing endurance among children in preschool period
identify statistical differences. Wilcoxon 
Rank Sum test showed that differences 
were statistically significant (Z = -5.83; 
p  = 0.001, r = 0.24). On average, the 
children experienced a higher intensity 
when endurance was implemented ac-
cording to the “method of game”. Given 
the differences between the minimum 
and maximum values, the effort would 
not be constant.
Table 3 shows heart rate average val-
ues of boys and girls obtained during 
uninterrupted walking and the “method 
of game”. We applied the Mann Whitney 
test to verify whether the differences in 
the average heart rate between boys and 
girls regarding the type of exercise were 
statistically different. From the results of 
Mann Whitney test, we can notice that 
there is no significant difference between 
girls and boys neither during uninter-
rupted walking nor during the “method 
of game”.
We analysed differences between age 
groups in the same way as we analysed 
differences between boys and girls.
Table 3: Average heart rate for boys and girls during uninterrupted walking and “method of 
game”.
Legend: N – number of replies; Min – minimum value; Max – maximum value; sig. - statistical 
significance – a p-value of 0.05 or lower was considered statistically significant.
N Mean 
value
Std. 
deviation
Min Max Mann-
Whitney 
test
“Method of game” Boys 38 143 14 110 178
0.491Girls 31 142 14 110 178
Total 69 143 14 110 178
Uninterrupted walking Boys 38 133 6 118 146
0.132Girls 31 130 4 123 136
Total 69 132 5 118 146
Table 4: Average heart rate for 3- and 5-year old preschoolers during uninterrupted walking and 
“method of game”.
Legend: N – number of replies; Min – minimum value; Max – maximum value; sig. - statistical 
significance – a p-value of 0.05 or lower was considered statistically significant.
N Mean 
value
Std. 
deviation
Min Max Mann-
Whitney 
test
“Method of game” 3-year 40 146 17 110 178
0.0205-year 29 139 7 128 150
Total 69 143 14 110 178
Uninterrupted walking 3-year 40 133 3 125 136
0.0025-year 29 130 7 118 146
Total 69 132 5 118 146
238
CARDIOVASCULAR SYSTEM
Zdrav Vestn | May – June 2021 | Volume 90 | https://doi.org/10.6016/ZdravVestn.2977
Table 4 shows that 3-year-old chil-
dren had higher heart rate average val-
ues during both types of exercises in 
comparison to 5-year-old children. 
The difference during the “method of 
game” was 7 strokes per minute, where-
as, during the uninterrupted walking, 
this difference was lower and equal to 2 
strokes per minute. The Mann Whitney 
Test indicates statistical significant dif-
ferences in average of heart rate be-
tween age during the “method of game” 
(Z = -2,34 p = 0.020, η² = 0.08) and un-
interrupted walking (Z = 3.17 p = 0.002, 
η² = 0.15).
4 Discussion
The study examined the reactions of 
the heart of preschool children to two 
different types of endurance exercise 
(i.e. uninterrupted walking vs “method 
of game”). Endurance itself is a func-
tional ability of an organism to perform 
a movement activity with moderate in-
tensity for a longer time (1,2). The reac-
tion of the body to given stress (strain) 
is called effort (22). Usually, the strain is 
determined by the speed of the move-
ment, length , frequency, type of exer-
cise, etc. In our study, the strain for the 
first exercise (uninterrupted walking) 
was 1 km walking for 13 minutes. For 
the second exercise (“method of game”) 
the reverse reaction (effort vs strain) 
was desired to achieve. The child should 
determine the strain themselves during 
the implementation of the “method of 
game” according to the effort they expe-
rienced at that moment.
The discussion is divided into three 
paragraphs according to the main re-
search question which one of the two 
exercises is more suitable for developing 
endurance in preschoolers. In the first 
part, the connection between the graphs 
and the Table 1 with the individuality 
of the exercise is shown. In the second 
paragraph, the results of Table 2 are re-
lated to the exercise intensity. The third 
paragraph, based on the results of Tables 
3 and 4, points out the universality of 
the “method of game” and its relation to 
Fartlek.
Measuring adult HR, the steady-
state is established by performing a 
cyclic movement at a constant speed 
on the aerobic threshold (24). By com-
paring our results obtained by measur-
ing HR during uninterrupted walking 
for 1 km at 4.6 km/h speed, all chil-
dren failed to reach steady-state. The 
heart rates of 3-year old children varied 
more frequently compared to 5-year 
old children. However, also all graphs 
of 5-year old children were not steady, 
as we could expect in adults. According 
to the first part of our results (Figures 
1-3 and Table 1), where we wanted to 
explore the possibilities of adapting the 
intensity to the individual, we noticed 
that children at the same time (“time 
stamp”) associated more variegated 
HR during the “method of game” than 
during uninterrupted walking. We con-
cluded that just by shortening the time 
of exercise and decreasing the distance 
of walking, we only shorten the time of 
developing aerobic activity. The reasons 
for the inability to perform exercises in a 
steady-state are sought in incompletely 
mastered walking as well as in the an-
atomical and physiological differences 
between children and adults. Dotan, 
Mitchell, Cohen, Klentrou, Gabriel and 
Falk (10) concluded that children have 
more muscle fibres of type 1. Because 
of that, they get tired more quickly. 
However, their muscle fibres also rein-
vigorate faster, which is an additional 
reason for more frequent pauses. The 
analysis of heart stroke curves measured 
239
ORIGINAL SCIENTIFIC ARTICLE
Developing endurance among children in preschool period
during the “method of game” might in-
dicate that this is exactly the reason for 
more frequent rests and quick increase 
of heart rate after rest. Krost, Mistovich 
and Limmer (11) also analysed the dif-
ferences in cardiac function between 
adults and children. According to them, 
the heart of an adult responds to an in-
creased strain by increasing both the 
heart rate and stroke volume, while 
a child’s heart can respond to the in-
creased strain only by increasing the 
number of heart strokes. This might be 
the reason for considerable fluctuations 
in heart rate.
In the second paragraph, the inten-
sity between exercises regarding strain 
determiner is discussed. According to 
the results from Table 2, children expe-
rienced a higher intensity when endur-
ance was implemented according to the 
“method of game” in comparison to un-
interrupted walking. Preschool teachers 
need to ensure safety when organizing 
uninterrupted walking. All children 
need to walk in a group with the same 
pace (17). Since they have different lev-
els of walking ability (6), different phys-
ical conditions (17), and differently de-
veloped cardiovascular and respiratory 
systems (10-14), the preschool teacher 
cannot take all these individual differ-
ences into account. As a rule, they select 
the child who has the least developed 
endurance and does not give others the 
appropriate intensity.
In the third paragraph, the results 
in Tables 3 and 4 are compared with 
Fartlek, which is considered as a uni-
versal method of endurance develop-
ment. Aerobic endurance development 
exercises are more and more sophisti-
cated and include strains for both aero-
bic and anaerobic metabolism. Roughly, 
they can be divided into three methods, 
namely: the uninterrupted exercise, 
interval exercise and Fartlek (which is 
the game of agility). Stöggl and Sperlich 
(24) concluded that the combination 
of all methods was important for aer-
obic endurance development in adult 
sportspersons. Gregorc and Humar (6) 
compared a pre-schooler’s heart re-
sponse with the »method of game« with 
adults’ heart responses with all three 
endurance development methods and 
concluded that a child’s heart response 
was most similar to adult’s heart re-
sponse when performing the Fartlek ex-
ercise. In our study, the 3-year-old chil-
dren achieved higher average heart rate 
during both exercises in comparison to 
5-year old children; during the “method 
of game” (Z = -2.34 p = 0.020, η² = 0.08) 
as well as during uninterrupted walking 
“(Z = 3.17 p = 0.002, η² = 0.15) (Table 4). 
However, a small variance was clarified. 
However, we believe that it would make 
sense to carry out uninterrupted walk-
ing with both younger and older chil-
dren in a different way. The preschool 
teacher should guide the children to safe 
walking paths on which they should be 
allowed short runs, breaks, alternating 
fast and slow walking. Based on the re-
sults of Table 3 (no statistically signifi-
cant differences between boys and girls 
were confirmed), it makes no sense to 
divide the children by gender.
This study has four main limitations: 
• the first is a relatively limited number 
of participants;
• the second is short-term observation, 
with no answer to long-term endur-
ance;
• the third is only two different types 
of exercises;
• and the fourth is measuring intensity 
only through heart rates.
Further prospective studies may an-
swer some of these questions.
240
CARDIOVASCULAR SYSTEM
Zdrav Vestn | May – June 2021 | Volume 90 | https://doi.org/10.6016/ZdravVestn.2977
5 Conclusions
The intention of this research was to 
explore the adequacy of using different 
methods for developing endurance for 
preschool children than for adults. By 
observing how children’s hearts respond 
to developing endurance using “method 
of game” and uninterrupted walking, we 
linked the results to some aspects of a 
child’s development characteristics and 
made three highlights:
• None of the children reached the 
“steady-state” HR during uninter-
rupted walking, especially among the 
3-year-old children.
• The “method of game” was associat-
ed with a higher average and variable 
heart rates compared to uninterrupt-
ed walking, which is also characteris-
tic of Fartlek.
• Rather than choosing a suitable type 
of endurance exercise, a different way 
of implementation is important.
References
1. Issurin VB. New horizons for the methodology and physiology of training periodization. Sports Med. 
2010;40(3):189-206. DOI: 10.2165/11319770-000000000-00000 PMID: 20199119
2. Vehovar M, Rajtmajer D. Funkcionalne sposobnosti mlajših otrok. Maribor: Univerza v Mariboru, Pedagoška 
fakulteta; 2011 [cited 2019 Feb 27]. Available from: http://rajtmajer.si/Rajtmajer.si_pdf/Funkcionalne_
sposob_mlajsih_otrok.pdf.
3. Robinson LE, Stodden DF, Barnett LM, Lopes VP, Logan SW, Rodrigues LP, et al. Motor Competence and 
its Effect on Positive Developmental Trajectories of Health. Sports Med. 2015;45(9):1273-84. DOI: 10.1007/
s40279-015-0351-6 PMID: 26201678
4. Pate R, Oria M, Pillsbury L. Fitness measures and health outcomes in youth. Washington (DC): National 
Academies Press; 2012.
5. Degens H. Physiopathology of muscle in relation to sarcopenia. Journal für Mineralstoffwechsel. 
2011;18(2):6-7.
6. Gregorc J, Humar A. Metoda za razvoj vzdržljivosti v predšolskem obdobju [diplomsko delo]. Ljubljana: 
Univerza v Ljubljani, Pedagoška fakulteta; 2013.
7. Antipuesto DJ. Difference Between Adult and Fetal Circulation. St. Louis: Nursing Crib; 2010 [cited 2019 
Mar 21]. Available from: http://nursingcrib.com/nursing-notes-reviewer/maternal-child-health/difference-
between-adult-and-fetal-circulation/.
8. Kliegman RM, Stanton BM, St. Geme J, Schor NF. Nelson textbook of pediatrics. 2nd ed. Philadelphia: 
Elsevier; 2016.
9. Samuels M, Wieteska S. Advanced paediatric life support. 6th ed. Manchester: Wiley-Blackwell; 2016. DOI: 
10.1002/9781119241225
10. Dotan R, Mitchell C, Cohen R, Klentrou P, Gabriel D, Falk B. Child-adult differences in muscle activation—a 
review. Pediatr Exerc Sci. 2012;24(1):2-21. DOI: 10.1123/pes.24.1.2 PMID: 22433260
11. Krost WS, Mistovich JJ, Limmer DD. Beyond the basics: pediatric assessment. Emerg Med Serv. 
2006;35(5):52-4. PMID: 16729526
12. Fleming S, Thompson M, Stevens R, Heneghan C, Plüddemann A, Maconochie I, et al. Normal ranges 
of heart rate and respiratory rate in children from birth to 18 years of age: a systematic review of 
observational studies. Lancet. 2011;377(9770):1011-8. DOI: 10.1016/S0140-6736(10)62226-X PMID: 
21411136
13. Agrawl S. Normal Vital Signs in Children: Heart Rate, Respirations, Temperature, and Blood Pressure. 
Complex Child E-Magazine. 2008-4. [cited 2019 Mar 21]. Available from: http://www.articles.complexchild.
com/march2009/00114.pdf.
14. Garber CE, Blissmer B, Deschenes MR, Franklin BA, Lamonte MJ, Lee IM, et al.; American College of Sports 
Medicine. American College of Sports Medicine position stand. Quantity and quality of exercise for 
developing and maintaining cardiorespiratory, musculoskeletal, and neuromotor fitness in apparently 
healthy adults: guidance for prescribing exercise. Med Sci Sports Exerc. 2011;43(7):1334-59. DOI: 10.1249/
MSS.0b013e318213fefb PMID: 21694556
241
ORIGINAL SCIENTIFIC ARTICLE
Developing endurance among children in preschool period
15. Ekelund U, Poortvliet E, Yngve A, Hurtig-Wennlöv A, Nilsson A, Sjöström M. Heart rate as an indicator of the 
intensity of physical activity in human adolescents. Eur J Appl Physiol. 2001;85(3-4):244-9. DOI: 10.1007/
s004210100436 PMID: 11560077
16. Sylta Ø, Tønnessen E, Seiler S. From heart-rate data to training quantification: a comparison of 3 methods 
of training-intensity analysis. Int J Sports Physiol Perform. 2014;9(1):100-7. DOI: 10.1123/ijspp.2013-0298 
PMID: 24408353
17. Jereb B, Burnik S, Karpljuk D. Heart rate responses to uphill walking in 9- to 10-yr old boys and girls. 
Kinesiologia Slovenica: scientific journal on sport. 2003;9(1):18-27.
18. Ojala K. UKK Walk test. Tester’s guide. Tampere: Institute Kaupinpuistonkatu; 2013.
19. Mora-Gonzalez J, Cadenas-Sanchez C, Martinez-Tellez B, Sanchez-Delgado G, Ruiz JR, Léger L, et al. 
Estimating VO2max in children aged 5-6 years through the preschool-adapted 20-m shuttle-run test 
(PREFIT). Eur J Appl Physiol. 2017;117(11):2295-307. DOI: 10.1007/s00421-017-3717-7 PMID: 28932901
20. Škof B. Šport po meri otrok in mladostnikov: pedagoško-psihološki in biološki vidiki kondicijske vadbe 
mladih. Ljubljana: Fakulteta za šport, Inštitut za šport; 2007.
21. Videmšek M, Berdajs P, Karpljuk D. Mali športnik. Ljubljana: Fakulteta za šport, Inštitut za šport; 2003.
22. Ušaj A. Kratek pregled osnov športnega treniranja. Ljubljana: Fakulteta za šport, Inštitut za šport; 2003.
23. Gregory J, Meško M. Analysis of the Possibility of Disseminating the “Game-Movement-Development” 
Approach in Early Childhood Education. Pol J Sport Tour. 2015;22(3):182-8. DOI: 10.1515/pjst-2015-0025
24. Stöggl T, Sperlich B. Polarized training has greater impact on key endurance variables than threshold, 
high intensity, or high volume training. Front Physiol. 2014;5(33):33. DOI: 10.3389/fphys.2014.00033 PMID: 
24550842