The period of life before adolescence is critical for the acquisition by young soccer players of many physical and technical features. During the course of prepubertal development, running speed, resistance, and strength improve (1,3,5,13,24). Likewise, the development of intellectual and motor skills leads to improved technical, tactical, and psychological abilities (8,28). Adequate training and acquisition of physical and motor skills during this period are decisive for the future progression of young soccer players because after this age, the selection process to play in a team is even harder.
Success in soccer is dependent on a variety of factors including the physical characteristics and physiological capacities of the players, their level of skill, their degree of motivation, and tactics employed by them against the opposition (28,30,38). Some of these factors are not easily measured objectively, but others can be tested using standardized methods and can provide useful information for coaches (18,19). Reilly and coworkers (31) reported that the most discriminating factors among elite and nonelite young soccer players were agility, sprint time, ego orientation, and anticipation skill. Other criteria, such as fat percentage, aerobic power, tolerance of fatigue, and ability to dribble the ball were also relevant.
Elite players are often selected at a very young age. Some studies have been conducted to investigate which factors influence the progression of young soccer players (11,26). However, the exact factors that affect the progression of these populations is not known.
In order to identify factors that may be relevant in the selection of a player in a given sport, our approach was to compare players selected as first team players (FTPs) with the corresponding reserves (21,35). Thus, in the present study, we compared the anthropometric and physiological characteristics of preadolescent soccer players selected to play on the first team with those who were reserves. We analyzed all the variables at the beginning and at the end of the season and examined bivariate correlations among growth/maturation and the seasonal changes in performance parameters of the players since the maturation status of a player has been considered to be particularly relevant to the progression of preadolescent and adolescent soccer players (11,26).
Experimental Approach to the Problem
To ascertain whether anthropometric characteristics are relevant to the progression of the soccer players, several body measurements were taken, such as height and weight, 6 skin fat folds, 4 diameters, and 3 circumferences. Body composition and somatotype were then calculated using various formulae.
To determine the physiological characteristics of the soccer players, we used Astrand's test to measure aerobic capacity. The validity of this test to estimate o2max was verified recently (23). Players performed a 30-m flat sprint to measure velocity, the same distance with 10 cones to measure agility, and a 30-m sprint dribbling around 10 cones with a soccer ball. To measure the power of the legs, players performed 3 different jump tests. Finally, salivary testosterone was measured as an index of the maturation status of each player.
A total of 66 soccer players aged between 10 and 14 years old belonging to 4 teams (each one corresponding to the year of birth) of the DENA project created by the Athletic Club de Bilbao (Spain) participated in this study. This professional club, playing in the first division of the Spanish league, has developed a talent identification project to select the best players in the area. These boys were selected because they are considered to be the best players in their region. All players were subjected to the same conditioning programs during the week that they performed twice weekly during the soccer season. In terms of conditioning, the soccer match on the weekend is the unique difference between FTPs and reserves, although integrants of the last group participate regularly, at least for a short period, in the matches. They did not participate in any specific strength training and the sports antecedents for both groups were very similar (they played soccer regularly since they were 8 years old). The season extended from November to May. Measurements were made at the beginning (November-December) and at the end of the competition period of the training cycle. Only players without prolonged (>3 weeks) injuries were analyzed.
The FTPs were the best 11 players according to the coach. The coaches were not aware of the purpose of our studies, and all the players in each age group underwent the same training program. The number of FTPs and reserve players in each team, their average ages at the beginning of the season, and the time playing soccer are shown in Table 1. The protocol was fully approved by the Clinical Research Ethical Committee of the local Hospital of Cruces (Bizkaia), and all subjects and their parents or guardians were fully informed and signed a consent form.
Each player's weight (in kilograms) and height (in centimeters) were determined. Skinfold measurements (in millimeters) were taken at 6 sites: triceps, subscapular, abdominal, suprailiac, thigh, and calf, using a skinfold caliper. Each measurement and the sum of the 6 of them were used for analysis.
The circumferences of the upper arm, thigh, and calf were measured (in centimeters), as well as 4 diameters (in centimeters): the biepicondyle of the humerus (elbow), the bistyloid of the wrist, the biepicondyle of the femur (knee), and the bimalleolus of the ankle. All measurements were performed by the same person, who holds an International Society for the Advancement of Kineanthropometry qualification (second level). Afterward, the weights and percentages of fat, bone, and muscle were calculated to determine body composition as follows: to estimate fat percentage, Faulkner's formula was used (10): fat percentage = sum of 4 skinfolds (tricipital + subscapular + supraspinal + abdominal) · 0.153 + 5.783. Therefore, fat weight (kg) = fat percentage · body weight · 100−1; bone weight was calculated using Rocha's equation (32): bone weight (kg) = 3.02 · (height2 · bistyloid diameter · biepicondyle diameter of femur · 400)0.712, where height and diameters are specified in meters. Consequently, bone percentage = bone weight · 100 · body weight−1; muscle weight (kg) was also estimated using Matiegka's formula (27): muscle weight = total weight − (fat weight + bone weight + residual weight), residual weight (kg) = total weight · 24.1 · 100−1 (39). Thus, muscle percentage was calculated as muscle weight · 100 · body weight−1.
All players performed 3 tests on the soccer pitch: a 30-m flat sprint, a 30-m sprint with 10 cones, and a 30-m sprint with the same cones dribbling a ball with their feet. The distance between 2 consecutive cones was 3 m. Sprint times were measured using electronic timing lights (Seiko System S129; Seiko, Tokyo, Japan). In order to run at maximal velocity, players had 15 m to accelerate before the first timing light.
Participants also performed 2 jumps in each of 3 jumping modalities using a jump mat (Ergojump; Bosco Systems, Globus, Italy). The squat jump was performed with the starting position of a squat: knees flexed at 90° and hands on hips. From this position, they made a maximal vertical jump landing with straight knees on the mat. For a countermovement jump, the starting position is standing straight and after flexing the knees to the squat position, players jumped vertically as high as possible. In the drop jump, soccer players jumped from a height of 40 cm to the jump platform, flexed to the squat position and followed with a maximal vertical jump. Jump heights were measured in millimeters. The best jump of each modality was selected.
Estimation of Aerobic Capacity
All players completed Astrand's test on a cycloergometer (Monark 834; Monark Exercise, Varberg, Sweden) starting the exercise at 50 W. Power was increased progressively over 2 minutes in order to obtain a heart rate between 120 and 170 b·min−1. Exercise continued at this power for 6 minutes. The average heart rate during the last minute was measured by a pulsometer (Advantage NV; Polar, Kempele, Finland) and was used to estimate absolute (L·min−1) and relative (mL·kg−1·min−1) maximal oxygen uptake by Astrand's nomogram (2).
Determination of Salivary Testosterone
Saliva samples were collected with a plastic straw. The saliva was let to flow down through the straw into a glass tube that was centrifuged at 3000 rpm for 5 minutes. Samples were frozen at −20°C and thawed before use. Testosterone was measured in duplicate by a direct immunoenzymatic determination using a specific kit, following manufacturer's instructions (Diametra, Segrate MI, Italy).
Date of Birth
We noted the date of birth (month) of soccer players, and this parameter was divided into 2 semesters: January to June (first semester) and July to December (second semester).
Test results for the FTPs and the reserve players were compared using the Student t-test. Two-way (beginning or end of the season, and FTP or reserve) analysis of variance and related samples Student t-test were also used to analyze the improvement or worsening of each group for the studied parameters. Linear regression analyses were performed to examine the bivariate relationship between growth/maturation and performance variables.
Analysis of date of birth of players in the whole group revealed that 72% had been born in the first semester and 28% in the second. This difference is even more marked in the FTPs (74% born in the first semester) than in the reserve players (68% born in the first semester).
Although the differences are not statistically significant, the FTP tended to be taller and heavier than the reserves. In addition, when we compared height and weight, team by team (Table 1), we found that the FTPs were higher in all the teams analyzed and heavier in 3 of 4 teams.
Table 2 summarizes the estimated means of the anthropometric characteristics in the FTPs and reserve players at the beginning and at the end of the soccer season. Even though no significant differences were found, it is interesting to note that the FTPs tended to have lower fat and higher muscle percentages. At the end of the season, the difference in fat percentage increased, but the differences were still not statistically significant (p = 0.06). In fact, fat percentage values increased in the reserves during the season, while they remained unaltered for the FTPs.
Table 3 summarizes the estimated means for the physiological characteristics of the 2 groups at the beginning and end of the soccer season. In both periods, the FTPs were found to be significantly faster in the 30-m flat (p < 0.05) and the 30-m cones test (p < 0.05). In addition to the differences found at the beginning of the season, the reserves performed more poorly than the FTPs on the drop jump test (p = 0.05) at the end of the season.
The FTPs improved their velocity during the season both on the 30-m flat and on the 30-m tests with cones, despite the fact that they performed more poorly on the countermovement jump at the end of the season (p < 0.05 by means of related sample Student t-test). In contrast, the reserve players did not improve significantly in the velocity test and their jumping capacity, as measured with the countermovement jump (p < 0.05), decreased more than that of the FTPs.
In order to ascertain whether the seasonal changes of the FTP and reserves were different between these 2 groups, we also performed 2-way analysis of variance. The FTPs improved more than the reserve players (p < 0.05) in the 30-m flat sprint. In the countermovement jump and the drop jump, reduced performance was attenuated in the FTPs compared to the reserves (p < 0.05). Although differences were not found to be statistically significant, these results always indicated that the FTPs were characterized by greater improvement or less worsening of measured variables than reserve players.
The FTPs had higher salivary testosterone levels than the reserves (10.15 ± 1.57 versus 8.76 ± 2.0 pg/ml), but the differences did not reach the level of statistical significance. Table 4 shows the Pearson correlations among growth/maturation and performance parameters. The increase in height during the season is negatively correlated with the difference in sprint time (tend − tbeginning) in the 30-m cones dash. This means that boys with more growth in height had more improvement (negative difference) in their times on this test. However, positive correlations were found among the increase in weight during the season and the magnitude of the countermovement jump. Salivary testosterone concentration was also positively correlated with improvement in the countermovement jump and drop jump tests and with the increase in relative o2max during the season. In addition, salivary testosterone concentrations were associated with improvement in 30-m flat sprint time (negative correlation). No other statistically significant correlations were found among other growth/maturation and performance parameters.
In the present study, we identified differences between FTPs and reserve soccer players, despite the fact that both groups undergo an identical training schedule. The FTPs were found to be taller and heavier than the reserves. These differences were not found to be statistically significant when we analyzed all the teams together, which is likely due to the fact that variability among children at these adolescent ages is quite high. However, when we compared variables within each team, we observed that in 3 of 4 teams, the FTPs were heavier and in the 4 teams analyzed, the FTPs were taller than the reserve players. This suggests that size is an important factor associated with progression in a soccer career at these early ages. Our results corroborate those of other studies that reported a trend of selecting taller and heavier players in under-18 teams (11,22,25).
The FTPs were faster than the reserves in the 30-m flat sprint and in the 30-m with 10 cones tests at the beginning of the season. The importance of this variable among elite soccer players was previously reported by Reilly et al. (31). In fact, although sprinting time in a soccer match only represents 8-13% of the whole time (36), these high-intensity runs may be decisive for winning the match. FTPs also performed better on the countermovement jump and drop jump tests at the end of the season. Speed and strength have also been reported to be discriminating factors among elite, subelite, and recreational soccer players (12). In contrast, we did not find any significant differences in o2max between the FTPs and the reserves, either at the beginning or at the end of the season. This result does not mean that aerobic capacity is not an important factor for good soccer performance because other studies have demonstrated that both young and adult elite players have an above average o2max (30). However, our results reinforce the hypothesis that an outstanding aerobic capacity is not necessary to be outstanding in soccer (19,30). Our results are also consistent with those of Young et al. (40) who reported that in the Australian Football League, starters had better sprint times than nonstarters. Sprint time is also an important factor for the career progression of these players. However, differences in o2max or in vertical jump between starters and nonstarters were not found to be statistically different (29).
The FTPs and the reserve players did not progress equally during the season. In general, differences between both groups were more marked at the end than at the beginning of the season. Thus, velocity, both in the flat sprint and in the sprint with cones, improved significantly in the FTPs. However, no significant improvement was found in the reserves. This fact may be especially relevant and worrying for the reserves because velocity is one of the most important parameters affecting performance in soccer (31) and one of the most relevant to differentiate among the FTPs and the reserves. In addition, in the reserve group, there was a clear tendency to increase fat percentage and also to lose jump capacity during the season, whereas these values remained unchanged for the FTPs.
The loss of physical fitness found in reserve players, in comparison to the FTPs, could be due to different degrees of growth or maturation. In fact, studies carried out with older soccer players did not find these seasonal differences in either the entire team (34) or among starters and nonstarters (21,35). In this regard, it has been reported that the interaction of sexual maturation, chronological age, and skeletal variables, in combination with height and/or weight, is the most significant predictor of many motor performance characteristics in adolescent players (6,20). Consistent with this hypothesis, we observed that most of the boys selected to play in the Athletic Club de Bilbao Center were born in the first rather than in the second semester of the selection year, and many studies have shown that a disproportionate number of elite young male soccer players are born early in the selection year (4,9,15-17,33,37). In the present work, we found that differences in certain performance parameters that occur from the beginning to the end of the season were associated with growth/maturation. Thus, boys with greater increases in height improve more in the 30-m cone sprint, and increases in weight are associated with greater improvement in the countermovement jump test. In addition, the concentration of salivary testosterone was found to be associated with improvement in 30-m flat sprint time (negative correlation) and higher seasonal differences in CMJ and DJ and in relative o2max. The association between testosterone levels and performance in explosive strength and speed has previously been reported by Bosco and coworkers (7). These findings would indicate that the different progression among FTPs and reserves could in part be associated with the different degrees of growth/maturation of each group. The higher salivary concentration and the height of the FTPs compared with the reserves, although not statistically significant, corroborate this hypothesis. Our results are coincident with previous work indicating that early maturing adolescent soccer players are at an advantage with respect to their later maturing counterparts (13,14,25).
In addition to growth/maturation, other factors such as a lack of motivation due to being excluded from the first team and/or a lower level of training because they did not play a match on the weekend could also be relevant to explain the different progression of each group. Any of these options may be a cause of concern because reserve soccer players who are repeatedly not selected for playing in successive seasons due to their lack of progression may eventually give up the sport.
In summary, in this work we analyzed differences among the FTPs and the reserves who belonged to a club that participates in the first division of the Spanish League. The FTP were faster and their progression during the season was better than the reserves. These differential characteristics may be associated with the advanced growth or maturation of FTPs.
In the present study, we analyzed anthropometric and physiological differences among FTPs and reserve soccer players who belonged to the same soccer team. The most discriminating factor between both groups was the sprint time for both the 30-m flat and the 30-m with 10 cones sprint tests. These results indicate that velocity is an important factor to be selected in an FTP at the ages of 10-14 years old.
The differences between the FTPs and reserves tended to increase from the beginning to the end of the season. Thus, only FTPs improved their sprint time. However, the reserves had poorer results in the jump test and a higher fat percentage at the end of the season than at the beginning. One factor that can explain the different progression of each group is the different growth/maturation status. Thus, changes in velocity with cones was associated with increasing height during the season and changes in certain jump tests and 30-m flat sprint were associated with the concentration of salivary testosterone and changes in weight. In addition, we detected a clear tendency of the trainers to select older players (born in the first months of the selection year).
Consequently, technical staff should take the present results into account and should not discriminate against younger or late-maturing players who may develop their abilities later. This discrimination is a cause for concern because it implies that many boys are not able to develop their sport potential or that they give up sports only because they are born late in the selection year or because they are late maturing. Moreover, innate talent in boys born in the second half of the year may not be given a chance to develop if trainers continue to select players in a discriminating manner and to be unaware of the present findings.
This work was facilitated by an agreement between the Athletic Club de Bilbao Foundation and the University of the Basque Country and supported by a grant of the UPV/EHU. L. Gravina and J. Zubero are recipients of grants from the University of the Basque Country.
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