Skeletal age and explosive strength in young volleyball players Idade óssea e força explosiva de jovens praticantes de voleibol

Identifying diferent anthropometric, physiological and motor aspects is essential to obtain success in sports practice. However, these components may be developed in same age subjects diferently, in part due biological development. he aim of the study was compare and correlate the explosive strength to biological maturation by sex, obtained by estimation of skeletal age based on anthropometric variables. he sample consisted of 239 subjects of both sexes, aged 10 to 13 years. Maturation was assessed by skeletal age predictive equation and upper and lower limbs explosive strength using medicine ball throw test and vertical jump, respectively. Skeletal age was assessed by skeletal age using a predictive anthropometrical based equation. Upper and lower limbs explosive strength were evaluated by medicine ball throwing test and vertical jump test, respectively. Post hoc analysis showed delayed biological development subjects had worst medicine ball throwing test performance than accelerated boys (p=0.001; d=0.96) and girls (p< 0.01; d= 2.01); regarding to vertical jump test, worst performance was also detected to maturational delayed boys when compared to accelerated ones (p=0.24; d=1.1) and girls (p=0.007; d=0.75). Regression analysis showed skeletal age explained variance of 36% and 19.2% for boys and 45.2% and 16.1% for girls upper and lower limbs explosive strength. Skeletal age is positively related to upper and lower limbs explosive strength and show higher performance for accelerated biological development young players when compared to delayed ones, independently from sex.


INTRODUCTION
To achieve success in sports, the identiication of diferent aspects such as anthropometric, motor and physiological are essential, making them the main parameters of selection and identiication of young athletes 1,2 .In intermittent sports, such as volleyball, athletes perform short-term high intensity eforts, with emphasis on explosive movements such as jump, hitting and blocking, what emphasizes muscular explosive strength as an important athletic attribute.Corroborating with this idea, Sheppard et al. 3 in a time motion study showed that players performed at least one jumping movement and frontcourt players performed four blocks jump and three spikes jump during a rally.Besides, functional capacities such as jump height and upper limbs explosive strength has been reported as good volleyball discriminant function predictor 4 .
Athletic readiness has been presented as a required condition to sports initiation 5 due to searching for immediate results.hus, based on talent selection and identiication system, young athletes with higher explosive strength performance may have a better selection chance.However, results achieved on early initiation may not relect on athlete's future success.herefore, it is necessary to identify parameters beyond athletics readiness during young athlete's development.During adolescence, same age subjects may have body size and physical performance distinguished regarding in part due to their biological development rate 6,7 , which varies according to sex 8,9 .For decades, studies highlighted biological maturity impact on functional performance, especially regarding to strength tests 10,11 .his works have reported accelerated biological development inluence on increased body growth, strength and power performance when compared to same chronological age less matured peers.Besides, performance differences associated to maturity may explain the high advanced maturity youth prevalence in selection process 2 .However, according to Malina et al. 1 these diferences tend to be reduced with age and eliminated, suggesting that maturation is an important tool to diagnose young people growth, development and physical potential.In this context, maturation appears as an important component to be identiied in athlete's selection.
Sexual maturation, somatic maturation and skeletal age are commonly used evaluation methods for maturational identiication.Skeletal age predicted by hand radiography is considered the most appropriate method to estimate biological maturation 12 .Even though it is referred as a gold standard to assess biological age, radiography practical application is very restricted, mainly because it demands high-cost equipment and specialized technicians for radiographs interpretation in addition, it increases subjects radiation exposure.Because of these limitations, our group recently developed a non-invasive, easily accessible and interpretive method based on a skeletal age predictor model using anthropometric variables 13 .herefore, this method allows a greater practical applicability for coaches and technicians estimate the young athlete's biological development.
Due to volleyball physical and functional demands, it is likely that biological maturity might play a signiicant role in youth players selection and performance.However, body size changes associated to accelerated maturation may impact on body displacement actions against gravity as in vertical jumps and throwing movements which characterizes explosive strength.herefore, it is necessary to identify how much biological maturation combined to body size predicts upper and lower limb explosive strength performance, sports success essential components.his could minimize future selection and orientation errors in young athletes talent selection process.his study was conducted in order to compare boys and girls upper and lower limbs explosive strength performance in diferent maturational stages, as well as to verify the relation between biological maturation assessed by skeletal age and upper and lower limbs explosive strength, in conjunction to body size parameters.It is hypothesized that both male and female accelerated players in maturity will show higher explosive strength compared to same chronological age less matured peers, besides, a positive relation of the skeletal age with these attributes will be identiied.

Sample
Subject sample consisted of 239 young volleyball players, both sexes, aged 10 to 13 years (12.03 ± 1.44 years), participants of a volleyball sport's initiation project in Natal -RN, Brazil.Sample selection was intentional and non-probabilistic and adolescents practiced, three training sessions per week, during two hour.Inclusion criteria included subjects participation on the sport's initiation project, between 10 and 13 years old, practicing volleyball for six months or more.hose individuals who refused to participate of the suggested tests, those with health problems that could impair test performance, missing from the collections for two days, as well as those ones with an injury history in the last three months were excluded from the study.Study was approved by local institution Ethics and Research Committee of Health Sciences Center (CAEE: 1249937/2015), following guidelines for data collection in humans, according to resolution nº 466/12 of 12/12/2012 of National Health Council, as well as the ethical principles contained in Declaration of Helsinki.

Study design
Present study is cross-sectional and correlational designed.Initially, all data collection procedures were explained to project coordinators and participants, and then a consent form was issued for responsible signature, as well as term of assent for study volunteers.After signed documents returning, subjects were submitted to anthropometric evaluation during the irst three days in order to estimate the biological development.Motor tests were then performed to identify upper and lower limb explosive strength, which also occurred over a period of three days.he data collection was done at project activities.

Anthropometric measurements
Body mass and stature were assessed using an electronic scale (Filizola® 110, São Paulo, Brazil), with a capacity of 150 kg, divisions of 1/10 kg and a precision of 100 grams and a stadiometer (Sanny ES2020®, São Bernardo do Campo, Brazil) with a 0.5 cm scale, respectively.Arm corrected perimeter and triceps skinfold, with Harpenden® adipometer (John Bull, London, England) with 0.2 mm precision and humerus and femur biepicondylar diameter, with a metallic caliper (Cescorf®, Porto Alegre, Brazil). he reliability of the measurements was tested by technical error of measurement (<5%) 14 and by test-retest coeicient (> 0.98).All procedures were performed by a single evaluator and followed guidelines of the International Society for Advancement in Kinanthropometry (ISAK) 15 .

Upper limbs explosive strength
Upper limbs explosive strength was determined by medicine ball throwing test according to recommended standards proposed by Gaya and Silva 16 , reported as a high reliability level test to evaluate upper limbs explosive strength 17 .he subject's was instructed to sit with his back fully supported on a wall, keeping his legs together and his knees extended.he medicine ball (2kg) was held close to the chest region and was thrown at evaluator's signal as far as possible.he marked distance was considered from the zero (wall) to the irst point of contact of the ball to the ground.For each test, the best performance between two pitches was considered.In case of trunk movement at throwing time, attempt was discarded and a new pitch was requested in order to avoid inluences in upper limbs strength measurements.All participants were familiarized with the test.

Lower limbs explosive strength
he protocol used to identify lower limbs explosive strength was counter movement vertical jumping test, proposed by Komi 18 , reported as a high validity and reliability measure test to estimate lower limbs explosive strength 19 .he instrument used was a contact platform connected to Jump Test Pro 2.10 software (Ceise®, São Paulo, Brazil).To perform the test, the patient was standing hands on waist, performing eccentric and concentric phase of the movement quickly, lexing the hip, knee and ankle joints.he subjects were instructed to land at same point of takeof, keeping their legs extended, in order to avoid knee lexion.To minimize diferences in measurement, the hands were held on the hips throughout the test.hree jumps were performed in a one-minute interval, and the best performance was recorded, with jumping height used to analysis.

Skeletal age and maturation
Subjects biological development was assessed through a mathematical model used to estimate skeletal age.Equation was determined by anthropometric variables using a multiple regression model.Only anthropometric variables that showed a signiicant correlation with the skeletal age obtained by wrist radiography, considered as gold standard, were included in the model.Predictor model was validated by Cabral et al., 13 for both sexes youth Brazilian population aged 8-14 years (r2 adjusted = 0.741; standard error = 1.24), according to the equation: Skeletal Age = -11.620+ 7.004(height) + 1.226.Dsex + 0,749(age) -0.068(Tr) + 0.214(Acp) -0.588(Hd) + 0.388 (Fd).
To determine maturation, skeletal age subtraction was used by chronological age in years (sum of months of life divided by 12). he maturation was stratiied according to the following cutof points for the study: delayed (result over 12 months negative for chronological age), normal (up to 12 months positive or negative for chronological age), or accelerated (Above 12 months positive in relation to chronological age) 9 .

Statistical analysis
Normal distribution of data was veriied by Kolmogorov-smirnov test.Data were reported on mean and standard deviation, and maturational stages on relative and absolute frequency.A univariate ANOVA was used to compare the dependent variables among the maturation levels.Eta squared partial (partial η2) was used to determine the efect size of the variance.When necessary, Bonferroni's post hoc test was used to ind the signiicant differences.Cohen's d was used to determine the efect size of the means.An efect size < 0.2 relects a negligible diference, ≥ 0.2 and < 0.5 relects a small diference, ≥ 0.5 and < 0.8 relects a moderate diference, and ≥ 0.8 suggests large diference.he relation between skeletal age and the upper and lower limbs explosive strength was veriied by Pearson's correlation.A coeicient r< 0.1 relects trivial correlation; 0.1 -0.3 small; 0.3 -0.5 moderate; 0.5-0.7 strong; 0.7 -0.9 very strong; 0.9 -0.99 almost perfect; and 1.0 perfect.In order to identify relationship of skeletal age (predictor variable) on dependent variables, upper and lower limb explosive strength, we used simple linear regression with skeletal age isolated and multiple regression adjusted by body mass and stature.Considering anthropometric variables in the model, the FIV index was veriied to diagnose the multicollinearity between the predictive variables (FIV <10).For each sex, two regression analyzes per dependent variable were performed.A post hoc statistical power analysis for diferences in explosive strength among the maturation levels was conducted to determine achieved power, based on the investigated sample size, an alpha of 0.05, and the achieved efect size.Regarding boys, for a univariate ANOVA analysis, achieved power for the main efect of group was 99% and 81% for upper limbs and lower limbs, respectively.Regarding girls, achieved power for the main efect of group was 100% and 84% for upper limbs and lower limbs, respectively.Data analysis was performed using Statistical Package for Social Sciences (SPSS) version 19.0 (IBM Corp. Released 2010, IBM SPSS Statistics for Windows (New York -USA) and the signiicance level set was p <0.05.

RESULTS
Table 1 reports comparison of physical and motor characteristics according to sex, as well as boys and girls distribution according to maturational stages.It is possible to identify that girls presented highest values for skeletal age compared to the boys (F (1.237) = 13.74;partial η 2 = 0.055), thus a higher prevalence of girls at accelerated maturational status was observed in relation to the boys focused at normal and late stages.Concomitantly, higher values were found for girls height when compared to boys, even with similar chronological ages (F (1.237) = 4.74; partial η 2 = 0.020).Figure 1 reports comparison between upper and lower limbs explosive strength of boys and girls according to their maturational stage.Diferences in upper limbs explosive strength performance among boys maturational stages were observed (F (2.98) = 10.8;p<0.001; partial η 2 = 0.181) and girls (F (2.135) = 26.6;p= <0.001; partial η 2 = 0.283), as well as boys lower limbs explosive performance force (F (2.98) = 10.8;p=0.007; partial η 2 = 0.095) and girls (F (2.135) = 5.5; p=0.005; partial η 2 = 0.075).Post hoc analysis showed delayed maturational development boys had the worst performance for medicine ball throwing test compared to normal group (p=0.001;d= 0.94) and accelerated ones (p=0.001;d=0.96), similar to results obtained for vertical jump test, delayed versus normal group (p=0.42;d=0.54) and delayed versus accelerated (p=0.24;d=1.1).For girls, explosive strength performance was lower for delayed maturational stage group when compared to normal (p=0.001;d= 0.97) and accelerated stage (p< 0.01; d= 2.01).Delayed maturational stage girls had worse lower limbs explosive strength performance when compared to accelerated (p= 0.007; d=0.75).
Table 2 shows correlation between skeletal age and upper and lower limbs explosive strength of boys and girls.It was observed a positive sig-niicant relation between skeletal age and upper and lower limbs explosive strength in both genders, which was observed moderate correlation coeficients for LLS in boys and girls, while strong correlation coeicients were found for ULS.

DISCUSSION
his study veriied the efect of young athletes maturational stages on upper and lower limb explosive strength performance.Despite the same chronological age group, our data show a strong efect of accelerated maturation on upper limb explosive force performance for both sexes, compared to delayed maturational stage subjects.Regarding to lower limbs explosive strength strong and moderate efect of accelerated maturation was observed for boys and girls, respectively, when compared to delayed maturational stages.After regression analysis, our indings demonstrated that biological maturation, estimated by skeletal age, alone, shared 36% and 45.2% of variance of upper limbs explosive strength performance for boys and girls, respectively.Besides, it was also observed that skeletal age alone showed only 19.2% and 16.1% of variance in lower limb explosive strength performance for boys and girls, respectively.Previous studies have reported similar data indicating a linear increase in explosive strength for young people with accelerated> normal> delayed maturational stage 5,7 .During adolescence, physical performance is correlated with biological development 1,6,10,11 .In this way, our indings predict that during biological development, there is an increase in upper and lower limbs explosive strength.In fact, some mechanisms can be attributed to explain the obtained values for young people explosive strength during biological development.hey include the largest responses of anabolic hormones, such as testosterone 20 and structural changes in muscle architecture, resulting signiicant changes in type I to type II ibers proportion 21,22 , observed in younger age.Volver et al. 23 , further complement that the anabolic androgenic actions in the pubertal period stimulate the development of type II muscular ibers, responsible for explosive force generation.However, because of their invasive nature, metabolic responses in children and adolescents according to maturational development are limited.However, based on the aforementioned studies, it is possible to hypothesize that lower hormonal circulation and amounts of type ibers II, may explain, in part, the diferences in explosive strength performance among maturational 24 .
Body changes in height and body mass inherent to maturational process 5,10 have been proposed by some authors to explain diferences in performance and muscle strength according to anthropometric variables 6,11 .However, after body mass and height inclusion to the model, a small correlation coeicient increase was evidenced, demonstrating that skeletal age is able to explain the greater variance in upper and lower limbs explosive strength, with lower variance shared for vertical jump.Previous studies of our group 10,25 showed similar relation between lower limbs explosive strength and biological development has presented lower coeicients when compared to upper limbs explosive strength.Till et al. 5 did not verify interaction efect for maturational group x time efect for vertical jump when they compared annually young rugby players performance in respective categories (under 13, under 14 and under 15) according to maturational stage.Probably, this lower relation of skeletal age with vertical jump performance is attributed to a greater neuromuscular factors contribution attributed commonly to tendon rigidity 26 .On the other hand, It was observed a low signiicant positive relation for this speciic volleyball actions, with a strong and moderate efect reported for boys and girls univariate analysis respectively.herefore, maturational inluence on lower limb explosive force performance is not yet clear in literature, suggesting future investigations in order to elucidate possible mechanisms that explain lower limbs explosive strength diferences in this population, mainly due to the importance of this component in sports. . .Ability to play the highest power of lower and upper limbs is identiied as critical to success in volleyball game and is often requested for athletes speciic game actions like spike, block, serve and set 26,27 .Among the discriminating functions of volleyball, block and spike in attack and counterattack phase are predictors that distinguish sets victory and defeat 28 .Additionally, it is possible to suggest that the upper and lower limb explosive strength performance plays an important role in this discriminant volleyball functions eiciency 26,29 it is likely that young athletes with better performance for these attributes may have advantages in sports selection process due to the importance of maturational development for this athletic component.Corroborating this idea, Torres-Unda et al. 2 , demonstrated that selected basketball athletes have greater maturational development than the non-selected.However, it should be pointed out that this selection criterion, using only athlete's physical and motor attributes, could induce a potential long-term planning mistake 1 .Recently, longitudinal design studies have shown that athletes at delayed development demonstrated higher increment for upper and lower limb power test performance compared to normal and accelerated development 5,30 .hese data suggest that a potential development window may exist for subjects with delayed maturation, contradicting current criteria for young athletes selection programs.herefore, the importance of monitoring biological development during young athletes' selection process is emphasized in order to avoid exclusion of a potential athlete in a near future, who can match or overcome their peers with accelerated development.
In conjunction with previous results our indings conirm the importance of evaluating maturational development for coaches and sports professionals during young athletes selection and sports orientation process considering that explosive strength performance is diferent according to the biological development.herefore, sports professionals have a simple greater practical applicability tool based on anthropometric measurements as an alternative to estimate the biological development according to gold standard method of x-ray of the wrist, and consequently improving choices during the young athletes training process.It is advisable to expect for better results from delayed young athletes during maturational process until they reach more advanced maturity levels in order to avoid an early exclusion of a potential athlete.In spite of study relevant information, some limitations must be pointed out, as transversal design, what became impossible to clarify a relation of cause and efect between maturation and athletes motor development.Besides that, our indings should be analyzed with caution, considering the use of indirect methods for explosive force evaluation, however, it is important to note that methods used for this study are widely used in literature and also have a high reliability degree.

CONCLUSION
It is evidenced that biological development, estimated by skeletal age, presents a signiicant positive relation to explosive upper and lower limbs explosive performance, although a smaller relation to lower limbs explosive strength was found.In this sense, our indings reinforce necessity to evaluate maturation during young athlete's selection processes, combined to other aspects, such as chronological age, in order to reduce potential choice misunderstandings, especially in those sports that require further upper limb potency development.It is also worth noting that this is a starting study, which proposes to longitudinally evaluate this sample in order to know if our model will be conirmed.

Figure 1 .
Figure 1.Comparison of explosive strength of upper and lower limbs according to the biological development of boys and girls ULS= upper limbs explosive strength; LLS= lower limbs explosive strength; *= statistical difference vs delayed (p<0.05);#= statistical difference vs normal (p<0.05).

Table 1 .
Characteristics of the sample according to sex.

Table 2 .
Correlation of skeletal age with the explosive strength of upper and lower limbs according to sex.

Table 3
reports a regression model with upper and lower limbs explosive strength variables considering skeletal age as an independent variable.Isolated, skeletal age shared respectively 36% and 45.2% of variance of upper limbs explosive strength performance for boys and girls; skeletal age shows lower regression coeicient for upper limbs explosive strength performance in both, boys (19.2%) and girls (16.1%).A variance of 41.4% and 49.4% was observed to performance boys and girls lower limb explosive strength, respectively, after anthropometric variables model adjustment, body mass, height and skeletal age (model 2).Whereas, lower limbs explosive strength there was a lower correlation coeicient, totaling 28.1% for boys and 21.9% for girls.

Table 3 .
Linear regression model for the variables explosive strength of upper and lower limbs by the variable independent skeletal age of boys and girls.