Many activities in soccer, such as sprinting, changing pace and direction, tackling, jumping, and kicking, are forceful and explosive and often constitute the most crucial moments of the game. Training to improve maximum strength usually relates to an improvement in power as strength provides a platform upon which power is expressed (24). Therefore, it is beneficial for a soccer player to have a high level of muscular strength. Increased leg strength may also prevent injuries by increasing tendon and ligament mobility and the cross-sectional area of the muscles (5).
Strength training for improving muscle performance is now an integral part of the professional soccer player's overall fitness program. A gain in muscle mass induced by specific leg muscle training can offer a significant advantage to elite soccer players by improving acceleration, speed and vertical jump height (30). However, soccer will never be a sport of pure, brute force, because the ability to produce force during a soccer match is not only dependent on the strength of the muscles involved in movements but also on coordination and timing. Complex movement patterns that usually require rapid and large changes in speed, and direction of movement also necessitate a high degree of agility. Agility is defined as the ability to change speed and direction rapidly and without loss of balance and is dependent on muscle strength, speed, balance and skill (8).
The benefit of strength conditioning in soccer training programs remains questionable as studies on the relationship between muscle strength and kick performance and the changes induced by training have demonstrated contrasting results (1,11,23,28). Contrasting evidence also exists on the relationship between strength qualities and sprinting performance (19,32). Furthermore, health and safety concerns are of obvious concern when younger players embark on strength training programs. However, it is uncertain whether simply playing soccer or undertaking traditional technical and tactical training can provide enough of a stimulus to sufficiently increase soccer-specific physical performance.
Standard strength and power tests may be poor predictors of sprinting performance and a better assessment could be based on more specific tests (19). The present study aimed at assessing the effectiveness of strength training on running speed and agility in young, professional soccer players, using field tests designed to simulate the specific demands of soccer. In young adult players, the production of testosterone enhances the development of force, and so physical training is most efficient at the end of puberty (21). Consequently, it can be assumed that the force component contribution can contribute to an increase in overall speed capabilities and, in particular, the specific combination of speed, agility and movement coordination involved in skills such as sprinting, turning and changing pace and/or direction.
Experimental Approach to the Problem
To test our hypothesis, we designed a longitudinal study to compare the performance of 3 groups of subjects before and after a short specific training program. The program involved technical training (the first group, denoted as “Re” for “reference”), or associated with additional movement agility and coordination exercises (the second group, denoted as “Co” for “coordination”). The latter program was additionally completed with weight lifting in the third group (denoted as “Sq” for “squats”). Exercises that require free weights translate to performance better than machines because they reflect the functional strength of the limb movements involved in soccer (25). Agility tests should be used in conjunction with single-sprint tests to obtain a thorough indication of player's speed capacity (22).
Twenty-six elite, male soccer players belonging to the training centre of a French professional club volunteered to participate in the study. All were of similar body mass and height (anthropometric characteristics are shown in Table 1). The players had all received similar coaching in various formats over the previous four years. The daily workload was monitored and was similar for all subjects. Full details of the study were provided to the players prior to their participation in the study, and informed consent was obtained in accordance with the ethical standards of the Helsinki Declaration (1975). We also obtained written consent from the parents of under-18 players.
All players were randomly assigned to 1 of the 3 groups and performed a 3-week, group-specific training program carried out during the final 3 weeks of preseason training (Table 2). We chose not to take the players' various playing positions into account, since there was no difference in body mass and height between the forwards and the defenders (12). Each group performed the training program once a day from Monday to Friday, followed by a rest day (Saturday) and a match day (Sunday).
In the reference group (Re, 9 subjects), the players conducted their usual, individual, “on-the-ball” technical exercises in order to develop ball skills. The 20-minute exercise program consisted of running and juggling with the ball with the legs, trunk and head (over distances of between 200 and 300 meters) and performing passes in various directions.
The subjects in the coordination group (Co, 8 subjects) performed a circuit designed to improve agility and movement coordination and foot supports (Figure 1). The players sprinted for 30 meters and then, for the next 12 minutes, ran around in various directions with changes in foot supports with and without the ball. The circuit included hurdles, hoops, boards, and Constrifoot equipment (Constrifoot SA, Hesingue, France) to provide a range of obstacles and difficulties. After completing the agility/coordination circuit, the players performed technical exercises for 8 minutes. Again, the program lasted 20 minutes total.
Subjects belonging to the “squats” group (Sq, 9 subjects) performed 3 sets of 3 squat repetitions at 90% of their respective, previously measured individual maximum values before performing the 12-minute coordination circuit described above. Each subject's “1 maximum repetition” strength was measured at the start of the program with a concentric squat machine (Plyopower Technologies, Lismore, Australia). After 3 warm-up lifts, the subject's near-maximal resistance was assessed. The resistance was then gradually increased until the subject could lift the resistance only once (1 maximum repetition). The foot position and bar height for the concentric squat were set so that the subject began the exercise with a knee angle of 90°. The subject had a 3-minute resting period between each attempt. This 15-minute exercise program helps strengthen the leg muscles and as with other training programs for developing maximal strength, it includes a low number of repetitions with a high load and high velocity movements (5).
We then assessed the players' for changes in running speed and agility/movement coordination, reactive speed and acceleration, using the following field-based tests:
- A single straight sprint test over 7.32 meters (ie, the breadth of the goal mouth). This involved covering the distance between two goal posts as quickly as possible.
- A single straight sprint test of 10 meters.
- A shuttle test: out and back over the course of 11 meters, plus a 16.50-meter sprint (the length of the goalkeeper's surface) with 2 changes of direction.
- A timed circuit developed by Bangsbo (4), a race, changes in foot supports, dribbling the ball with changes in direction and over obstacles and, finally, striking the ball into a goal (over a total distance of 31.10 meters). The stopwatch was started when the player's feet left the starting area and was stopped when the ball had been struck (Figure 2).
We measured the time to completion of each test with Ergotimer photoelectric cells (JFB Medical, Bessenay, France). Each test was performed before initiation of group-specific training (P0) and then again at the end of each week of the program (denoted as P1, P2, P3), to assess potential performance changes over time. Each player was tested on 4 different occasions with at least 5 days between tests. All tests were performed at a similar time in the morning.
The reliability of the tests was defined by calculating the intraclass correlation coefficient, or ICC, according to the formula previously defined by Fleiss (13), the standard error of measurement (SEM), which is defined as the square root of the within-subjects mean square error (27) and the coefficient of variation (CV), as described by Bland (6). The coefficients were then stratified according to the values chosen by Currier (10): 0.90-0.99 = high reliability; 0.80-0.89 = good reliability; 0.70-0.79 = fair reliability; and <0.69 = poor reliability.
Data were tested with analysis of variance (ANOVA). When F-values were significant, Student t-tests for unpaired data (intergroup comparisons) or paired data (intragroup comparisons) were then conducted. Subscripts under F and t-values indicate degrees of freedom. To assess the influence of the duration of the training program and to discard a possible effect of intersubject variability in performance, each player served as his own control. The significance level was set to 0.05. Values are given as means, with their standard deviations.
The test-retest reliability of the parameters describing the players' running speed and coordination are shown in Table 3. The ICC and CV values showed good reliability in most tests, with high reliability for the 10-meter speed test. Our results show that performance in the 7.32-meter short sprint test differed according to the test session (F3.92 = 34.38; P < 0.001) and was significantly greater (P < 0.001) for P2 when compared with P0 (t25 = 7.01), P1 (t25 = 7.60), and P3 (t25 = 8.84). This increase in speed reaction was the same (0.21 seconds) in all 3 groups of players. We did not observe any significant interaction (F6.92 = 0.36; P = 0.903) between groups and the test session. There was no intergroup effect in the test of 10-metre speed (F2.92 = 2.14; P = 0.124). Similarly, the training programs undertaken did not modify this parameter (F3.92 = 2.00; P = 0.120).
Figure 3 compares time to competition between groups for the shuttle test. A significant reduction in performance in the shuttle test was observed in the Sq group (F3.92 = 10.61; P < 0.001). The time increase for the Sq group was 0.27 seconds (t66 = 3.93; P < 0.001) when compared with the Co group and 0.17 seconds (t70 = 2.76; P = 0.008) when compared to the Re group + Re/Co: (t66 = 2.75; P = 0.008). There was neither a significant effect of the test session (F3.92 = 0.84; P = 0.477) nor a group-session interaction (F6.92 = 0.27; P = 0.949).
Figure 4 presents the results for the timed circuit test, for which there were significant effects of group (F2.92 = 4.81; P = 0.010) and test session (F3.92 = 9.53; P < 0.001). This group effect is mainly explained by the difference between the Re and Co groups (t66 = 2.91; P = 0.005) observed before the start of the training program (P0). The performance of all 3 groups significantly increased after the second week of training (P2) and significantly increased after the second week of training when compared to P0 (P1: t25 = 7.01; P < 0.001, P2: t25 = 6.37; P < 0.001, P3: t25 = 6.03; P < 0.001), revealing a positive, long-term effect of training on the performance. This response reached 0.53, 0.84, and 1.16 seconds for the Sq, Co, and Re groups, respectively. There was no significant group-test session interaction (F6.92 = 0.39; P = 0.881).
In the present study, we tested the effects in young adult professional soccer players of a specific leg strength conditioning program on field-based sprinting and agility performance. Most previous studies have dealt with adult players and only focused on 1 or 2 soccer-related activities such as single sprint speed, vertical jumping and acceleration capacities (18,31). Tests for evaluating strength progression in soccer players often use protocols that do not always reflect the sports specific movements actually involved in soccer and may under-estimate training progress where movement patterns differ from those performed during training (2). Similarly, components of physical performance such as maximal running speed, acceleration and agility are independent variables involving different contributing physiological and biomechanical factors and should involve specific testing when working with elite soccer players (22). In the present study, specific soccer-related performance was tested in terms of speed reaction and velocity and acceleration in the short sprint tests whilst agility was measured through a complex, timed test circuit which takes into account many of the movements involved in soccer play. The different field-based tests of performance performed in the present study displayed good or high reliability, and can thus be used with confidence in future assessments of soccer specific performance.
A soccer match involves repeated short sprints, changes in pace and direction with varying foot supports, backwards or sideways runs, and vertical jumps. It has been suggested that an increase in the force of muscular contractions may improve acceleration and speed in these critical physical skills (14). Our results generally indicate that a short period of specific leg strength, agility and technical training undertaken by young adult professional soccer players did not significantly improve performance in field-based short sprint tests of acceleration, speed reaction and in a sprint shuttle test with changes of direction. The gain brought about by the strength conditioning program in a test of agility was highly comparable to that obtained from specific training programs based on improving agility and movement coordination and/or individual game technique.
The lack of improvement demonstrated in the sprint tests over short distances by the strength conditioning group may be explained by the results of a previous study which concluded that maximum strength was more related to maximum sprinting speed than starting ability (32). Furthermore, experienced resistance-trained athletes may have a smaller window for adaptation for maximal strength development which may have been the case in the present study. Improving the power to weight ratio as well as plyometric training involving countermovement and loaded jump-squat training may be more effective for enhancing sport speed in elite players (9). The lack of progress in the sprint tests by the group performing additional muscle strengthening work could also be explained by an insufficient test duration, which was limited by the working practices of the participating club to 3 weeks. The effectiveness of strength training work for young adult subjects mainly depends on their production of testosterone, which enhances rapid muscle force by acting on fast fibers. This increases explosive force, provided that the duration of training ranges from 8 to 10 months or more when a highly significant level of performance is needed (7). Neural adaptations also play a major role in developing force though it is suggested such adaptations are limited to a starting period of 6-8 weeks of strength training (15). However, gains in performance in soccer players can be noticed within one week of starting a strength conditioning program (20) and the authors nevertheless felt that the short experimental period would be long enough to examine whether strength conditioning could improve short sprint performance.
The muscle reinforcement program diminished performance in the shuttle test involving sprints with changes of direction. This disparity is supported by the work of Young et al. (33), who reported an inconsistent relationship between leg muscle strength and change-of-direction speed. The present findings may be explained by the findings of Aagaard et al. (1), who described a force profit transfer and muscle power where a soccer player's functional efficiency is due more to the contribution of the abdominal muscles (which transfer the force) than to the quadriceps (strengthened in the squat exercise). Our results again emphasize that an agility training program including coordination and foot support control exercises can be more beneficial for short speed and changes in direction, since the “Co” group's performance was better, albeit non significantly, than that of the other two groups.
Soccer players tend to demonstrate their greatest improvement in performance in the 15- to 17-year age bands and the utilization of specific soccer-related fitness drills, as in the present study, is recommended to allow further progression in young elite players and prevent stagnation in the development of physical performance after 18 years of age (29). In the standardized, timed circuit (4), we observed an improvement in performance (ranged between 0.53 and 1.16 seconds) after 3 weeks of training, regardless of the type of training program undertaken. Increased force development is closely related to balance (15), which may have improved in all groups through the training of agility, movement coordination and/or technical skills. However, the improvement observed in the agility test due to added strength training of the lower limb muscles did not offer a significantly greater advantage over that obtained through agility and coordination training. In a previous study on advanced soccer players, no significant link was found between the level of power output and movement coordination (26). It appears that operational neuromuscular coordination may be more important than muscular reinforcement for improving motor task performance in soccer players. A strength-based training program does not help players anticipate the tactics and actions of opponents or help coordination and timing of movement skills. If match specific agility tasks are built into drills using the ball and also during sprint and speed training, neuromuscular coordination can be improved and agility enhanced (8). However, effective coordination is related to an alternation of the various muscle contraction modes (17). It has been reported that an eight week eccentric strength training program enhanced agility performance through greater force development during changes in direction due to improved eccentric breaking and concentric push off phase capacity (3). A future study on the present players may examine the impact of other forms of strength training such as jump squats (using both eccentric and concentric phases of contraction and over a longer duration) on running speed and agility performance in place of the present weight conditioning program. This form of training has recently been shown to elicit significant strength gains and improvements in physical performance (16).
The results of this study on in professional soccer trainees indicate that a short period of strength training did not significantly enhance performance more than an identical period of agility and coordination training in field tests of soccer specific sprinting and agility. From these results, it would be imprudent to suggest that strength conditioning practices should be replaced by agility and coordination training and more football related activities in young professional adult players. Our study has shown however, that soccer-specific running and agility drills may be an effective alternative to a basic strength-conditioning program. The drill used in the present report provided a similar fitness-related stimulus to that obtained from leg strength conditioning. However, such drills also have the added advantage of developing technical skills and may help avoid overloading the developing musculoskeletal system. The agility training practices employed in the present study could be used effectively to compliment the traditional pre-season strength/power training mesocycle. Similarly, microcycles aimed at helping transition from mesocycle to mesocycle are important in avoiding staleness and aiding recovery. Agility and coordination type training circuits similar to those undertaken in the present study may be beneficial in this period by providing variety and a sufficient stimulus to maintain physical fitness. In the same way, strength training is the traditional domain of the preseason in soccer and if stopped after preseason, a reversal of the adaptation process occurs and the player can lose any previous gains. Therefore, the implementation of speed and agility exercises as used in the present study may avoid reversibility and maintain fitness gains obtained through a preseason strength program.
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