Changes in maximal oxygen uptake during growth and development in girls who actively participate in basketball and nonathletes girls : a longitudinal study

Background/Aim. It is well known that continuous engagement in physical activity is important for normal growth and development of children. Maximal oxygen uptake (VO2max), as a measure of functional state of the organism, is largely affected by level of physical activity, but it remains unclear to what extent it can be improved during childhood. The aim of the study was to evaluate dynamics of changes in aerobic capacity, anthropometric and body composition characteristics in active and non-active girls over a period of 3 years. Methods. A total of 48 young girls were included in the study. Girls were divided into 2 groups: training group consisted of 25 girls who played basketball (age 13.84 ± 0.94) and non-training group of 23 girls who were not involved in any organized sports (age 13.83 ± 0.98). Anthropometric and body composition characteristics were measured in order to monitor somatic growth during the study. VO2maxvalues were obtained by performing cardiopulmonary exercise testing on a treadmill. All parameters were measured every 6 months during 3-years period. Results. ANOVA analysis showed a significant time and group interaction effect on VO2max (p < 0.001), body mass index (BMI) (p < 0.001) and fat percentage (FAT%) (p < 0.01). Also, there was an obvious increase in VO2max within both groups due to growth and development itself (p < 0.001). Conclusion. The main finding of the study was an increase in VO2max due to growth and development. The girls who actively participated in basketball had higher level of aerobic capacity compared to non-active girls. Furthermore, continuous basketball training led to maintaining normal body composition in terms of FAT% and BMI, which altogether may imply that organized physical activity has a positive influence on evaluated characteristics.


Introduction
Physical activity is an important factor for normal growth and development of children since continuous exercise enhances the functional state of entire organism and has beneficial effects on body composition and basic motor skills 1 .It is essential to adopt proper habits in terms of physical activity during childhood in order to prevent sedentary lifestyle and, consequently, all the concomitant diseases (cardiovascular diseases, type 2 diabetes, obesity, etc.) [1][2][3][4] .
Aerobic capacity represents the functional state of the entire organism, but largely depends on the ability of the heart and lung to deliver an adequate amount of oxygen to our muscles in order to produce energy for work.Maximal oxygen uptake (VO 2max ) is the measure of aerobic capacity and cardiorespiratory fitness per se.It is significantly affected by genetic factors, gender, age, and level of physical activity.Although it is predominantly genetically determined, active participation in sports can lead to full expression of genetic potential and subsequently to an increase in aerobic capacity, especially in period between age 18 and 25 5 .On the other hand, VO 2max values are increasing during growth and development of children due to increase in body size, but it remains unclear to what extent aerobic capacity can be improved by engaging in continuous physical activity in such early and sensitive period of life 2, 3, 6, 7 .There are limited data available on changes in VO 2max during growth and development of both active and inactive children, especially longitudinal data are lacking for young girls.
Therefore, the study was designed to evaluate dynamics of changes in aerobic capacity, anthropometric and body composition characteristics in 2 groups of girls over a period of 3 years.The obtained results should provide the basis for a discussion on the individual influence of growth and development as well as basketball training on the evaluated parameters.

Subjects
A total of 48 young girls were included in the study.Girls were divided into 2 groups: the training group consisted of 25 girls who played basketball (age 13.84 ± 0.94 at the beginning of the study; age 16.32 ± 1.21 at the end of the study) and the non-training group of 23 girls who were not involved in any organized sports (age 13.83 ± 0.98 at the beginning of the study; age 16.65 ± 0.88 at the end of the study).The girls from the training group were recruited from 3 local basketball teams in Belgrade, Serbia.At the baseline of the study they have already played basketball 3.65 ± 1.67 years on average and had 5.7 ± 2.13 h of training per week.The non-training group consisted of girls who attended the same elementary schools as their peers in the training group.Both groups were homogeneous in terms of age, gender and socioeconomic status.Written informed consent was obtained through a letter given to parents explaining study goals, procedures, and methods.The protocol was in accordance with the Declaration of Helsinki for research on human subjects and was approved by the Ethical Committee of Sports Medicine Association of Serbia.

Measurements
Participants were evaluated at outpatient sports medicine clinic, under the auspice of the Sports Medicine Association of Serbia, every 6 months (April/October) during a period of 3 years (total of 6 tests were carried out).
Physical/somatic growth of the young girls was estimated by obtaining anthropometric and body composition characteristics.Height was measured by Seca stadiometer to the nearest 0.1 cm.Arm span was obtained by using tape measure technique.Tanita Body Composition Analyzer BC-418MA was used to measure body weight, body mass index (BMI), body fat percentage (FAT%), and free fat mass (FFM).
VO 2max values were obtained by performing cardiopulmonary exercise testing (CPET) on a treadmill (H-P-COSMOS).Before the CPET, participants passed sports medical examination and were eligible for carrying out maximal progressive test.The subjects were equipped with a face mask, heart rate monitor (COSMED Wireless HR Monitor) and ECG device (Quarck T 12x, Wireless 12-lead ECG) in order to perform the test.The initial speed and inclination were set at 2.5 km/h and 3°, respectively.Every 30 seconds treadmill speed was increased by 0.5 km/h, while the inclination remained constant throughout the test.The protocol was created, as described above, taking into account the age of participants and the fact that most of them did not use the treadmill before.A very low speed at the beginning of the test, with only 0.5 km/h of increase in 30 sec, has provided enough time to get familiar with the treadmill and walking and running technique.The treadmill inclination was set in order to amortize steps on a treadmill and avoid local muscle fatigue and aches.Oxygen consumption kinetics was measured continuously by using breath-by-breath analysis technique (Quark CPET system manufactured by Cosmed).Heart rate was monitored by both COSMED Wireless HR Monitor and ECG device.A test was considered maximal if participants achieved 90% or more of predicted maximal heart rate for age and gender.The 220 -age equation was used in order to calculate maximal heart rate predicted values (Tanaka,  2001).Furthermore, a plateau in oxygen consumption despite increased workload, a respiratory exchange ratio grater than 1.00 and reached volitional exhaustion were also criteria for the end of the test.All the tests were performed by trained personnel and the test equipment were routinely calibrated with both volume and gas calibration every fifth test.

Statistical analysis
Statistical analysis was performed with the software IBM SPSS Statistic version 20.0.All data were assessed for normality (one-sample Kolmogorov-Smirnov test).Paired samples t-test was used to compare measured characteristics within the groups at the beginning and the end of the study.The subject's baseline and final characteristics between the groups were compared using independent-sample t-test.A comparison of continuous variables was performed by general linear model 2-way analysis of variance (ANOVA) with repeated measures, with group (research group vs. control group) as the between-subjects effects, and time (baseline vs. end of the observational period) as within-subjects effects.Descriptive statistics of data for both groups were given as ґ ± SD.A p-value ≤ 0.05 was considered statistically significant.

Results
The anthropometric and body composition characteristics for both groups are presented in Tables 1 and 2 respectively.

Within the group changes
Main effects of time on anthropometric and body composition parameters are shown in Table 2.A significant increase in FAT% was noticed in the non-training group after 3-years follow-up in comparison to baseline values (paired samples t-test, p < 0.01), while there were no significant changes in the training group in terms of this parameter (paired samples t-test, p > 0.05).Furthermore, Figure 1 shows an obvious increase in VO 2max values within the both groups throughout the study (ANOVA repeated measures; main effect of time, p < 0.001).In addition, Figure 2 and 3, respectively, present trend changes in BMI, FAT% and VO 2max values within the training and non-training group over time.
Between the group changes and time and group interaction effects on measured anthropometric and body composition parameters are shown in Table 2.At the baseline of the study, there was no differences between the active and inactive girls regarding FAT% (p > 0.05), but final measurements revealed significant difference in terms of this characteristic (independent sample t-test, p < 0.01).Furthermore, ANOVA analysis also showed a significant time and group interaction effect on VO 2max (p < 0.001).

Discussion
The main finding of the present study was an increase in VO 2max in both study groups over time.Furthermore, the participants from the training group achieved higher VO 2max values compared to their non-active peers.
Representative data of VO 2max values in the young girls were scarce, presented within a wide range of age (i.e.13-19), and vary among studies depending on sample size and measurement methods 3,8 .Our data are highly comparable with those reported in a study including 92 active and 75 non-active Spanish girls (41.85 mL/kg/min and 39.03 mL/kg/min in 12 to 18-years-old active and non-active girls, respectively) 4 .On the other hand, higher VO 2max values were obtained among Indian girls (54.4 ± 5.00 and 48.70 ± 3.21 ml/kg/min in 12 to 17-years-old active and non-active girls, respectively) 9 and yet, lower in 6 to14-years-old Serbian girls (30.4 ± 9.6 mL/kg/min) probably due to younger age of evaluated participants 10 .Compared for normative data for VO 2max , the training group girls were graded as excellent (39.0-41.9mL/kg/min) at the beginning of the study and as superior (> 41.9 mL/kg/min) by the end of the observational period.Aerobic capacity of the non-training group was graded as good (35.0-38.9mL/kg/min) throughout the entire study.
Earlier studies reported a gradual increase in VO 2max through childhood and adolescence due to growth, development, and a subsequent increase in body size and muscle mass, which enhances oxygen utilization.Accordingly, maturation, age, and body composition may account for most of differences in VO 2max values in pre-pubertal and pubertal children 3,11 , with only weak to moderate training-induced increase inVO 2max during this sensitive period of life 3,7,8,11 .In line with our findings, a gradual increase in aerobic capacity goes in favor to growth and development.The continuous increase in body height, arm span, body weight, BMI and FFM over time clearly contributed to better oxygen utilization and an increase in VO 2max values.On the other hand, the recent research on this matter 7,9,12,13 showed a strong association between regular engagement in physical activity and aerobic capacity in children, which might explain higher VO 2max values in training group compared to non-training one.The higher level of aerobic capacity in the trained group of girls could be associated with an increased respiratory volumes, stroke volume and cardiac output, which are in favor of both morphological and functional adaptations of heart and lungs to organized and continuous physical activity 7 .
Furthermore, being involved in regular basketball training led to maintaining normal body composition in terms of FAT% and BMI during growth and development 10 .It is well known that lower FAT%, with normal BMI and greater FFM are also correlated with higher cardiorespiratory fitness 14 .In addition, athletic training at a non-elite level did not affect growth in somatic sense 15 .Growth spurt and changes in weight were the same for both groups, since there was no significant time and group interaction effect on these parameters by the end of the observational period.
In addition, according to Robbins et al. 15 , by the age of 13, only about 22% of girls meet World Health Organization recommendations of at least 60 min of physical activity every day 1, 16,17 and almost 28% are already overweight or obese.These findings indicate that young girls would benefit from continuous involvement in exercise programs, such as those designed to increase physical activity 15,18,19 .The obtained results could be in a favor of positive effects of physical activity on aerobic capacity, body composition and consequently cardiorespiratory system of young girls itself.

Limitations
Although the present study has certain advantages such as direct measurement of VO 2max and evaluation of a large number of parameters over a period of 3 years, it also has some limitations.At the beginning of the study there was already statistically significant difference in terms of VO 2max, body weight and FFM between the two evaluated groups.The complex interaction between basketball training and growth and development could be properly evaluated only if there were no significant differences between the groups in terms of baseline measurements.The results obtained by this comparative longitudinal study design might only imply that active participation in basketball led to an increase in aerobic capacity beside growth and development.In addition to frequency of basketball training (hours/week of training) obtained for the training group, the level and intensity of physical activity could have been measured by accelerometers and heart rate monitors for both evaluated groups in order to get better insight into influence of organized physical activity on monitored parameters.

Conclusion
According to the results presented in the study, maximal oxygen uptake gradually improves with an increase of body size due to growth and development in young girls.It is demonstrated that girls actively engaged in basketball training have higher aerobic capacity and maintain normal values of FAT% and BMI during growth and development.These findings may imply that organized physical activity has a positive influence on evaluated characteristics.

Fig. 3 -
Fig. 3 -Trend changes in BMI, FAT% and VO 2max values within the non-training group over time.BMI -body mass index; FAT -body fat percentage; VO2max -maximal oxygen uptake.

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E F E R E N C E S 1. Morrow JR Jr, Tucker JS, Jackson AW, Martin SB, Greenleaf CA, Petrie TA.Meeting physical activity guidelines and health-related fitness in youth.Am J Prev Med 2013; 44(5): 439-44.2. Saygin O, Zorba E, Karacabey K, Mengutay S. Gender and maturation differences in health-related physical fitness and physical activity in Turkish children.Pak J Bioll Scis 2007; 10(12): 1963-9.

Test VI: performed measurements during period of 3 years on every 6 months (April/October). Table 2 Within and between the group changes in terms of anthropometric and body composition characteristics and VO 2 max values during the study
a Time (

baseline vs. end of the observational period) refers to within-subjects effects.
b Group (training vs. non-training group) refers to between-subjects effects.c Time*