In sports such as soccer, in which numerous bursts of explosive activity are required, explosive strength determines high-level performance (19,20,31). Moreover, players need an important specific strength on soccer tasks (20). Subsequently, one of the most important aims of training programs should be to improve soccer-specific strength, which can be defined as the ability of a player to use muscle strength and power effectively and consistently in soccer-specific tasks during the soccer game (2).
Several authors have emphasized that kicking is one of the most important skills in soccer (3,23,36). Its effectiveness depends on various factors, such as maximal strength of the muscles involved, rate of force development, neuromuscular coordination, linear and angular velocities of ankle in the kicking leg, and the level of coordination between agonist and antagonists (7,10,16,17,19,20,37). Although some authors identified a relationship between the strength of the lower limbs and ball speed in both male and female players (16,17,19,20,36,37), there is a lack of information concerning the effects of a strength training program on the characteristics of a task such as kicking. This is especially the case in female players, for whom there are, to our knowledge, no related studies available in literature. In male players, the effects of strength training on kicking performance are controversial. Although some studies reported an increase in performance after the application of training programs involving explosive strength (8,15), maximal strength (32), isokinetic strength (11), or mixed technical and strength training (19,20), others found the opposite (1,34).
Training exercises based on a stretch-shortening cycle are an established technique for enhancing athletic performance. Ball speed is also affected by the stretch-shortening cycle characteristics of the muscles involved when kicking (20). However, only 1 study cited above used plyometric training (15). Plyometric exercises are composed of an eccentric loading immediately followed by a concentric contraction (28). These exercises induce neuromuscular adaptations to the stretch reflex. This reflex is initiated during the eccentric loading phase and can facilitate greater motor-unit recruitment during the subsequent concentric contraction (6,28).
Plyometric training has been advocated for sports that require explosiveness to increase skills such as vertical jumping ability. There are studies that demonstrated increases in this task after a plyometric program in female athletes (22,26) and in male soccer players, both in youth (9) and in adult age categories (27). However, the results obtained in female soccer players are scarce and controversial. Chimera et al. (5) carried out a 6-week plyometric program with female soccer players exercising twice a week. The plyometric group carried out plyometric exercises in addition to the normal soccer training program. The results did not demonstrate significant improvements in jumping ability for the plyometric group. On the other hand, Siegler et al. (30) developed a 10-week plyometric, intermittent, high-intensity anaerobic program with 34 female soccer players. They registered gains in speed and jumping ability in comparison with players of the control group, who continued with their regular aerobic soccer conditioning program. Several authors (12,21) noted that the discrepancy in the results of previous studies may be caused by different research protocols such as different durations of training methods, different status of the subjects, or different training loads.
Because of this lack of related studies and the presence of controversial results, the knowledge about the conditioning of female soccer players should be extended rather than merely implementing extensions of current male training programs. Therefore, the aim of the present study was to determine how explosive strength, kicking speed, and body composition are affected by a 12-week plyometric training program and a subsequent 5-week detraining period in elite female soccer players. More specifically, we hypothesized that a plyometric program implemented during the second part of the competitive season, which replaced the standard regular soccer conditioning program, would increase jumping ability and kicking speed in adult experienced female soccer players. We also hypothesized that these gains could be maintained by means of regular soccer training only.
Experimental Approach to the Problem
Twenty elite female soccer players participated in the study. They were divided into 2 groups according to the training program: control group (CG) and plyometric group (PG). The independent variable was the treatment effect of the plyometric 12-week training program that was focused on the strength of the lower limbs. The dependent variables were body mass, fat mass, muscle mass, countermovement jump (CMJ) height, drop jump (DJ) height, kicking speed with dominant leg, and kicking speed with nondominant leg. Each variable was measured on 4 occasions: 1 week before the start of the training program, after 6 weeks of training, 1 week after the end of the training period, and 5 weeks after the end of the program (detraining period). Two-way analysis of variance (ANOVA) with repeated measures was conducted to assess training-related effects.
Before the start of the intervention, the coach and the women were fully informed about the aims of the study. They provided written informed consent and completed a form giving personal, medical, and training details. All procedures described in this study were approved by the Ethical Committee of the University of Leon (Spain).
Twenty healthy female soccer players participated in the present study. They played for the same team in the Spanish National Women's First Division (Primera División Nacional Femenina). There were no group differences with regard to soccer-related experience. They were divided by playing position and then randomly allocated either to the CG or to the PG. All players involved in the study attended all the sessions. Following are descriptions of the 2 groups:
- - CG: 10 players (1 goalkeeper, 2 center backs, 2 full backs, 2 midfielders, 2 wide midfielders, and 1 forward) (age 23.0 ± 3.2 yr; weight 56.9 ± 7.4 kg; height 161.5 ± 5.4 cm). They had 5.4 ± 3.8 years of experience in soccer and trained on average 10 hours a week and played a match weekly.
- - PG: 10 players (1 goalkeeper, 2 center backs, 2 full backs, 2 midfielders, 2 wide midfielders, and 1 forward) (age 22.8 ± 2.1 yr; weight 58.5 ± 9.3 kg; height 163.0 ± 7.0 cm). They had 5.2 ± 3.2 years of experience in soccer and trained on average 10 hours a week and played a match weekly.
As mentioned previously, the specific training program was implemented during the second part of the competitive season (i.e., February, March, and April) after 6 months of training (i.e., from August until January). During the intervention program, the team trained 4 times a week. On Mondays, Wednesdays, and Fridays, the session lasted approximately 120 minutes and on Thursdays only 70 minutes. At each training session, the CG and the PG performed the warm-up and technical and tactical program together. Training games on Thursdays and competitive matches on Sundays were also carried out together. The physical conditioning program was different for the 2 groups. CG women followed the regular standard soccer physical conditioning program. This program was completely replaced by a plyometric program to improve the strength of lower limbs for PG players. After the intervention of 12 weeks, PG and CG continued with their normal soccer training together. During the study, players were not allowed to perform any other training that would impact the results. The training regimen of the team during the study is shown in Figure 1.
Plyometric Group Training
Although all women had previous experience in this type of training, a trained instructor gave specific instructions and showed illustrations of each plyometric exercise before the first session. As illustrated in Figure 1, the plyometric training took place after the warm-up, 3 days a week for 12 weeks (36 sessions distributed on Monday, Wednesday, and Friday), supervised by the same coach. Each plyometric session took 40 to 60 minutes to perform, and the training regimen was based on 3 different exercises, always coming after the procedures described below:
- - Monday: Series of 5 jumps over hurdles of 60 cm height spaced apart at 45 cm intervals with a rest of 30 seconds after 5 jumps and 4 minutes after 20 jumps.
- - Wednesday: Series of 5 DJ in stands of 50 cm with a rest of 1 minute after 5 jumps and 5 minutes after 20 jumps.
- - Friday: Series of 5, 8, or 10 horizontal jumps with a rest of 40 seconds after 5, 8, or 10 jumps and 4 minutes after 20 to 30 jumps.
Training sessions were carried out on a hard synthetic floor. Each week, the number of jumps was the same on Monday, Wednesday, and Friday. After every 2 weeks of training, the number of jumps decreased to ensure a proper adaptation to the program. After 12 weeks, the total number of jumps was 3,240. Intensity in terms of height/length and speed was always maximal. During the jumps over hurdles and DJ, women were instructed to perform a free knee flexion position to ensure an individually and preferred chosen knee flexion angle to achieve the optimal jumping height. In all the exercises, players were asked to minimize ground contact. The plyometric program is described in Table 1.
Control Group Training
As explained previously, during the 12-week study, the CG continued with its regular standard soccer physical conditioning program after the warm-up on Mondays, Wednesdays, and Fridays. The program included fartlek, speed endurance, core stability training, reaction speed, and stretching. It also included general strength training (exercises of various muscle groups, e.g., knee extension and flexion, hip abduction and adduction, and ankle plantarflexion-dorsiflexion, where external resistance was provided by using portable rubber bands, ropes, or with the assistance of another soccer player), but it did not include weight training.
Before the initial testing, each player was familiarized with the testing protocol. To standardize testing procedures, the same trained test leaders carried out the entire test procedure using identical order and protocol. All players were tested on 4 separate occasions: T1, 1 week before the start of the training program; T2, after 6 weeks of the training program. The test session replaced the training on Wednesday in that week: T3, 1 week after the end of the intervention program; T4, 5 weeks after the end of the intervention program (detraining).
All the anthropometric measurements were taken by 1 trained anthropometrist assisted by a recorder in accordance with the standardized procedures of International Society for the Advancement of Kinanthropometry. Testing was carried out in a standardized order after a proper calibration of the measuring instruments. Height and body mass were measured for each woman using a Holtain Ltd. stadiometer (95-190 cm, accurate to 0.1 cm) and a Seca Electronic balance (0-150 kg, accurate to 0.1 kg). To estimate body composition, 6 skinfolds (triceps, subscapular, suprailial, abdomen, front thigh, and medial calf) and 2 diameters (wrist and femur) were taken using a Holtain (British Indicators, Ltd.) limiting calliper (0-40 mm, accurate to 0.2 mm) and a Lafayette calliper. Subsequently, fat mass, bone mass, residual mass, and muscle mass and their respective percentages were calculated to evaluate body composition, using the formulas of Faulkner (13), Rocha (29), Würch (38), and Matiegka (25).
Explosive Strength-Jumping Ability
Before the start of the test session of the explosive strength, each woman carried out a standardized 15-minute warm-up period. The jumping ability of the players was evaluated with a jumping mat (SportJUMP System; DSD), which showed a positive significant correlation (Rxy = 0.998, p < 0.001) with the Ergojump Bosco System and also a positive significant correlation (Rxy = 0.994, p < 0.001) with a Dinascan 600 M force plate (14). The women performed 2 different jumps, a CMJ and a DJ (starting from 40 cm), both with the arms kept on the hips. Five attempts were carried out for each type of jump. The best result was used for the statistical analysis. The rest between the trials was 40 seconds in the CMJ and 60 seconds in the DJ.
After a short recovery of 3 minutes after stretching exercises, players practiced the kicking test for 10 minutes to familiarize themselves with the test procedure. Kicking performance was estimated from maximum ball speed during shooting. The speed, expressed in km/h, was measured with a Stalker´s type hyperfrequency radar (Stalker Professional Radar, Radar Sales, Plymouth, MA, USA) set up 30 cm above the ground behind a goal. A ball with a standard size and inflation pressure following the rules of Féderation Internationale de Football Association was used. It was always placed at the same point (5 m distance). The reliability of the results offered by the radar gun with the current measuring protocol was previously validated using a high-speed camera and a 2D photogrammetric system Kinescan/IBV 2001. This pilot study revealed a positive significant correlation (Rxy = 0.994, p < 0.05) between the results registered by the radar gun and those recorded by the video system. To standardize, we used a 2-step run-up. Participants were asked to kick the ball as fast as possible toward the radar gun, using the instep of the dominant and the nondominant leg alternatively. They were told that kicks that missed the radar gun could be repeated and that they should not decrease the speed to improve accuracy. Each woman performed 10 trials with each leg, and the best result was used for statistical analysis. The rest between trials was 40 seconds. The radar gun was always calibrated immediately before the sessions according to the instructions given in the user's manual.
Normality of distribution was tested by means of the Kolmogorov-Smirnov test. Standard statistical methods were used for the calculations of the means and SD. Student's t-tests were carried out to determine differences among the 2 groups' initial values in all variables analyzed. Training-related effects were assessed using two-way ANOVA with repeated measures (group x time). When a significant F-value was achieved by means of Wilks lambda, Tukey post hoc procedures were performed to locate the pairwise differences. Bonferroni adjustment for multiple comparisons was used. The p < 0.05 criterion was used for establishing statistical significance.
The Kolmogorov-Smirnov test suggests that all variables were distributed normally (p > 0.05). Results of comparative analysis (Student's t-test) between CG and PG in all the variables at baseline revealed that there were no statistically differences before the start of the training program (Table 2). Table 3 presents the data for the anthropometric features, explosive strength, and kicking speed for both groups on every test occasion (T1, T2, T3, and T4).
Analysis of variance revealed no significant time x group interaction effects for body mass, (F(3,16) = 4.880), body fat (F(3,16) = 2.085), and muscle mass (F(3,16) = 2.364).
For explosive strength, ANOVA showed that there were significant interaction effects both for CMJ (F(3,15) = 3.578, p < 0.05) and DJ (F(3,15) = 8.434, p < 0.05). Tukey post hoc tests indicated that, for PG, there were significant differences between T1, on the one hand, and T2, T3, and T4, on the other hand, both in CMJ and DJ. There were no significant differences among T2, T3, and T4. Graphic display of the data clearly demonstrates the treatment effect that was produced by the plyometric training program (Figures 2 and 3).
Similar to explosive strength, ANOVA results showed that there were significant interaction effects for kicking speed with dominant (F(3,14) = 11.225 p < 0.05) and nondominant leg (F(3,15) = 3.368 p < 0.05). For PG, Tukey post hoc tests located the differences between T1 and T3 and between T1 and T4 in kicking speed both with dominant and nondominant leg. There were no significant differences between T1 and T2, T2 and T3, and T3 and T4. Figures 4 and 5 show the evolution in kicking speed during the study for the 2 groups.
Myer et al. (26) stated that female athletes may specially benefit from multicomponent neuromuscular training such as plyometry because they often show lower baseline levels of explosive strength compared with their male counterparts. However, as explained previously, there are no related studies concerning the effects of a strength-training program on the characteristics of a task such as kicking in female soccer players. Furthermore, although weight training is very common in male soccer players, it is not that common in female soccer players, even in elite women teams. Most of the time this is because of a lack of facilities, and consequently it could be interesting to find valuable strength training protocols that can be directly applied on the field. Plyometric training could be one of the alternatives (9). However, the results obtained about the possible influence of plyometric training on tasks such as jumping ability in female players are controversial. Therefore, our aim in this study was to determine how explosive strength and kicking speed were affected by a plyometric training program. As we expected, the main findings of the current study indicated that a 12-week plyometric training program focusing on the lower limbs, in addition to the regular soccer training, increased explosive strength of the lower limbs as well as kicking speed with both dominant and nondominant legs.
The current results revealed that there was a gain in jumping ability in the PG, whereas the training effects for this variable were not significant in the CG. These results are in agreement with those of Siegler et al. (30), who reported gains in jumping ability after a 10-week plyometric program in female players. Conversely, Chimera et al. (5) did not find significant increases in jumping ability after a 6-week plyometric program. The difference in frequency of training could be the reason of the discrepancy in results (12,21).
Markovic (21) suggested in a meta-analytical study that there may be a positive transfer of the effects of plyometric training on vertical jump ability to other athletic performance, which could include kicking. Results of the present study are in agreement with this statement because it was shown that the plyometric training program caused significant increases in kicking speed between pretraining and post-training values for PG, whereas there were no differences for CG. Bangsbo (2) stated that there was a relationship between improvements in general strength of the lower limbs and increases in ball velocity. Although there are no studies about the influence of strength training on kicking performance in female players, many researchers also found that specific strength training improved kicking performance in male players (8,11,19,32).
These data also revealed that although 6 weeks of plyometric training was enough time to produce significant improvements in explosive strength, players needed 12 weeks to produce significant increases in kicking speed. Several researchers (8,11,19) who also showed increments in ball speed after strength training noted that these improvements were not fully determined by the increase in muscle strength, suggesting that the transfer of the gains in strength to the specific skill is also a crucial factor. Manolopoulos et al. (19,20) confirmed the importance of mixing strength training with technical training to transfer the gains given the fact that a successful soccer kick depends on a precisely coordinated action of the leg muscles rather than on isolated muscle strength. Manopoulos et al. (19) attributed the improvements in kicking speed after strength training to a change in some kinematic variables such as linear velocity of the distal segments and the position of the body throughout the shot. These changes possibly display an adaptation in the kicking movement after the gains in strength and may be the result of an altered stretched-shortening cycle of the musculature involved (19). In the present study, we did not register these variables, but the improvements in kicking speed could be caused by the increased transfer of energy from proximal to distal segments, which may have contributed to a higher ball speed value after the plyometric training intervention program. Therefore, players may need time to convert their gains in explosive strength to the changes in kinematic variables and eventually translate it in a higher kicking speed. This could be the reason for the lack of significant improvements in kicking speed after 6 weeks of plyometric training.
On the other hand, the fact that significant improvements in jumping ability already appeared in the first 6 weeks could be related to the initial level of jumping ability in the sample. Several authors pointed out that the initial low level can explain the magnitude and the velocity of the gains in jumping performance (12,18). Results obtained in T1 were not comparable with those registered in other female players of the same level (27,30,35,37), and this could be one of the reasons why they rapidly obtained improvements in vertical jumping ability.
Although several authors have reported significant improvements in vertical jump using plyometric training in male (24) and female athletes (22,26) and male soccer players (9,27), there is still a discrepancy about the factors influencing these improvements. Many authors suggested that muscular performance gains after plyometric training are attributed to a neural adaptation located in the nervous system rather than to morphologic changes (4,18,28,33). According to these authors, neuromuscular factors such as increasing the degree of muscular coordination and maximizing the ability to use the muscles' stretch-shortening cycle appear to be more important than changes in fiber size. Results of the present study are in agreement with the latter because there were no interaction or time effects with regard to the muscle mass component. Nevertheless, we did not assess neuromuscular variables. Further studies that focus on neuromuscular factors are needed to corroborate that statement. The improvement of muscular coordination after the training period would be partly related to the specificity of movements used during the training program (9). This is especially the case for the velocity because training at a specific velocity improves explosive strength around that particular velocity (4,20,33). The specificity of exercises and the knee flexion angle during strength training could also be considered a possible factor influencing the gains in jumping ability (4,18)
With regard to the detraining period, our findings revealed that the detraining period did not affect the changes registered during the plyometric program. Other researchers also found that a detraining period in which the regular training of a specific sport is maintained allows an athelete to maintain the gains previously achieved (18). In soccer players, Diallo et al. (9) discovered no significant differences in jumping performance after 8 weeks of reduced training (after a 10-week plyometric program). The fact that players' explosive strength and kicking performance can subsequently be maintained at a high level by means of sport-specific training only and that supercompensation can give even greater strength after the program is important to plan the season (32).
Finally, it must be noted that our results revealed no significant effects on body fat. Similarly, Potteiger et al. (28) did not find changes in body mass and body composition after an 8-week plyometric training in physically active males. They attributed this lack of change to the short duration of their study. However, the present study was longer. The period of the season when the training intervention was implemented (i.e., the 2nd part of the competitive period) may explain the lack of differences between pre- and post-training values in anthropometric features. Probably, effects in body composition could be observed after the preseason period in the beginning of the training season.
Plyometric exercises have gained popularity in training, and their use by strength and conditioning specialists is increasing because they can be applied on the field. On the basis of the results of the present study, it could be concluded that a 12-week plyometric training can improve explosive strength in female soccer players, and, most importantly, these improvements can be transferred to soccer kick performance in terms of ball speed. Therefore, female soccer professionals must notice that the plyometric training is beneficial in increasing the ability to use explosive strength effectively in a specific task. However, soccer coaches must also take into account that plyometric training should be combined with regular soccer training to transfer the gains in explosive strength to the kinematic parameters of the kicking movement. Moreover, players need time to transfer these improvements in strength to the specific task. To be effective, plyometric training must incorporate specific exercises in terms of velocity or knee angle, and intensity must be maximal in terms of height/length. Exercises should be carried out in a ballistic way, minimizing the ground contact. On the other hand, 3 days of training per week was proven to be an effective training frequency to improve both explosive strength and kicking speed. Furthermore, regular soccer training can maintain the improvements from a plyometric training program for several weeks. Although, as mentioned previously, weight training is not common in female soccer players, it must be taken into account by professionals that explosive weightlifting combined with plyometric training might augment the results obtained in the current study and that it could be more effective than isolated plyometric training.
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