Physiological Profile of Brazilian Elite Soccer Players: Comparison between U-17, U-20 and professionals

Possible diff erences in the physical values related to characteristics of the age group and the play positions in soccer. The aims to analyze the isokinetic muscle strength profi les, anthropometric, neuromuscular and cardiovascular characteristics, comparing age, categories and play positions. 105 players divided into U-17, U-20, Professional and six play positions. The isokinetic test, anthropometric measurements, counter-movement jump test with arms and yo-yo intermittent recovery test were performed. Signifi cant diff erences (p<0.05) were found in the professionals compared to U-20 and U-17 for body mass, height, body fat, VO2max and for U-17 in relation to professionals in the isokinetic test. Signifi cant diff erences were also found between play positions (p<0.05) for body mass, height, VO2max and isokinetic. Soccer athletes playing in diff erent positions have diff erent profi les of physiological characteristics and U-17 have a lower level of strength compared to professionals, suggesting the importance of training specifi city.


Introduction
One of the main factors aff ecting soccer performance is muscle strength (Brown & Weir, 2011). Knowledge of strength levels in players is therefore important for a proper training prescription in order to gain optimal performance (Hamzeh & Head, 2004). Th e strength of soccer players has been previously studied by measuring squats and vertical jumps (Dauty, Bryand, & Potiron-Josse, 2002;Silva et al., 2012;Wisløff , Helgerud, & Hoff , 1998), as well as by specifi c strength tests using isokinetic dynamometry (Agre & Baxter, 1987;Gioft sidou et al., 2008;Hamzeh & Head, 2004;Hom, Steen, & Olstad, 2005). Studies have demonstrated muscle strength increases as young athletes grow and develop (Le Gall, Laurent, & Rochcongar, 1999), and that muscle balance is fundamental for young players in order to advance to levels of excellence using specifi c training (Hom, Steen, & Olstad, 2005).
Normally, two factors tend to cause injuries in soccer play-ers. Th e fi rst is the agonist-antagonist imbalance between the quadriceps and hamstrings. Th e second is the contra lateral strength imbalance between the dominant and non-dominant limbs (Daneshjoo, Rahnama, Mokhtar &Yusof, 2013). Th ese imbalances have been identifi ed as a common occurrence and can potentially increase the risk of injury (Hamzeh & Head, 2004;Lehance, Binet, Bury, & Croisier, 2008;Bulatović & Kezunović, 2011;Tsikanos, Paschoalis, & Valasotiris, 2016). Th e incidence of knee injuries due to soccer practice has become a concern to clubs and athletes of all age groups, both in individuals who practice this sport recreationally as well as in professional athletes. Young people age 16-20 years are involved in intense periods of technical, tactical, and physical training with the aim, to be promoted to the professional squad in the long-term (Gioft sidou et al., 2008). Consequently, this prolonged and intense training may lead to muscle imbalance and injuries.
PHYSIOLOGICAL CHARACTERISTICS OF YOUNG ATHLETES | C. HERDY ET AL.
Studies demonstrate signifi cant cardiovascular, anthropometric, maturation, and physical diff erences among age categories (Lago-Peñas, Casais, Dellal, Rey, & Domínguez, 2011;Silva, Bloomfi eld, & Marins, 2008) and between fi eld positions in amateur and professional athletes (Keiner, Sander, Wirth & Hartmann, 2015). At each position, the player uses a specifi c style of play which directs the development of a particular musculoskeletal profi le (Hencken & White, 2006). Th e availability of reference values for knee extensor and fl exor strength has become an important additional factor for improving training prescriptions and the prevention of injuries. Th us, the purpose of this study was to examine the muscle strength, anthropometric and cardiovascular profi les of athletes from an elite soccer club in the under 17 years of age (U-17), under 20 years of age (U-20), and adult professional (AP) categories, and to explore relationships with their playing positions.

Subjects
Th e sample was composed of 105 male soccer players from a fi rst division club in Brazil participated in the study. Th ey were divided into three age categories: U-17, U-20 and AP. Th eir playing positions were recorded as goalkeeper, external defenders, central defender, defensive midfi eld, midfi elder and forward. Th e players had at least fi ve years of training experience in high performance soccer. To be included in the study, all subjects had to have performed isokinetic testing before the study and have no medical history that could be aggravated by participation in resistance training. Participants were also directed to abstain from any nutritional or ergogenic supplements. Th e Faulkner protocol was used for body fat analysis (Faulkner, 1968), the counter-movement jump test (with arms) was used for vertical jump height (Silva et al., 2012) and the yo-yo intermittent recovery test was used to estimate the maximum oxygen consumption (VO 2max ) (Castagna, Impellizzeri, Chamari, Carlomagno, & Rampinini, 2006). All subjects were instructed to refrain from participating in any physical training during the study period, except those prescribed as part of the investigation. We obtained the assent and consent from all participants and their respective legal guardians and this study followed international standards for working with human beings (World Medical Association, 2013) and was approved in Research Ethics Committee of the Federal University of Rio de Janeiro, with the number 34315478.7.0000.5257, in accordance with Resolution 196/96 of the Brazilian Health Council.

Isokinetic testing
An isokinetic dynamometer (Technogym®, REV9000, Italy) was used for strength assessments. Subjects performed fi ve sub maximal familiarization tests. Th e order of the limb testing was randomly selected. Th e concentric tests of the hamstrings and quadriceps were performed at 60º/s with fi ve maximal repetitions. A standardized pretest procedure was employed, where all athletes received instructions on the test purposes, as well as on the procedures for performing the test. A 5-min warm up was performed using a cycle ergometer at a speed of 80 rpm and power at 50W. Th e environmental conditions during the tests were: ambient temperature 21.8±1.20 ºC (20-24), barometric pressure of 702.7±1.4 mmHg (700-705) and relative humidity percentage 52.1±14.9% (28-71). Th e isokinetic strength test was undertaken in the seated position (120 o hip angle). Subjects were stabilized by belts attached to the chest, hip, and thigh of the tested limb. Participants crossed their arms over the chest to minimize involvement of upper body musculature. Th e lateral femoral condyle was aligned with the axis of rotation of the dynamometer while the ankle cuff was attached proximally to the lateral malleolus. Gravitational correction was performed to account for limb mass. Subjects were tested by the same examiner who encouraged the athlete with verbal commands throughout the test to perform with their maximal eff ort.

Statistical Analyses
All data are presented as mean (± SD). A Kolmogorov-Smirnov test (Bartlett criterion) was performed to assess the normality and homoscedasticity of the data. All variables presented normal distribution and homoscedasticity. An ANOVA with repeated measures was performed with Bonferoni post-hoc tests when appropriate, to examine the diff erences between the fl exors and extensors knee torque measures of the dominant and non-dominant limbs and to identify possible diff erences between the groups. Th e signifi cance level was set at p<0.05. Data were analyzed using SPSS Version 21 (SPSS Inc., Chicago, IL, USA), which analysis was used to determine the diff erences in 6 play positions.

Results
Data on the composition of the sample containing the values of age, anthropometric data (body mass and stature), fat percentage, vertical jump and maximum VO 2 (Mean ± S.D.), can be observed in Table 1     pared to the forwards (p<0.05). Signifi cant diff erences (p<0.05) were found between the U-17 and AP players for knee fl exion of the dominant and non-dominant knee fl exion, and for the dominant H:Q ratio (Table 3).
In addition, the goalkeepers demonstrated signifi cant differences compared to all other playing positions (Table 4) (p< 0.05).

Discussion
Th e main conclusion of the present study is that soccer athletes in diff erent playing positions have diff erent isokinetic strength, anthropometric, and cardiovascular profi les. In addition, the current study confi rmed players in the U-17 category have lower strength levels compared to professional adult athletes. Th us, in line with Hom et al. (2005) and Lehance et al. (2008), we found U-17 athletes had lower peak torque values than adult professionals. In addition, the current data demonstrated goalkeepers were distinguished by higher strength values compared to the other positions. Th e height variation in the goalkeepers could be attributed to the fact that taller players were chosen for this position in addition to being more eff ective in placing the ball back in play because they perform numerous goal kicks during training and games.
Our results corroborate the fi ndings of a recent study by Tsikanos et al. (2016) examining 275 professional soccer players that reported highest knee extension isokinetic strength values for goalkeepers at three angular velocities 30º/s, 60º/s, and 90º/s. In this sense, with respect to the fi eld positions, we demonstrated signifi cant diff erences between goalkeepers and most other positions examined. For instance, our data suggest the bilateral strength (H/H) in the forwards was signifi cantly lower than in goalkeepers. Lago-Peñas et al. (2011) also highlighted the fact goalkeepers were diff erent from other soccer athletes given their training specifi city. However, confl icting with our data, Tsikanos et al. (2016) did not report diff erences among fi eld positions aft er examining peak torque relative to body mass in professional Greek soccer players. Th ese confl icting results were probably because Tsikanos et al. (2016) studied professional athletes and analyzed four positions while in the present study we also analyzed the U-17 and U-20 categories in six diff erent positions. In addition, Tsikanos et al. (2016) only analyzed the dominant leg, which may have been a limiting factor for inferences. Th us, our results point to the need of specifi c training approaches for diff erent age categories and playing positions. Silva et al. (2012) reported the level of muscle power in soccer athletes, and examined diff erent categories and positions using the vertical jump test. No signifi cant diff erences were reported among the U-17, U-20 and adult professional categories, although it was noted there were diff erences between midfi elders and defenders. Tourney-Chollet et al. (2000) also found signifi cant diff erences related to forwards and midfi elders hamstrings, though the study examined only three playing positions of adult teams. While their ages diff ered across the three categories, test subjects were highly trained in the present study.
Th e present data demonstrated signifi cant diff erences between central defenders and central midfi elders' hamstrings peak torque of the dominant leg. Soccer is a complex sport and playing and training in a certain position can stimulate diff erent power levels. However, this relationship is not well described in the literature and needs further investigation. Th e observation and analysis of categories and positions played on the fi eld is of great importance for professionals who aim at reaching the best results and developing injury prevention protocols.
In the case of VO 2max AP had lower values than younger athletes. Although athletic individuals demonstrate a non-linear decline upon decreasing or ceasing training, the normal age-related decline in VO 2max is reported at approximately 10% per decade regardless of activity level (Hawkins & Wiswell, 2003). Th is demonstrates that older professional soccer athletes need to pay greater attention to the aerobic training component. Previous studies have observed little diff erence between the playing positions in men (Lockie, Lazar et al., 2016). However, the respective measurements were: anthropometrical measures, jump assessments, linear non-linear speed tests or featured only indirect muscular endurance tests. Future investigations involving the measurement of physiological, chemical and functional parameters (Lago-Peñas et al., 2011) are suggested to extrapolate the real relationship between the ages and categories of soccer players.
In conclusion, the U-20 did not present signifi cant statistical diff erence for the physical characteristics evaluated in relation to the AP. While, U-17 do not have the same strength, anthropometric and cardiovascular profi les compared to AP. Th is fi nding can be explained by the fact that athletes in this age group are still in the process of growth and maturation incomplete, which would aff ect isokinetic strength levels. In addition, players in diff erent positions have diff erent physical and physiological characteristics (particularly goalkeepers and forwards), suggesting the importance of specifi c training for each position. Professionals involved with sports training, strength, conditioning, and rehabilitation are therefore alerted to review the training and rehabilitation procedures used with younger soccer players and to consider tailoring these to the athletes playing position.