Players in competitive soccer require high levels of power, speed, and agility to perform explosive movements such as heading, shooting, sprinting, and dribbling (36). A sprint bout occurs approximately every 90 seconds in competition, each lasting an average of 2 to 4 seconds (33), whereas sprinting constitutes 1%-1% of the total distance covered during a competition (33). On the other hand, players have to run approximately 10 to 12 km during a game depending on their playing positions (32), and, thus, approximately 98% of the total energy is derived from aerobic metabolism (1). Therefore, previous studies have mainly focused on the improvement of soccer players' aerobic endurance by using different training methods such as small-sided games (20), high-intensity interval running (10), and specific dribbling training (26). However, the most decisive events during competition are represented by the aforementioned explosive movements (19). In this regard, it is important to improve soccer players' power capacity while maintaining an appropriate level of aerobic endurance. Nevertheless, previous study about muscular strength and power training in soccer players is limited to adults (27), and no information is available in the literature concerning young soccer players. In addition, it is of great concern to soccer coaches that such a training program may induce negative effects on aerobic endurance, but these studies only examined the training effect on explosive performance such as sprint and jump, and no information of its effects on aerobic endurance was reported. Therefore, studying the effect of muscular strength and power training among young soccer players is needed to fill in the research gap. In addition, both aerobic endurance and explosive movements should be measured in the study to examine the training effects on power and endurance capacities.
Strength is defined as the maximal force a muscle can generate, usually during slow movement velocity (36). Power (or explosive strength) is the product of force and movement velocity, and it refers to the ability of the neuromuscular system to produce the greatest possible impulse in a short time period (36). It is believed that by increasing muscular contraction force at high speed, explosive performance can be improved (3). This improvement can be achieved by explosive strength training, which significantly increases the neural activation and adaptation of the trained muscles (16). In addition, it can probably increase the number and firing frequency of the active motor unit of the muscle trained or change the recruitment pattern of the motor units in the fast-twitch muscle fibers (16). Moreover, explosive strength training has been reported to improve running economy (i.e., reduced energy cost at the same work intensity) (29) as a result of improved mechanical efficiency (37) and muscle power (29). In addition, combining strength training and power training improves explosive performance and power-related skills to a greater extent than any of the 2 training modalities alone (11). The consequence of any eventual improvement of strength and power in soccer players can help them improve the performance of their short-lasting efforts during a game, most likely contributing to the general soccer performance.
Although strength training appears beneficial for soccer players (38), not every soccer team is able to include it in their annual training plan. However, because of the importance of the explosive efforts in the general outcome of a soccer game, it is believed that strength and power training are necessary for soccer players to make them physically well prepared for competition. In this context, even if the traditional indoor strength training center is not available, some portable and relatively inexpensive training equipment can provide positive training effects. However, to our knowledge, information regarding the effects of on-field combined strength and power training (CSPT) using portable training equipment is not available in the literature. Such study provides scientific rationale for soccer coaches and strength and conditioning specialists to include a strength training program by using the portable training equipment. Consequently, a feasible and cost-effective on-field strength training program could be available for most teams.
Therefore, the purpose of the present study is to examine the effects of a 12-week on-field CSPT on physical performance among young soccer players in the preseason phase. Because it has been shown that strength and power training is insufficient to decrease the mitochondrial density and does not harm the aerobic performance (15), we hypothesize that a 12-week on-field CSPT can improve the players' explosive performances and reduce running cost (RC) with no concomitant interference on aerobic capacity.
Experimental Approach to the Problem
To examine the effects of a 12-week preseason on-field CSPT on players' physical performance, all the players participated in the pretest to measure their baseline performances. After that, the experimental group (EG) underwent a 12-week strength and power training together with their normal soccer training, whereas the control group (CG) only performed soccer training. The CSPT was conducted twice a week in the soccer pitch, and each lasted for 1 hour. At the end of the study, all the players participated in the post-test to examine the training effects of the 12-week CSPT.
In the present study, we selected test items that have been reported to have high discriminating power among young soccer players and are related to match performance. In this context, it has been shown that maximal vertical jump was the most discriminating variable for young soccer players (13). In addition, Reilly et al. (34) compared elite and subelite soccer players aged 15 to 16 years and found that sprint time was the most discriminating measurement. Specifically, 30 m sprint (with 10 m lap time) has been suggested as standard sprint test for soccer players (36). Moreover, shooting is a very important skill in soccer, and one of the ways to score that leads to team and individual success, but to our knowledge, no study has been conducted in young soccer players to determine the importance of that ability. Finally, aerobic capacity has been reported to distinguish elite and subelite soccer players aged 12 to 16 years (34). With the abovementioned reasons, in the present study, we assessed players' lower-body explosive power (maximal vertical jump and ball-shooting), running speed (30 m sprint with 10 m lap time), intermittent aerobic performance (Yo-Yo intermittent endurance run [YYIER]), and continuous aerobic capacity (laboratory O2max).
At the beginning of the study, 62 young male Chinese soccer players participated in the study. These players were members of a regional U-14 representative teams competing at the highest level of competition for their age category in Hong Kong. The season lasts for 28 weeks, during which the formal match was played once a week with 11-a-side on a pitch of regular size (approximately 50 × 100 m) and lasted for 70 minutes (2 × 35 min with 5 min rest between halves). During the preseason, they performed soccer training twice a week, with each session lasting for approximately 2 hours. Each training session was generally composed of a 15 minute warm-up, 30 minute technical training, 30 minute tactical training, 40 minute simulated competition, and 5 minute cool down. None of the players had any background in regular strength and power training. They were divided into 2 groups matched on body mass, height, and age at the pretest: EG (n = 31) and CG (n = 31). During the study, 3 players from the EG and 8 players from the CG quit because of injury or illness not connected with the intervention training regimes. Therefore, at the end of the study, the remaining players were 28 for the EG and 23 for the CG. Their body mass, height, body mass index, age, and soccer experience are shown in Table 1. The study was conducted according to the Declaration of Helsinki, and the protocol was fully approved by the clinical research ethics committee. All players and their parents were properly informed of the nature of the study without being informed of its detailed aims. Each of the players and their parents or guardians were informed of the experimental risks, and both signed an informed consent document before the investigation.
Each player visited the laboratory to perform the O2max test, and, after 5 days but no more than 7 days, they performed all the other tests in the soccer pitch wearing soccer sportswear in the following sequence: maximal vertical jump, ball-shooting, 30 m sprint, and the YYIER. Players had a 20 minute warm-up comprised of a slow jog and static and dynamic stretching before the test. There was a 30 minute rest between tests for full recovery (40). Water breaks and extra rest time were allowed upon the request of the players.
Maximal Vertical Jump
Players in the unshod condition started from an upright standing position and performed vertical countermovement jumps on a portable platform (Just Jump System, M-F Athletic Company, Cranston, RI, USA). Jump height was determined based on the flight time (40). Each player performed 3 jumps interspersed with 1 minute rests in between, and the best (highest) jump was used for the analysis (40).
Players performed a maximal velocity instep place kick to a stationary ball. A ball with a standard International Federation of Association Football size and inflation was kicked 4 m toward a target of 1 × 1 m in size (40). The players were asked to shoot the ball as hard as possible. Five shots were allowed for each player with 1 minute rests in between (40). Ball speed was measured by a radar gun (Sports Radar Gun SRA 3000, Precision Training Instrument, IL, USA) located 0.3 m near the stationary ball and pointed toward the target according to the instruction manual. The shot that hit the target and produced the highest ball speed was selected for analysis.
30 m Sprint
Players were asked to complete a 10 minute specific warm-up including several accelerations to decide which foot to set on the starting line for the sprint start. Players had to start from a standing position placing their forward foot just behind the starting line and their rear foot on the starting pedal. They performed a 30 m sprint with a stationary start (5), and the timing started as soon as the rear foot left the pedal. Speed was measured with an infrared photoelectronic cell (Speedtrap II Wireless Timing System, Brower Timing System, Draper, UT, USA) positioned at 10 m and 30 m from the starting line at a height of 1 m. There were 3 trials in total, and a 3 minute recovery was allowed between each trial (5). The best (fastest) 30 m sprinting time and the associated 10 m sprinting time were selected for analysis (5).
Yo-Yo Intermittent Endurance Run: Level One
Because soccer includes high-intensity, intermittent bouts of exercise, which stresses the anaerobic glycolysis metabolic pathway (9), the YYIER is close to the soccer exercise pattern. In this test, players had to perform a series of 20 m shuttle runs at a pace set by an audio metronome, with a standard rest interval between shuttles (5 s shuttle of 2.5 m by walking-active recovery) (4). The speed for the shuttles was progressively increased, and the test was terminated when the player was unable to maintain the required speed. The distance covered in the shuttles was recorded for analysis, but the distance covered during the rest interval was excluded (4).
The O2max protocol was previously used to test soccer players (6). Players ran on a 5.5% slope motorized treadmill (cos10198, h/p/cosmos, Nussdorf-Traunstein, Germany) for 4 minutes at 7 km·h−1, followed by a 1 km·h−1 increment every minute until exhaustion. Running cost was measured as the average O2 during the last 30 seconds at 7 km·h−1, and the corresponding heart rate (HRRC) during this period was also recorded (6). Cardiorespiratory variables were determined using a calibrated breath by breath system (MetaMax 3B, Cortex, Leipzig, Germany). Heart rate was determined every 5 seconds from a portable monitor (Polar, Kempele, Finland). Blood lactate was collected 3.5 minutes after the O2max test, with 25 μL samples of capillary blood withdrawn from the fingertip. Blood lactate concentration was subsequently measured using an enzymatic method (YSI 1500, Yellow Springs Instruments, Yellow Springs, OH, USA). The following criteria were met by all players when the O2max was tested: (a) a leveling off of O2 despite an increase in treadmill speed; (b) a respiratory gas exchange ratio greater than 1.1; and (c) blood lactate greater than 6 mmol·L−1. The O2max is determined as the average of the last 30 seconds of the test, whereas the maximal heart rate (HRmax) is the highest value attained at exhaustion (5).
On-Field Combined Strength and Power Training
One of the investigators, who is a Certified Strength and Conditioning Specialist of the National Strength and Conditioning Association (United States) designed and supervised the training program throughout the study. This type of strength and power training induces minor muscular hypertrophy (16) and does not interfere with the development of aerobic endurance (29). The on-field training equipment included medicine balls from 1 to 3 kg, commercial weight bags (Powerbag, UK) with a weight of 5, 7.5, 10, 15, 20, and 25 kg, and 30 cm height minihurdles. An initial load was selected to make all the players perform the determined repetitions with the correct movement pattern, that is, 10 repetitions for most exercises in the general adaptation phase, except for the sit up, which was 15 repetitions. The investigator monitored and ensured that the load was appropriate for every individual to prevent skeletal and muscular injuries. The loads were progressively increased every 2 weeks unless a player was unable to perform the movement correctly or unable to complete 1 set, and that player was then trained with the previous load. In the general adaptation phase (wk 1-4), circuit training was adopted in which 10 exercises were split into 2 circuits (Figure 1). The players moved to another exercise after 1 set, and this served as the rest interval, which was very short (<30 s). In the strength phase (wk 5-8), exercises with external weights were performed for 4 sets each with 6 repetitions to maximize strength gain (2), whereas exercises with body weight were performed with more repetitions (instead of 6) to reach the desired training intensity that strengthen the trained muscle groups. The players finished the exercises in a straight set (1 exercise after the other). In the power phase (wk 9-12), plyometric exercises, which consisted of the stretch-shortening cycle to stimulate the stretch reflex of the muscles to maximize muscle recruitment in the shortest period of time, were included (31).
Data were expressed as the mean ± SD. Independent samples t-tests were used to examine the differences between the 2 groups in the baseline measurements (pretest) (Table 2), and it was found that initial group differences existed. Therefore, the pretest values were considered as covariates to determine significant differences between the post-test adjusted means in the EG and those of the CG. The multivariate analysis of variance (MANOVA) was used to examine the differences between the 2 groups over the 2 tests: the test was the within-subject factor with 2 levels (pretest and post-test), and the group was the between-subject factor with 2 levels (EG and CG). Significant test effects in the same group were further analyzed using paired samples t-test, whereas significant group effects were analyzed using independent samples t-test. Pearson correlation was used to examine the relationship between variables. Significant level was defined as p < 0.05.
Effect size was also used in the present study to provide information on the magnitude of treatment effect (35). Rhea (35) recently proposed a new scale for determining the magnitude of effect sizes in strength training research. In this classification, Rhea (35) took the training status of the participants into consideration by separating them into 3 groups: untrained (consistent training less than 1 yr), recreationally trained (consistent training from 1-5 yr), and highly trained (consistent training of more than 5 yr). Because the players in this study had prior soccer experience of 3.7 to 4.2 years, the scale for “recreationally trained” was selected for interpretation: trivial (effect size <0.35), small (0.35-0.80), moderate (0.80-1.50), and large (>1.50). This is also in accordance with their weekly training volume of only 2 sessions a week.
The reliability of each test was assessed by intraclass correlations coefficient (ICC) and coefficient of variance (CV). The results show that these tests were highly repeatable: vertical jump (ICC = 0.97; CV = 4.0%; n = 51); ball-shooting speed (ICC = 0.99; CV = 3.5%; n = 51); and 30 m sprint (ICC = 0.97; CV = 2.1%; n = 51). Although the repeatability of YYIER and O2max tests cannot be calculated from the present study, previous studies have shown that the CV of incremental treadmill O2max was less than 5% (30); the CV of Yo-Yo intermittent exercises was 4.9% (23), and there was no significant difference between test-retest distance coverage (p > 0.05) (4).
The MANOVA showed significant training effects of the CSPT on EG when the pretest served as the covariate, F(10, 30) = 2.18, p < 0.05 (Table 2). There were significant differences between the EG and the CG in the vertical jump (F = 5.05, p < 0.05), ball-shooting (F = 7.14, p < 0.05), and YYIER (F = 4.13, p < 0.05) after a 12-week CSPT. The CSPT significantly increased the vertical jump height by 3.3 cm, (5.9%, p < 0.001), ball-shooting speed by 4 km·h−1 (5.2%, p < 0.001), improved the 10 m and 30 m sprint by 0.10 seconds (4.9%, p < 0.001) and 0.11 seconds (2.3%, p < 0.01), respectively, and improved the YYIER by 451 m (20%, p < 0.001). In addition, the CSPT reduced the RC at 7 km·h−1 by 2.21 ml·min·kg−1 (5.1%, p < 0.05) and reduced the HRmax by 2 beat.min−1 (1%, p < 0.01), whereas the HRRC, O2max, and the corresponding running time remained unchanged.
In terms of practical significance (Table 3), the 12-week CSPT had moderate overall effect (mean effect size = 0.85) on the 5 performance tests in the present study. Specifically, there was a moderate effect on the vertical jump, ball-shooting, 30 m sprint, and YYIER; small effect on the 10 m sprint, RC, and O2max; and trivial effect on the HRRC, HRmax, and running time (Table 3).
Significant correlations were found between power performances in the EG. The vertical jump height had moderate correlation with the ball-shooting speed (r = 0.61, p < 0.01) and 10 m (r = −0.70, p < 0.001) and 30 m (r = −0.76, p < 0.001) sprint times. The ball-shooting speed also had moderate correlation with 10 m (r = −0.53, p < 0.01) and 30 m (r = −0.65, p < 0.001) sprint times. In addition, the YYIER had significant correlations with 10 m (r = −0.47, p < 0.01) and 30 m (r = −0.43, p < 0.01) sprint times, ball-shooting speed (r = 0.51, p < 0.001), and vertical jump (r = 0.34, p < 0.05).
The primary results of the present study demonstrated that after the 12-week CSPT, players significantly improved their explosive performance (vertical jump, ball-shooting, and sprint), intermittent aerobic endurance (YYIER), and reduced RC. However, this training program did not significantly alter the players' O2max level and HRRC. Moreover, the CSPT had a moderate overall effect (mean effect size = 0.85) on the 5 performance tests performed in the present study.
Compared with the pretest, the CSPT significantly increased vertical jump height (5.9%, 3.3 cm, p < 0.001), which was in agreement with the previous studies on youth soccer players using explosive strength training (15,27). The improvement in the vertical jump height after the explosive strength or plyometric training of these studies were 7% or 2.8 cm (27) and 5.1% or approximately 2 cm (15), respectively. On the other hand, previous studies showed no improvement in the vertical jump after strength training when slow or normal contraction speed was used in the training (14,38). These results suggest that the speed of movement rather than the resistance or load is more important and positively affects the jump performance of young soccer players.
The CSPT significantly increased ball-shooting speed (5.2%, 4 km·h−1, p < 0.001), whereas soccer training did not yield a similar effect. Shooting is a multijoint activity that depends on various factors such as timing and transfer of energy between body segments involved in the shot (24). Nonetheless, the muscles are directly responsible for increasing the speed of the foot (24). Manolopoulos et al. (25) reported that after a 10-week combined strength/plyometric and kicking training in amateur adult players, ball-shooting speed significantly increased from 90 km·h−1 to 101 km·h−1 (∼12%, p < 0.05), and the strength of the vastus medialis of the swinging leg also increased significantly (30%, p < 0.05). In addition to the contribution of the improved strength and power, high movement speed in the CSPT group of the present study might also have improved ball-shooting performance. It has been reported that the increased linear velocity of the foot and ankle and angular velocity of all joints improves shooting performance (25). Moreover, there was a higher correlation between kicking performance and knee extension in high angular velocity (r = 0.90) among junior soccer players compared with that in low angular velocity (r = 0.61) (21).
The CSPT significantly decreased the 10 m sprint time (4.9%, 0.1 s, p < 0.001), and this corresponds to 0.9 km·h−1 improvement in speed or 0.49 m faster over 10 m. Likewise, the CSPT significantly decreased the 30 m sprint time (2.3%, 0.11 s, p < 0.01), corresponding to the 0.5 km·h−1 improvement in speed or 0.68 m faster over 30 m. These results agreed with those in a previous study that reported a significant improvement in the 30 m sprint (3.5%, 0.15 s, p < 0.05) after a 13-week combined strength and speed training in youth soccer players (22). Moreover, it has been reported that a short-distance sprint is highly dependent on the players' ability to generate muscular power (8), especially in the ankle, knee, and hip extensions (12). Therefore, in the present study, high pull, power clean, and weighted squat jump were supposed to have provided the greatest effect in sprint performance because these exercises consisted of simultaneous triple-extension of the ankle, knee, and hip joints. Moreover, sprint time was significantly correlated with vertical jump (10 m: r = −0.70, p < 0.01; and 30 m: r = −0.76, p < 0.001) and ball-shooting speed (10 m: r = −0.53, p < 0.01; and 30 m: r = −0.65, p < 0.001). These results suggest a possible transfer from the gain in the leg muscular power into the sprint performance, which has been reported previously by Gorostiaga et al. (15).
The CSPT significantly improved the YYIER performance (20%, 451 m, p < 0.001), and this corresponded to the approximately 3 minute improvement in the YYIER (from 17.5 min to 20.5 min). After the CSPT, no significant change in the O2max was observed, but the submaximal RC significantly decreased (from 43.16 to 40.95 mL·min·kg−1, p < 0.05), which could be attributed to the improved mechanical efficiency after the CSPT program (37). This agreed with Paavolainen et al. (29), who showed that the improvement of aerobic performance was caused by the decrease in RC. Moreover, it has been reported that trained athletes with similar O2max level could have up to 20% difference in RC (18), which means that aerobic performance may be affected not only by central factors related to O2max but also by peripheral factors such as muscle power (28). In agreement with this, improved muscle power after the CSPT was observed in the present study, and the YYIER had significant correlations with 10 m and 30 m sprint times, ball-shooting speed, and vertical jump. Therefore, we conclude that decreased RC and improved muscle power both improved the YYIER.
After the CSPT, no significant differences were observed in the O2max and corresponding running time compared with the pretest values. It has already been shown that strength and power training induces minor muscular hypertrophy; thus, it is insufficient to decrease the mitochondrial density, and the oxidative potential does not harm the aerobic performance (15). In addition, RC is significantly decreased after the CSPT because of the improved mechanical efficiency, but no change was found in the HRRC, indicating that the CSPT did not increase the cardiac output (i.e., HR × stroke volume). Nonetheless, in the present study, the fact that the YYIER was improved after the CSPT but the O2max and the associated running time remained unchanged appears to provide contradictory results. Although the O2max and YYIER were considered measurements of aerobic capacity, they imposed different physiologic stresses on the players. The duration of the O2max test was approximately 10.5 minutes, and the running speed at the end was 13 km/h. On the other hand, the durations of the YYIER were 17.5 minutes (including 4.8 min active rest) in the pretest and 20.5 minutes (including 5.8 min active rest) in the post-test at 12 km·h−1 and 14.4 km·h−1, respectively. Furthermore, the O2max test is a continuous exercise, but the YYIER is an intermittent exercise. It has been reported that continuous exercise has a higher rate of fat oxidation, whereas intermittent exercise has a higher rate of carbohydrate oxidation (7). In addition, there was a higher oxyhemoglobin concentration remaining in the working muscle groups in intermittent intense exercise at exhaustion compared with continuous intense exercise (7). These results suggest that a larger proportion of energy was derived from the anaerobic metabolic pathway during intermittent exercise. Therefore, the CSPT program used in the present study enhanced anaerobic and explosive performances and resulted in improved YYIER but not O2max level. Nevertheless, it is believed that the concurrent soccer training also plays a role in maintaining the YYIER and O2max level in both groups.
During the preseason period, coaches can use the combined muscular strength and power training to enhance young soccer players' explosive performances (e.g., maximal vertical jump, ball-shooting, 30 m sprint) and aerobic endurance (e.g., YYIER). It has been found that success in youth soccer is associated with good aerobic endurance (39). Furthermore, players with superior aerobic endurance perform better during the match, with increased distance coverage, greater involvement with the ball, similar technical performance despite a significantly higher exercise intensity, and increased number of sprints without altering technical skills (17).
The present study showed the effectiveness of using portable training equipment for the combined training. Its feasible and cost-effective use is warranted in young soccer players. Moreover, for young soccer players who do not have prior experience with strength and power training, a general adaptation phase is scheduled to ensure proper movement technique and safety. Furthermore, we suggest performing the combined training in the preseason period rather than in-season because the high training load induced by the combined training together with scheduled competitive matches may result in insufficient recovery/rest or overtraining. As a result, the applicability of CSPT, together with regular soccer training, could be performed during the preseason with no concomitant interference on aerobic capacity and endurance performance.
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