Authors: Nuri Topsakal
Nuri Topsakal, PhD
Duzce University Faculty of Sport Sciences,
Department of Coaching Education, Istanbul, Turkey
Mailing address: Duzce Universitesi Spor Bilimleri Fakültesi Konuralp Yerleşkesi
Telephone: +90 544 308 25 03
Fax: + 90 (380) 542 1365
Nuri Topsakal is an assistant professor for the University of Düzce Faculty of Sport Science. His areas of research interest are Sports & Exercise Science and Sport Performance.
Relative age effect-enhanced physical fitness reference standards for Turkish youths who live in Istanbul
Purpose: This study aims to form physical fitness reference standards based on the relative age and gender variables of Turkish female and male children between the ages of 7 and 13.
Methods: The sample of this study consisted of 13,863 children (nfemale = 5580; nmale = 8283), between the ages of 7 and 13 from 32 districts of Istanbul. The relative age factor (formed by dividing a one-year period into four subgroups) was considered in the formation of norm tables, which were based on anthropometric measurements and motor tests according to gender. All percentile values were calculated according to gender and age quarter group, with all percentile tables including 5th to 95th percentile values.
Results: The physical fitness parameters of the male and female children improved as they aged. Only the females at certain ages (11-13 years) showed no improvements in BMI, 10-20m sprint, and standing long jump values.
Conclusion: This study formed percentile norm tables that had values ranging between 5 and 95 by using the anthropometric and physical fitness test results obtained based on the standard values related to gender and relative age of Turkish children between the ages of 7 and 13.
Keywords: Turkish Children, Relative Ages Effects, Physical Fitness, Norm Tables, Percentiles
Physical fitness is related to hereditary and environmental factors (10) and involves components related to muscular strength and muscular endurance, flexibility, speed, quickness and durability (34) and body composition (25).
Physical fitness is recognized as a significant indicator of health in childhood and adulthood. A low physical fitness level in childhood and adolescence poses a risk of increase in the rate of obesity and cardiovascular diseases. In addition, precautions should be taken to prevent certain problems, such as skeletal health issues, low quality of life and poor mental health, all of which can potentially arise from low physical fitness level (10).
Muscular strength, one of the basic components of physical fitness, is a strong indicator of health in adolescence and later on, in adulthood. Studies have reported that the muscular strength of young individuals is in a negative relationship with the cardiovascular risk factors, such as blood pressure, total cholesterol, high-density lipoprotein cholesterol, low-intensity lipoprotein cholesterol and triglycerides or insulin resistance (21). This information has increased the importance of evaluating muscular strength in detail to understand the health status of children and adolescents. Despite the proven benefits of a high fitness level for human health, the performance levels of children and adolescents in fitness tests have decreased in the last 30 years (10), an indication that the studies in this field need to be evaluated with a different perception.
Norm tables have been developed in studies conducted in different countries, including Australia, the United Stated of America, Belgium, France, Spain, Hungary, England, Russia and Portugal, (4-6,10,14,19,26,28-32) based on the percentage curves related to children’s and adolescents’ physical fitness levels, age groups and gender. Studies involving the development of physical fitness norm tables for children and adolescents started in Turkey in 1978. However, over the last 40 years, studies have generally been limited to small independent groups or to the activities performed on a larger scale in a single sports field. The norm tables that have been generated in these studies are related to the physical fitness levels of children (8-14 years of age) who do not play sports, (8,9,22) to the athletic skills of children between the ages of 10 and 12 (female and male) and of adolescents between the ages of 13 and 17 (male), (2) to the sports skills, orientation and talent in children’s Olympics of children between the ages of 7 and 12, (18) and to physical performance and body composition of male children between the ages of 7 and 14 who play football (27).
In addition to Şahin’s study (27), the norm tables that have been developed from all the research related to sports in Turkey (2,8,9,18,22) have been generated based on the conventional one-year-based age group delineations. However, as the age difference between the children of the same age born in January and December can affect physical and mental development (11), it is important to group norm tables in monthly, quarterly or four-month periods. The importance and effect of birth month for children of the same age reflects the concept of Relative Age Effect (RAE); that is, the advantage of earlier birth in the same year is referred to as RAE or “Birth Date Effect” (35). The fact that the RAE is related to physical and mental development (11) has recently increased its popularity in the field of sports sciences (37). Moreover, it is important to determine the normative values related to physical fitness level by using a reliable standardized test methodology (34). Norm tables related to physical fitness level that are developed by considering the RAE will enable a more sensitive evaluation of children’s development during the period covering childhood and adolescence. However, there are only a limited number of studies related to the formation of gender-based norm tables that are divided into younger age periods and examine the RAE. This study aims to form norm tables affiliated with the fitness levels of Turkish male and female children between the ages of 7 and 13 based on gender and relative age variables.
Rationale of the Population Survey
Istanbul is Turkey’s largest city receiving huge migration from each region. The Sports Directorates Istanbul Metropolitan and city district Basaksehir Municipality implemented physical fitness tests within the scope of a social responsibility project in order to increase children’s physical activity levels and orientate them to sports. The tests were conducted on children studying in the schools of these districts or coming to the sport facilities of the municipalities. The permission of the above institutions has been obtained for the publication of the test results. The author was consulted for the choice of the tests performed, training of the coaches, implementation of tests and recording of data. Since the participants were under the age of 18, the purpose of the study was explained to their families before the tests and their written consent was obtained. All children participated in the study voluntarily. Before the test, the children were informed about the application of the tests. During the tests, children who did not want to continue the test were allowed to quit. Before the study, theoretical education and practical training on the measurements and tests that would be performed for this study was provided to sports trainers who were employed by the municipalities and had voluntarily agreed to support this study. In these training sessions, applied anthropometric measurements and motor tests were performed on children in the 7-13 years age group. After the education and training were completed, the measurements and tests were initiated. This experimental study was carried out between 2015 and 2019.
Measurements and tests were performed under similar conditions in the sports facilities of the schools and municipalities. Anthropometric measurements (body weight and height) and motor tests [speed (10 m – 20 m), agility-quickness (zig-zag running), strength (handgrip strength, standing long jump and sit-ups)] were applied on the participating children in this study. Tests were completed on the same day for each child. All children participating in this study were asked to not engage in any physical activity on the day of the test and to arrive at the test in a rested state. They attended the tests in suitable sportswear.
The population of this study consisted of 1,521,057 male and female children between the ages of 7 and 13 (female:739,131; male: 781,926), while the sample consisted of 13,863 children (nfemale = 5580; nmale = 8283) from 32 districts (out of 39 in total) of Istanbul (33). The sample size corresponds to 0.91% of the population of this study (females=0.76%, males=1.06%). Children’s ages (in years and months) were determined by subtracting their birth dates from the test date (Age-month = (test date – birth date) / 365.25). The RAE was taken into account when dividing each age group into four sub age groups, and each participating child was evaluated in accordance with their age groups. For example, 7-year-old-children were grouped by their birth dates and months as follows: 7.1 = 7 years 1, 2, 3 months; 7.2 = 7 years 4, 5, 6 months; 7.3 = 7 years 7, 8, 9 months; 7.4 = 7 years 10, 11, 12 months.
The International Society for the Advancement of Kinanthropometry (ISAK) protocol (20) was used in this study. Body weight (kg) and height (cm) were measured with the children barefoot and clothed in underwear using an electronic scale (Polar Balance Baskule, Finland) with 100 g precision and a range of 0–150 kg and a portable stadiometer (Seca 225, Seca, Birmingham, UK) with 0.1 cm precision and a range of 70–200 cm, respectively. Body mass index was calculated as body weight (kg) divided by height (m) squared.
Measurement of motoric characteristics
Speed, agility, strength, explosive strength and muscular resistance were tested in this study. Accordingly, speed was measured by having the participants complete a 10 m. and 20 m. run. Agility was measured by having them do a zig-zag run. Handgrip strength was measured by having them squeeze a handgrip device. Explosive strength was measured by having them do a standing long jump. Finally, abdominal muscular resistance was measured by having them do a 30 sec. sit-up test.
The speed (10 m-20 m) and zig-zag run tests were performed with the photocells set at a sensitivity of 1/1000 (Fusion Sport, Smartspeed Pro). The time it took for the participants to run 10 m. and 20 m.(sec/ms), and the time it took them to complete the zig-zag test track (5 m x 3 m) (sec/ms) were recorded. Tests were repeated twice, with breaks of 2-5 minutes given between each test. The best results were used for the evaluations. The handgrip strength test was repeated twice for both hands using a digital hand dynamometer (Smedley; Takei, Tokyo, Japan), and the best result was recorded (kg). In the 30 sec sit-up test, children were asked to keep their knees bent (at approx. 90°) and hands crossed on their chests, and the maximum number of sit-ups they were able to perform in 30 seconds was recorded using a digital chronometer (Technos, YP2151/8P, BR). The trainers only recorded the number of sit-ups that were determined to be valid (i.e. raising the body from the ground until getting to a fully upright position). For the standing long jump test, the distance the children reached on the horizontal plane by jumping on both feet without a running start was recorded in cm. starting with their toes on the line, the children jumped, and the distance (cm) from where their heels were and the starting line was measured. The tests were repeated twice, with the best results being used in the evaluations.
The characteristics of the participants were described as means, standard deviations, and number. All percentile values were calculated according to gender and age quarter group (set as quarters of every year, and once a child’s measurements were completed, their current age was immediately calculated, and recorded as age and quarter – e.g. 7.1 means age 7 and in the first quarter of his/her age). All percentile tables include 5th, 10th, 20th, 30th, 40th, 50th, 60th, 70th, 80th, 90th and 95th percentile values. The SPSS 20.0 (IBM Inc., Chicago, IL, USA) package program was used to perform statistical analyses.
Among the 13,863 participating children between the ages of 7 and 13, 59.8% were male (n = 8283), while 40.2% were female (n= 5580). The number of participants corresponding to anthropometric and motor test parameters, mean figures, and standard deviation values by age and gender are presented in Table 1 and 2.
Results from the body weight (Table 3), height (Table 4), and body mass index (BMI) (Table 5) measurements and from 10m the sprint (Table 6), 20m sprint (Table 7), agility, zig-zag (Table 8), standing long jump (Table 9), handgrip-right (Table 10), handgrip-left (Table 11) and 30-sec. sit-up (Table 12) tests were used as reference values for gender and age groups (each age was divided into four sub groups) and percentile tables (P5, P10, P20, P30, P40, P50, P60, P70, P80, P90 and P95).
The physical fitness parameters of both the male and female children improved as they aged. Only the females at certain ages (11-13 years) showed no improvements in certain parameters; these included their BMI values (from the age between 13 years and 4 months and 14 years), their 10 m. and 20 m. sprint values (from the age between 11 years and 10 months to 14 years), and their standing long jump values (from the age of between 12 years and 4 months to 14 years).
Although norm tables for the physical fitness values of children have been globally developed (4-6,10,14,19,26,28-32), the number of relevant studies conducted on this issue in Turkey is limited. Normative values related to gender and relative age are needed to evaluate and interpret the physical fitness statuses of the current generation of Turkish children. Furthermore, there has only been a limited number of participants involved in the studies that have been conducted since 1978 on the development of physical fitness norm tables for children in Turkey (2,8,9,18,22), and the age groups were evaluated in one-year periods in these studies. The fact that the norm tables developed in the studies performed with more participants on the basis of the relative age (2,18,27) were related to only certain sports rather than to the general population indicates that more studies need to be conducted in this field.
Considering this shortage of studies and participants, this study aimed to form physical fitness reference standards based on the relative age and gender variables of Turkish female and male children whose ages range between 7 and 13. This study consisted of 13,863 children between the ages of 7 and 13 (1% of the population), and the relative age factor (formed by dividing a one-year period into four subgroups) was considered in the formation of norm tables, which were based on anthropometric measurements (body weight and height) and motor tests [speed (10 m – 20 m), agility-quickness (zig-zag running), strength (handgrip strength, standing long jump and sit-ups)] and arranged according to gender.
To date, this is the only study to have been conducted with a large population of Turkish children (between the ages of 7 and 13), where the relative age was taken into consideration in forming the percentile tables to be used for evaluating the physical fitness status. Considering the age group and norms in this study, the discussion is limited in the sense that there is no previous study by which to compare this study. Therefore, comparison with other Turkish and international studies involving similar methods was made to establish physical fitness norms for the children and adolescents.
Şahin (27) generated tables for the norms (within 5-95% segments) related to physical performance (10-20-30 m sprint, agility, explosive strength, endurance) and body composition (height, weight, BMI, fat percentage) based on the RAE for organized footballers between the ages of 7 and 14 in Turkey. It was found that the norm parameters based on age groups in the tests of height, weight, 1-20m sprint and standing long jump performed with male footballers under the same protocols were better than those of the male children in this study. These results are parallel with the results of the study by Hammami et al. (16) where trained adolescent footballers had better results than those of the untrained adolescents. On one hand, the differences between the physical fitness values of trained and untrained children can be attributed to the fact that growth and development is affected by training (1,13,17). On the other hand, the weight and BMI values of the children in Şahin’s (27) study were lower than those of the children in this study, which supports the aforementioned assumption. This can be explained by increasing physical activity decreases body weight and BMI, while limited physical activity increases weight (15).
Another study on the development of norm tables in Turkey is the Turkish Norm Values in Athleticismconducted by Bayraktar et al. (2) between 2005 and 2007. They conducted this study with 4080 female and male children who were between the ages of 10 and 12 and did not play sports, and they developed norm tables based on gender and age distribution for the parameters of height, weight, BMI, vertical jump height, handgrip strength, standing long jump, throwing medicine ball while standing, flexibility (sitting and laying), sit-ups for 30 seconds and 60 seconds, 10-20-30 m sprint, 1000 m walking/jogging, and endurance. Evaluations were reported in percentile tables as low (1-25 percentile), normal (30-50 percentile), high (60-75 percentile) and very high (80-99 percentile). In comparing the similar parameters in the study by Bayraktar et al. (2) with those of this study, it was found that the mean figure of each age group in all tests (handgrip strength, standing long jump, 20 m sprint), save for the 30-second sit-up test, and was better in this study. The difference in the 30- second sit-up test results could be attributed to the difference in test protocol.
Another study was performed by Coşan and Demir (8) in Turkey between 1993 and 1999 to examine the physical fitness norms of 5518 female and male children between the ages of 8 and 14 who did not play sports. Each of the ten tests (weight, sitting height, height, arm span, vertical jump height, 30 m sprint, 200 m run, standing long jump, throwing a one kg medicine ball, sitting, and laying) performed in this study was scored a maximum of 50 points, and a general mean value was calculated by dividing the total result by 10. A scale with scores ranging from 4 to 10 (unsuccessful-perfect) was developed for each age group, and norm tables were created based on the scores. Anthropometric and motoric characteristics of the children between the ages of 8 and 14 were collectively evaluated, and the sports-related orientation of these children was determined. Although this study presented the norm tables in terms of the relative age variable regarding the physical fitness levels of female and male children who did not play sports, no comparison could be made with the study by Çoşan and Demir (8) because they formed the norm tables based on scores.
Physical fitness is a determining factor for morbidity and mortality. De Miguel-Etayo et al. (10) have reported that low physical fitness levels in children and adolescents are related to an increase in body fat percentage and cardiovascular risk factors, and poor skeletal and mental health. On one hand, the reference values obtained in the present study from the norm tables may be important for identifying children with low physical fitness levels who have medical issues and are in the risk group (10). On the other hand, the literature presents different opinions about whether or not the relative age affects teams’ selections of elite athletes (7,12,23,24,38). Those who are relatively older within the same age group might have advantages in terms of maturation and physical development. As relatively older children are more compliant to the instructions from their coaches, they get more playing time, which in turn allows for greater skill development of the children (3). Therefore, the effect of RAE has often been the subject of studies related to talent selection and sport orientation (23,24,36). However, if RAE is not considered when performing risk evaluations for health, inaccurate results may be found in cases where children who were born in January and December are evaluated under the same standards because they are of the same age, ignoring the fact that they are in a different sub age group. Thus, the norm tables formed concerning physical fitness level were generated by considering the four sub age groups according to the RAE. The norm tables presented in this way are believed to contribute to the literature insofar as they will enable risk evaluations to be performed more accurately.
Despite performing no statistical comparisons between the groups, the norm tables (Table 3-12) created in this study indicate that the results of each age group improved as the age increased. This improvement was found to be clearer between the first quarter and third and fourth quarter of age, a finding which can be associated with factors such as maturation and physical and mental development (3,11).More detailed studies on this topic should be conducted.
The most significant aspect of this study was that the team performing the tests and measurements received prior training and did not have access to any of the data resulting from the tests and measurements until all procedures had been completed. The pre-tests and measurements were viewed as part of the training and therefore were not included in the database. Moreover, particular attention was given to meeting the objective that the fitness centers and municipality centers used for tests and measurements had the same conditions, with little to no fundamental differences. The participating children were informed before the tests to wear proper sportswear for their visit to the centers. In this way, the effort was made to minimize the standardization-related limitations for collecting data. Another important aspect of this study was that the data were collected from 32 districts (out of 39) of Istanbul. As Istanbul receives migration from many Turkish cities, the sample can be considered as a reflection of Turkey’s general population. The authors of this study managed to access a prodigious amount of data from almost all regions in Istanbul, securing results from a sample that was socio-economically and socio-culturally diverse. Due to the limited opportunities in the fitness centers where the tests were performed and considering the medical risks, cardiovascular tests were excluded. The absence of such tests to evaluate the cardiovascular status of children can be reported as one of the study’s limitations. Moreover, the absence of a statistical presentation of the differences between the groups and genders can be regarded as another limitation of this study.
This study presented the standard values related to the physical fitness tests of Turkish children between the ages of 7 and 13 as norm tables, with the percentile values ranging between 5% and 95%. In presenting these values, the children’s gender and RAE were taken into consideration.
The norm tables are presented as percentile values which enables the determination of whether the Turkish children are above or under the mean value related to physical fitness and anthropometric characteristics. Therefore, medical and sports professionals working with children between the ages of 7 and 13 will be able to identify children by their physical fitness levels. In addition, as these norm tables are presented in terms of the relative age, trainers, teachers, families and those working on the physical fitness of children will be able to perform evaluations that are more detailed. It is recommended that future studies statistically compare the differences in physical fitness components by gender and age groups and present the results. Moreover, studies related to the development of different motoric characteristics and norm tables in relation to various age groups should be conducted.
APPLICATIONS IN SPORT
Physical fitness is recognized as a significant indicator of health in childhood and adolescence while low physical fitness levels dispose to obesity, skeletal health issues, low quality of life and poor mental health. The establishment of norm tables for these critical periods will enable parents, coaches and educators to monitor the development of the fitness level of children.
The author kindly thanks the Sports Directorates of İstanbul Metropolitan Municipality and Başakşehir Municipality that supported this project.
The author would also like to thank the participants and the coaches of the Sports Directorates of İstanbul Metropolitan Municipality and Başakşehir Municipality.
The author owes special thanks to Ani Agopyan,PhD.,Cengiz Karagözoğlu,PhD., Meral Küçük Yetgin, PhD. from the Marmara University, Faculty of Sport Science (Istanbul, Turkey) and Taylan Balcıoglu, MSc, Fenerbahce University Faculty of Sport Science (Istanbul, Turkey).
- Acikada, C. (2004). Training in children. Acta Orthop Traumatol Turc, 38:16-26.
- Bayraktar, I., Pekel, H.A., Yaman, M. & Aydos, L. (2010). Atletizmde Türkiye norm değerleri. [Norms of Turkey in track and field]. Deliceoğlu, G., Yaman, Ç. (Eds.), Ankara: Ata Ofset Matbaacılık. (in Turkish).
- Birch, S., Cummings, L., Oxford, S.W. & Duncan, M.J. (2016). Examining relative age effects in fundamental skill proficiency in British children aged 6–11 years. Journal Strength Cond Res. 30:2809-15. doi: 10.1519/JSC.0000000000000526.
- Castro-Piñero, J., González-Montesinos, J.L., Mora, J., Keating, X.D., Girela-Rejón, M.J., Sjöström, M. & Ruiz, J.R. (2009). Percentile values for muscular strength field tests in children aged 6 to 17 years: influence of weight status. Journal Strength Cond Res. 23:2295-310. doi: 10.1519/JSC.0b013e3181b8d5c1.
- Castro-Piñero, J., González-Montesinos, J.L., Mora, J., Keating, X.D., Sjöström, M. & Ruiz, J.R. (2010). Percentile values for running sprint field tests in children ages 6–17 years: influence of weight status. Res Q Exerc Sport. 81:143-151. doi: 10.1080/02701367.2010.10599661.
- Catley, M.J. & Tomkinson, G.R. (2013). Normative health-related fitness values for children: analysis of 85347 test results on 9–17-year-old Australians since 1985. Br Journal Sports Med. 47:98-108. doi: 10.1136/bjsports-2011-090218.
- Cobley, S.P., Schorer, J. & Baker, J. (2008). Relative age effects in professional German soccer: a historical analysis. Journal Sports Sci. 26:14:1531-8. doi: 10.1080/02640410802298250.
- Cosan, F. & Demir, E. (2000). Physical fitness norms of Turkish children (Türk çocuklarının fiziki uygunluk normları). Mengütay S. (Eds), Istanbul: Mart Matbaacılık. (in Turkish).
- Cosan, F. (2005). Athletes resource project for the Olympics. Scientific foundations of athletics infrastructure studies. (Olimpiyatlar için sporcu kaynağı projesi. Atletizm altyapı çalışmalarının bilimsel temelleri). Demir A. (Eds), Istanbul: Uğur Basım ve Yayınevi. (in Turkish).
- De Miguel-Etayo, P., Gracia-Marco, L., Ortega, F.B., Intemann, T., Foraita, R., Lissner, L., Oja, L., Barba, G., Michels, N., Tornaritis, M., Molnár, D., Pitsiladis, Y., Ahrens, W., & Moreno, L.A. (2014). Physical fitness reference standards in European children: the IDEFICS study. International Journal of Obesity (2014) 38, S57–S66; doi:10.1038/ijo.2014.136.
- Delorme, N., Boiché, J. & Raspaud, M. (2010). Relative age effect in elite sports: methodological bias or real discrimination? European Journal of Sport Science, Sci. 10:2:91-6. doi: 10.1080/17461390903271584
- Deprez, D., Coutts, A. J., Fransen, J., Deconinck, F., Lenoir, M., Vaeyens, R., & Philippaerts, R. (2013). Relative age, biological maturation and anaerobic characteristics in elite youth soccer players. International journal of sports medicine, 34(10), 897-903.
- Diker, G. & Muniroglu, S. (2016). Examination of selected physical characteristics of 8-14 age group football players according to age groups. (8-14 yaş grubu futbolcuların seçilmiş fiziksel özelliklerinin yaş gruplarına göre incelenmesi). SPORMETRE Beden Eğtimi ve Spor Bilimleri Dergisi. 14:45-52. (in Turkish)
- Eisenmann, J. C., Laurson, K. R. & Welk, G. J. (2011). Aerobic fitness percentiles for US adolescents. American journal of preventive medicine, 41(4), S106-S110.
- Grabara, M. (2012). Analysis of body posture between young football players and their untrained peers. Human movement, 13(2), 120-126.
- Hammami, A., Randers, M. B., Kasmi, S., Razgallah, M., Tabka, Z., Chamari, K.. & Bouhlel, E. (2018). Effects of soccer training on health-related physical fitness measures in male adolescents. Journal of sport and health science, 7(2), 169-175.
- Koşar, N. S. & Demirel, H. A. (2004). Physiological characteristics of child athletes. Acta orthopaedica et traumatologica turcica, 38, 1-15.
- Kuter, F.O. & Arabacı, R. (2008). Examining the I. level results of the talent selection in sports and directing to sports project for the Olympic games (Bursa sample). (Olimpiyatlar için sporda yetenek seçimi ve spora yönlendirme projesi, 1.aşama sonuçlarının incelenmesi (Bursa örneği)). E-Journal New World Sci Academy, Health Sci, 3:B001, 18-28. (in Turkish).
- Ljach, W. (1997). High-performance sport of children in Russia. Leistungssport, 27, 37-40.
- Marfell-Jones, M. J., Stewart, A.D. & De Ridder, J.H. (2012). International standards for anthropometric assessment. Wellington, New Zealand: International Society for the Advancement of Kinanthropometry;.p.51-5
- Martínez-López, E. J., Manuel, J., Suárez-Manzano, S., & Ruiz-Ariza, A. (2018). Analysis of the effect size of overweight in muscular strength tests among adolescents: reference values according to sex, age, and body mass ındex. The Journal of Strength & Conditioning Research, 32(5), 1404-1414.doi: 10.1519/JSC.0000000000001967.
- Mengutay, S., Demir, A. & Cosan, F. (2002). Athletes resource project for the Olympics. (Olimpiyatlar için sporcu kaynağı projesi). Mengütay S. (Eds), Istanbul: Mart Matbaacılık. (in Turkish).
- Mujika, I., Vaeyens, R., Matthys, S. P., Santisteban, J., Goiriena, J. & Philippaerts, R. (2009). The relative age effect in a professional football club setting. Journal of sports sciences, 27(11), 1153-1158.
- Mulazimoglu, O., Cihan, H., Erdogdu, M. & Sirin, E.F. (2013). The relative age effect on infrastructure and professional team of some football clubs in Turkey. (Türkiye’deki bazı futbol kulüplerinin profesyonel ve alt yapı takımlarında bağıl yaş etkisi). SPORMETRE Beden Eğtimi ve Spor Bilimleri Dergisi, 11:105-11.
- Pescatello, L. S., Riebe, D. & Thompson, P. D. (Eds.). (2014). ACSM’s guidelines for exercise testing and prescription. Lippincott Williams & Wilkins. Section 1: Health appraisal and risk assessment 1 9th ed. Philadelphia, PA.
- Ruiz, J. R., Rizzo, N. S., Hurtig-Wennlöf, A., Ortega, F. B., Wàrnberg, J. & Sjöström, M. (2006). Relations of total physical activity and intensity to fitness and fatness in children: the European Youth Heart Study. The American journal of clinical nutrition, 84(2), 299-303.doi: 10.1093/ajcn/84.1.299
- Sahin, M. (2017). A norm study: Body composition and physical performance of 7-14 year old children playing football in Turkey. Türkiye’deki 7-14 yaş futbol oynayan çocukların fiziksel performans ve beden kompozisyonu norm çalışması. Unpublished doctoral dissertation. Marmara University, Institution of Health Science, Istanbul.
- Saint-Maurice, P. F., Laurson, K. R., Kaj, M., & Csányi, T. (2015). Establishing normative reference values for standing broad jump among Hungarian youth. Research quarterly for exercise and sport, 86(sup1), S37-S44.doi: 10.1080/02701367.2015.1042416.
- Saint-Maurice, P. F., Laurson, K. R., Karsai, I., Kaj, M., & Csányi, T. (2015). Establishing normative reference values for handgrip among Hungarian youth. Research quarterly for exercise and sport, 86(sup1), S29-S36.doi: 10.1080/02701367.2015.1042354.
- Sandercock, G., Voss, C., Cohen, D., Taylor, M. & Stasinopoulos, D. M. (2012). Centile curves and normative values for the twenty metre shuttle-run test in English schoolchildren. Journal of sports sciences, 30(7), 679-687.doi: 10.1080/02640414.2012.660185.
- Santos, R., Mota, J., Santos, D.A., Silva, A.M, Baptista, F. & Sardinha, L.B. (2014). Physical fitness percentiles for Portuguese children and adolescents aged 10–18 years. Journal Sports Sci, 32:1510-8. doi: 10.1080/02640414.2014.906046.
- Silva, G., Aires, L., Mota, J., Oliveira, J., & Ribeiro, J. C. (2012). Normative and criterion-related standards for shuttle run performance in youth. Pediatric exercise science, 24(2), 157-169. https://doi.org/10.1123/pes.24.2.157
- Turkiye Istatistik Kurumu TUIK (2019). Bilgi Dağıtım ve İletişim Dairesi, Bilgi Dağıtım Grubu. Available at: http://tuik.gov.tr/Start.do, (accessed 21.05.2019).
- Vanhelst, J., Labreuche, J., Beghin, L., Drumez, E., Fardy, P. S., Chapelot, D., … & Ulmer, Z. (2017). Physical fitness reference standards in french youth: the BOUGE program. The Journal of Strength & Conditioning Research, 31(6), 1709-1718.doi: 10.1519/JSC.0000000000001640.
- Vincent, J., & Glamser, F. D. (2006). Gender differences in the relative age effect among US Olympic Development Program youth soccer players. Journal of sports sciences, 24(4), 405-413.doi: 10.1080/02640410500244655.
- Votteler, A., & Höner, O. (2014). The relative age effect in the German Football TID Programme: Biases in motor performance diagnostics and effects on single motor abilities and skills in groups of selected players. European journal of sport science, 14(5), 433-442.doi: 10.1080/17461391.2013.837510.
- Wattie, N., Schorer, J., & Baker, J. (2015). The relative age effect in sport: A developmental systems model. Sports Medicine, 45(1), 83-94.. doi: 10.1007/s40279-014-0248-9.
- Wiium, N., Lie, S. A., Ommundsen, Y., & Enksen, H. R. (2010). Does relative age effect exist among Norwegian professional soccer players. International Journal of Applied Sports Sciences, 22(2), 66-76.doi: 10.24985/ijass.2010.22.2.66