The process of training and educating young soccer players includes instruction in the numerous important components of a professional soccer performance, including the development of high levels of technical and tactical skills and a sufficient physical capacity. During a 90-minute match at an average intensity of 80–90% of the maximum heart rate (HRmax) (19,34), top-level players covered a distance of approximately 10–12 km (14,27). Previous studies (3,6) have shown that the intensity (the mean HR values and total distance covered) in the first half of a match is higher than in the second half. Although the average intensity is close to or above the anaerobic threshold, most of the energy is generated from aerobic processes, thereby highlighting the significance of aerobic capacity indicators such as the V[Combining Dot Above]O2max and the anaerobic threshold. The maximal oxygen uptake of professional male soccer players ranges from 58 to 65 ml·kg−1·min−1 (19,32). In general, young players have a lower V[Combining Dot Above]O2max than adults (33). However, Chamari et al. (10) suggested that a V[Combining Dot Above]O2max corresponding to 70 ml·kg−1·min−1, should be a goal for boys younger than 15 years, who are in soccer training.
Monitoring of the duration and intensity of training sessions is a basic task of coaches working with young players. Because of methodological difficulties, there are few longitudinal investigations (24) of young soccer players that describe the applied training loads and record their impact on physical performance. Therefore, keeping records of implemented training loads and activities may provide valuable feedback and information.
A complete record of the applied training load should include data on the intensity and duration of the exercises and sessions. However, few reports include detailed training content. In their study, McMillan et al. (24) specified the following drills: warm-up, stretching, endurance running, small-sided games, technical training, strength training, and match play. In addition, Reilly (31) stated that traditional training components include warm-up jogging, calisthenics (flexibility and agility), running, circuit training, skills practice, drills, games, and recovery. No published studies have presented a universal classification system for training drills; therefore, a new solution is required.
The selection of training drills has a major influence on the effectiveness of young players' training. The structure of the training loads is directly dependent on the age of the players, and the proportion of technical drills and exercises performed during training changes as players mature. Clearly, the main purpose of a child's training is to develop technical skills and general fitness. After puberty, the periodization of the training of young soccer players becomes similar to that of adults, and enhancing the levels of endurance, strength, and speed becomes essential for further progress.
Another important aspect of monitoring training loads is the manner in which they are recorded. One of the most popular methods for monitoring exercise intensity is the rating of perceived exertion (RPE) scale proposed by Foster (17). Other methods are based on a relationship between the work intensity and HR. Banister (4) created a method that expresses the training load in training impulse (TRIMP) units. Variables such as the training session duration (D), HRmax, and average HR values during rest (HRrest) and exercise (HRex) are used to determine the training load. Another method based on the HR was proposed by Edwards (15), in which the exercise intensity is divided into 5 HR zones (50–60% HRmax, 60–70% HRmax, 70–80% HRmax, 80–90% HRmax, and 90–100% HRmax). Each zone is graded with indices from 1 to 5. The intensity of each activity is evaluated by multiplying the time spent in each zone by the zone index. Alexiou and Coutts (1) showed that a RPE correlates significantly with the HR-based methods of recording the training load during different soccer activities. Methods based on both the HR and AT (anaerobic threshold) are also used (16,20). The use of these criteria is advisable because of the value of the indicators, such as the AT, LT (lactate threshold), and VT (ventilatory threshold), in classifying exercise intensity through-out the training process.
The duration of a standard soccer season differs by country but typically includes basic periods such as a preparation period, a competitive season, and a transition period. Because of climatic conditions, the cycle described above occurs twice a year in some leagues. The level of physical fitness varies according to the phase of the season. Indeed, most studies have shown that the lowest level of physical capacity is observed during the preparation period, and improvement is generally noticed after the completion of the preseason training. However, at the end of the season, physical fitness levels might decrease (11,26), be maintained (24), or improve (29).
The accurate training process in soccer should ensure equal level of physical fitness in both, the first squad players (FSP) and substitute players (SP). Many coaches find it difficult because the league match is the most intensive effort for the players (1). Therefore, the compensation training sessions performed by SPs seem to be advisable.
Investigating the changes in physical capacity over a soccer season and searching for indicators that might have a positive or negative influence on players' fitness are important and practical tasks. The main purpose of this study was to examine the effect of applied training loads on the aerobic capacity of young soccer players over 1 soccer season. Additionally, the quantity of played league games that influenced on the changes of the players' physical capacity was examined. It was hypothesized that the suggested structure of the training loads would maintain or improve the players' aerobic capacity and might be adopted into the training regimen of the young soccer players.
Experimental Approach to the Problem
An annual training load schedule was designed before the start of a soccer season. The season included 2 periods (spring and autumn) separated by a 6-week summer break. The training loads were used during the spring preparation period (SPP), the spring competitive season (SCS), the autumn preparation period (APP), and the autumn competitive season (ACS). The intensity of exercise was divided into 4 zones: (a) Aerobic performance (AP, below AT), (b) Mixed aerobic-anaerobic performance (MAAP, at or just above the AT intensity), (c) Anaerobic lactate performance (ALP, above the AT intensity for longer than 90 seconds), and (d) Anaerobic nonlactate performance (ANLP, short high-intensity activities). Heart rate monitors (Polar Electro OY, Kempele, Finland) were used to facilitate the classification of the exercises. Additionally, the training content was divided into 15 groups of drills (Table 1) to specify the load registration. The training load was expressed as the duration in minutes that the players spent in each zone. Maximum V[Combining Dot Above]O2 data were collected on 3 different occasions: at the beginning of the SPP, in the middle of the season (the end of the SCS), and at the end of the season (immediately after the ACS). Moreover, sprint tests, shuttle run tests, and Wingate tests were conducted at the beginning and end of the season in this study to investigate the influence of applied training loads on the anaerobic capacity of the players. Additionally, the changes of the physical capacity were evaluated after dividing the players into 2 groups (FSP and SP).
Nineteen young male soccer players with more than 6 years of training experience participated in this study (Table 2). During the soccer season, the team played 30 league games (2,700 minutes). After the season, the players were divided into 2 groups (FSP, n = 10; SP, n = 9). The players, who performed at least 70% of total games (1,890 minutes) were allocated to the FSP, whereas participants, who played less than 40% of total game time, were considered as SP. Goalkeepers were excluded from the study. All the subjects passed medical examinations and had their sportsmen medical cards. The players and their parents were fully informed about the potential risks and benefits, and they provided written informed consent before the players' participation in this study. The Ethical Committee of Regional Medical Chamber in Gdansk granted an official approval for all of the study procedures.
A typical training microcycle during the competitive season included 4 training sessions and 1 league game each week. Moreover, during the ACS, the players who played less than a half of the league match performed the compensation training session involving small-sided games. The training structure throughout spring preparations varied and was partitioned into 3 phases: a general preparation period (3 weeks; 15 training sessions), a directed preparation period (2 weeks; 15 training sessions and matches), and a soccer-specific preparation period (4 weeks; 18 training sessions and matches). A 12-week spring season with a regular structure of weekly training microcycles was followed by a short (1 week) transitional period. The APP was shorter (3 weeks) than the SPP and involved general (1 week; 7 training sessions and matches), directed (1 week; 6 training sessions and matches), and soccer-specific (1 week; 6 training sessions and matches) preparation periods. The 15-week ACS training structure was identical to that of the SCS.
The dominance of aerobic intensity exercises was observed in all of the training periods, and the proportions of the applied loads were very similar in the SPP, SCS, and ACS (Tables 3 and 4). The large number of matches played in a short period of time during the APP resulted in differences in intensity levels (45% of the exercises were performed with a mixed intensity).
There were 112 training sessions and matches during the spring season (SPP and SCS) and 92 in the autumn season (APP and ACS). During the spring season, the players performed more general warm-ups than in the autumn season, but the durations of the general and specific warm-ups were similar (666 and 593 minutes, respectively). Despite the greater number of training sessions in the spring season, the subjects performed less continuous running in spring than in autumn (627 and 659 minutes, respectively). The spring season involved more small-sided games than the autumn season, but the durations of league, cup, and control matches were comparable (1,995 and 2,050 minutes, respectively). The durations of the applied training drills are presented in Figure 1. A typical weekly training load during a competitive season is presented in Table 5. Its structure was constant during both the SCS and ACS but differed from the structure of the weekly training loads during the SPP and APP.
All the tests were conducted before the SPP and after the ACS. Additionally, the aerobic capacity was measured in the middle of the season (before the APP). With the exception of the sprint and shuttle run tests, which took place on an indoor track to avoid the influence of wind, all testing procedures were conducted in the morning in a laboratory with a temperature of approximately 20–21° C. Individuals were asked to abstain from drinking caffeinated beverages for 3 hours before the test. All the tests were performed within 5 days (day 1—V[Combining Dot Above]O2max , day 3—Wingate test, day 5—sprint and shuttle run tests).
Maximum Oxygen Consumption (V[Combining Dot Above]O2max)
All the subjects performed a continuous graded exercise test on an electronically braked cycle ergometer (Oxycon Pro; Erich JAEGER GmbH, Hoechberg, Germany) to determine the maximal oxygen uptake. The participants began pedaling with a relative load of 1.5 W·kg−1 for 5 minutes. After this phase, the workload increased by 20·W every minute until exhaustion. The highest value of oxygen uptake maintained for 15 seconds was determined as the V[Combining Dot Above]O2max. The AT values were obtained using the V-slope method (5).
Before the test, all of the players completed a warm-up involving 10 minutes of pedaling followed by stretching exercises. A 30-second all-out effort test was performed on a cycle ergometer (Monark Ergomedic 894 E, Varberg, Sweden) with a relative load corresponding to 7.5% of the subject's body mass. All the participants were verbally encouraged to pedal as fast as possible. The peak power recorded is presented as a relative value (watts per kilogram).
All the sprint tests were conducted in an indoor facility on a running track with ambient temperature between 19 and 20° C. Before the test, the subjects completed a 20-minute warm-up involving a minimum of three 30-m sprints. The times were recorded by photocells (TAG Heuer, La Chaux-de-Fonds, Switzerland) positioned at the starting and finishing line of each distance. The participants started from a standing position with their front leg on the starting line, and they performed 2 maximal attempts with a 4-minute rest period between the trials. The best (the shortest) time of the 2 sprints at distances of 5 and 30 m was chosen for subsequent analysis.
Shuttle Run of 150 m
A shuttle run (Figure 2) was performed after the sprint tests in the same facility. The time of the run is a useful indicator of speed endurance.
All of the data sets were assessed using the Shapiro-Wilk test for normality. Levene's test was used to check the homogeneity of the variances. The reliability of the data was determined using the Cronbach's alpha reliability coefficients. A repeated-measures analysis of variance was used for the dependent variables (V[Combining Dot Above]O2max). A t-test for the dependent variables was used to determine any significant changes in the times of the 5-m sprint, the 30-m sprint, the 150-m shuttle run, and the relative peak power. Statistical significance was set at p < 0.05. All of the statistical analyses were performed using STATISTICA 9.0 (Statsoft, Cracow, Poland).
The relative peak power at the end of the season (11.96 ± 0.75 W·kg−1) was significantly higher (p < 0.000) than before the preparation period (11.24 ± 0.78 W·kg−1). A significant improvement in the 150-m shuttle run time was also observed at the end of the season. Interestingly, the 5-m sprint time at the end of the autumn season was significantly (p < 0.04) worse than before the SPP. No significant changes in the 30-m sprint time were noted (Table 6). The HR/AT values and the percentage of the HRmax at the anaerobic threshold did not reach statistical significance at any point during the season. No significant (p < 0.05) changes were observed in the V[Combining Dot Above]O2max during the season (Figure 3). In the FSP, the level of aerobic capacity after the SCS was significantly higher than in SPP (p < 0.04) and end of the season (EOS) (p < 0.00). In the SP, the level of V[Combining Dot Above]O2max after the season was significantly higher than in SPP (p < 0.02) and middle of the season (MID) (p < 0.00). Moreover, the level of SP aerobic capacity in the middle of the season was significantly lower (p < 0.003) when compared with FSP (Table 7). No changes in speed and power were observed.
The purpose of this study was to analyze the changes in the aerobic capacity of young soccer players as a result of imposed training loads during 1 soccer season. Additionally, measurements of anaerobic capacity were taken at the beginning and at the end of the season to assess the influence of the suggested training structure on all components of physical fitness. The training load was recorded using the method based on dividing the intensity of exercise into 4 zones. Moreover, the training drills were categorized into 15 groups to enable a detailed assessment of the training content. The weekly training structure during both the SCS and ACS was constant and oriented toward improving or maintaining the aerobic capacity on an elite level. No significant changes in the V[Combining Dot Above]O2max were observed during the season (Figure 3). The levels of power and speed endurance increased significantly with the coincident decrements of the 5-m sprint time.
Detailed planning of an entire season's training structure rarely occurs in soccer. Usually, coaches only prepare general training programs and do not specify the proportion of time spent in training drills. Furthermore, there is a paucity of studies in the literature that accurately evaluate the type and quantity of imposed training loads. There are at least 2 reasons for this problem. First, some coaches are reluctant to share their work and achievements. Second, a comprehensive analysis of applied training loads requires a large amount of systematic and meticulous work. The methods used in this study to assess the training load and content allow coaches to evaluate each exercise easily.
The subjects' training loads were sufficient to maintain an aerobic capacity at a level adequate for elite soccer players. The lowest value of the maximal oxygen uptake (57.85 ± 7.94 ml·kg−1·min−1) was observed at the beginning of the preseason preparations. Similar results were presented by Caldwell and Peters (7), who found that the V[Combining Dot Above]O2max of their subjects at the beginning of the preseason period (56 ml·kg−1·min−1) was significantly lower than at other times during the season.
In the middle of the season, the participants in our study reached the maximal oxygen uptake level of 60 ml·kg−1·min−1, which is suggested by Reilly et al. (32) as the minimum level for elite soccer players. The AT levels expressed as a percentage of the V[Combining Dot Above]O2max before the preseason period, in the middle of the season, and at the end of the season were 77.36, 73.66, and 74.36%, respectively. Casajus (8) found that the AT of Spanish professional soccer players was between 77 and 79% of the V[Combining Dot Above]O2max. Numerous observations have shown similar aerobic capacity levels in young soccer players. Vanderford et al. (35) noted a V[Combining Dot Above]O2max for 15–16 years old soccer players of 56.2 ml·kg−1·min−1 and an AT of 61.2% of the V[Combining Dot Above]O2max. Comparable values of the maximal oxygen uptake were observed by Rahkila and Luthanen (56 ml·kg−1·min−1) (30). However, the AT was 85.7% of the V[Combining Dot Above]O2max. A superior level of aerobic capacity in young soccer players was noted by McMillan et al. (25), who found that after 10 weeks of high-intensity aerobic interval training, the V[Combining Dot Above]O2max increased from 63.4 to 69.8 ml·kg−1·min−1.
Merely maintaining a high level of aerobic capacity, as was performed by the subjects in our study, is not enough. At this age, players are highly capable of improving their endurance through the modification of the training structure. However, an increase in low-intensity exercise may decrease the level of power and speed of a player. Therefore, the percentage of each training drill should be carefully considered.
At the beginning of the SPP, the V[Combining Dot Above]O2max level in FSP and SP groups was similar. The aerobic fitness of the FSP increased after the SCS and significantly decreased toward the end of the season. These findings are in line with previous researches (7,26). The decrease in FSP group at the end of the season may have been caused by the fatigue or game overload. Moreover, to enable full recovery of the most exploited players, coaches reduced their training load. This may have affected the decrease in aerobic fitness (7). The highest level of V[Combining Dot Above]O2max in SPs was noted at the end of the season. In contrast to FSP, the analysis of the individual changes suggest that particular improvement was observed between MID and EOS. This may be an effect of compensation training sessions applied to the SPs. These training sessions were performed only during the ACS and usually contained different forms of small-sided games.
A comprehensive training program should improve a player’s ability to exert himself both aerobically and anaerobically. The Wingate test is a widely accepted measure of power output and anaerobic capacity (22). The relative peak power of the analyzed players increased during the soccer season to 11.96 W·kg−1, which is an average value according to the classification proposed by Zupan et al. (37). These results are comparable with the findings of Al-Hazzaa et al. (2), who reported that the mean relative peak power of adult Saudi elite soccer players was 11.88 ± 1.3 W·kg−1.
Although the Wingate test is typically used to determine anaerobic capacity, it does not reflect the performance requirements of those sports involving intermittent high-intensity activities (13). The shuttle run test performed in this study includes 9 turns and 150 m of running and appears to yield an accurate evaluation of the speed endurance of soccer players. Because of its simplicity, this test may be widely used by not only soccer coaches but also by coaches of other sports as well. The significant improvements in the shuttle run test results suggest that the exercises selected in this study for the purpose of developing speed endurance and coordination were appropriate.
In soccer, speed is as important as aerobic capacity. During a 90-minute soccer match, speed abilities are expressed in sprint bouts. Although sprint bouts occur rarely and do not significantly correlate with the level of a player's technical skills (28), they could have a crucial influence on the final results of a game. According to Bradley et al. (6), the English Premier League outfield players sprint between 152 and 346 m (speed > 25.1 km·h−1), depending on their playing position, which corresponds to 1.5–3% of the total distance covered in a game. In our study, no significant improvement in speed was observed during the course of the season. Moreover, the 5-m sprint time decreased significantly to 1.13 seconds after the season. This decrease is greater than that achieved by young Spanish soccer players (18) and by professional players from Germany (23). The 30-m sprint times in this study were comparable to those achieved by young soccer players based on the research of Chamari et al. (9), but they were worse than the elite soccer players from other countries (12,23,36). In our opinion, the worsening of the 5-m sprint time over the course of the season might be the result of deterioration in running technique rather than of the players' ability to use high-energy phosphates because a coincident statistically significant improvement in relative peak power based on the Wingate test was also observed.
Dividing the training intensity into 4 categories (AP, MAAP, ALP, and ANLP) allows for a detailed description of training loads. The aerobic performance category includes activities such as low-intensity continuous running, technical-tactical exercises, and active recovery. The league, cup, and control games were allocated to the MAAP category, and the small-sided games and high-intensity training (above the AT) were allocated to the ANLP category. The ANLP category involved drills such as speed and strength training.
Impellizzeri et al. (21) gave examples of the weekly training loads of an Italian professional soccer team. During weeks with 2 matches (Wednesday and Sunday), the most intensive session was conducted on Friday, whereas the high-impact training sessions were performed on Wednesday and Thursday during weeks with 1 match (Sunday). In both examples, there was no training session on Monday. Unfortunately, the authors did not demonstrate the influence of this type of training structure on the players' physical capacity. In our study, the most intensive training session of each week was conducted on Thursdays, when the small-sided games were played (usually 4 vs. 4, 4–6 games, 3–5 minutes each). The main goal of these games was for the players to reach an intensity corresponding to the anaerobic threshold or higher. Thus, the small-sided games were classified as anaerobic lactate drills. On Friday (1 or 2 days before each match), a short tactical game and short speed exercises were usually performed. When analyzing the duration of the applied training drills, a lack of individual and group technical exercises was observed (drill 11). Although the players were developing their technical skills together with tactical competencies (drill 12), it is reasonable for young soccer players to practice their individual techniques in isolated forms as well.
Success in soccer is dependent upon many factors. Applied training loads have a crucial influence on the players' physical capacity. In this study, we assumed that the suggested training structure would be conducive to the maintenance or improvement of the players' aerobic capacity. The V[Combining Dot Above]O2max values obtained during the season showed that the aerobic capacity level was sustained and that this goal was therefore partially attained. For further improvement of V[Combining Dot Above]O2max, a quantitative modification of the high-intensity aerobic exercises seems to be required. However, the proportion of time spent on training drills should be precisely established to avoid decreasing the players' power and speed. Although the training structure presented here cannot be considered a recommended pattern according to which young soccer players should be trained, it might provide helpful guidelines for soccer coaches.
This study results suggest that 1 high-intensity training session a week, without playing the league game, is insufficient to improve the aerobic capacity in young soccer players. However, compensation training sessions applied to SPs may have a positive influence on their aerobic fitness.
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