Nowadays, female soccer shows a great deal of similarity to male soccer in terms of struggle, and properties based on technique-tactic and conditions. Actions such as power, running, direction changes, jumps, and short-distance sprint belong to lower extremity. During a soccer match, short duration actions such as jumps, direction changes, and accelerations, all of which occur once in 4–6 seconds, are repeated 1,000–1,400 times (35). Technique in a soccer game is exhibited by speed, power of lower extremity, and actions in which explosive power comes to the fore. As frequent use of these actions can influence the result of a game and optimal high performance (5,35), the need to develop these properties by different training occurs.
One of the training methods that includes these actions and that is used for power development is plyometric exercises. These kinds of exercises are characterized by stretch-shortening cycle (10,22). It can be used for upper- and lower-body exercises. Especially for lower body, plyometric training (PT) is a method to improve vertical jump ability and leg muscle power (23), which is included in various types of jumps, like countermovement jumps (CMJ), the drop jump, and squat jump, hopping, alternate-leg bounding, and stretch-shortening cycle (10). Also, PTs are found to increase kicking speed (26,33), as well as strength, muscle power (26), coordination, agility (26,36), speed, and acceleration time (16,31,34); however, there are also reports about neutral impacts (6) or negative impacts (20).
The controversial and scarce impacts of PT on female soccer players must also be pointed out. Chimera et al. (6) reported that 2 times per week for 6 weeks of PT increased small (5.8%) but insignificant improvement in vertical jump in collegiate female soccer and field hockey players. Also, studies show that if adults perform PT twice a week, then jumping and speed performance will increase (6,7). But, Rubley et al. (32) reveal the effect of PT in vertical jumps in children in 1 day a week program. However, technical training of female soccer players during the competition season, the effects of implementing PT, especially the low-frequency PT (once a week), could not be fully determined.
As a result, contradictory and different results of PT, which insures speed and strength development that are important factors in female soccer, show that more studies must be done in this area. Moreover, as the studies on soccer are oriented at male soccer players and less study exists about female soccer players, more studies on this subject are required. Despite the opinion that PT is sufficient twice per week (27), we hypothesize that 8-week low-frequency, high-volume and low-intensity (nondeep jump) PT, 1 day per week, will significantly increase jumps and sprint performance in female soccer players.
Experimental Approach to the Problem
In the present study, a group-assigned, mixed-model experimental design was used to determine the effect of 8-week low-impact low-frequency PT on the leg power, and jump and sprint performance of female soccer players. According to the preliminary test results, the 18 elite female soccer players were divided into two groups as control and plyometric. One group who completed only regular soccer practicesserved as the control group (CG); the other group who completed once-weekly PT, served as the plyometric training group (PG). The effectiveness of the PT program was evaluated with a pretest and posttest.
A total of 18 female soccer players, 15 to 22 years of age (18.4 ± 2.7 years), from the University Sports Club female soccer team that plays at Women Second League volunteered to participate in the study. Anthropometrics and training characteristics of soccer players are presented in Table 1 (mean ± SD and range). Participants were chosen from the players who had at least 4 years of training history; and they were excluded from participation in this study if they had not previously engaged in a formal PT program and had an orthopedic injury in the past 6 months.
Groups were divided into plyometric (n = 9) and control (n = 9). Participants were homogeneous in terms of their training history and physical properties (height, weight, body mass index [BMI]). All players participated in all the training; goalkeepers were not included in this study. Approval from Marmara University Ethical Committee was received. All participants and their parents were informed of the purpose, benefits, and potential risks of the participation. Written consent was obtained from all participants, and parents gave written consent for the participants younger than 18 years old.
All the tests selected to determine the training effects were carried out before and after 8 weeks of training intervention. The tests were assessed in a single session (i.e., 15:00 and 18:00 hours) in the same condition with a test being completed by anthropometric measurements, sprint (20 m) and strength (triple jump, CMJ, standing board jump) tests, respectively, and 1 minute of rest was allowed between each test. All players were familiarized with all experimental tests before baseline performance. The players were requested not to perform strenuous exercise in the 24 hours before testing and avoid drinking or eating at least 3 hours before measurements. These tests were performed on natural grass in a soccer pitch, and the participants were wearing soccer kit. Before the tests and after completing anthropometric measurement, participants performed the standardized warm-up, which consisted of jogging, multidirectional movements, sprints, dynamic stretching exercises, and ball drills in 10 minutes.
The anthropometric variables (body height and body mass) were measured according to the instructions of the Anthropometric Standardization Reference Manual (19).
Sprint times were evaluated during 20-m maximal running on a grass track (3) and recorded by infrared photoelectric cells (Newtest 2000 Sprint Timing System; NewtestOy, Oulu, Finland) having a precision of 0.01 seconds. Two trails were given and recovery time between sprints was set at 2 minutes. During the recovery period, the subjects walked back to the starting line and the best sprint time was used for statistical analysis (11).
The players performed 3 CMJ to realize maximal height on a vertically using jumping mat (Newtest 2000 System; NewtestOy). During the jump test, participants were instructed to place hands on their hips to minimize contribution of the arms during the jumps. Three maximal jumps height (cm) were recorded. Between jumps, a break of 30 seconds was provided. The best trial was included into further analysis (12).
Triple Hop Distance Test
During the triple hop distance (THD) test, 3 consecutive jumps were performed with the same leg, an attempt to jump as far as possible to reach the maximal horizontal distance. These tests were performed until 3 successful hops were obtained for each leg, with the starting order of randomly either the right leg or the left leg assigned to the participants. For the best distance (cm), dominant and nondominant leg was recorded from 3 trails. This was used for analysis (13,28).
Standing Broad Jump
The test was performed according to Eurofit test battery (1). Participants were instructed to jump forward as far as possible and land with the feet together and stay upright. The test was repeated twice and the best trial was recorded.
The CMJ height was then used to determine peak power (PP) by the formula given in Lara et al. (17) and this equation was used for national level of female players. PP = (53.6 × Jump Height [cm]) + (67.5 × Body Mass [kg]) − 2,624.1.
Participants of both groups had at least 2 years of experience for PT with the same coach and to participate in high-impact PT. It has been suggested that athletes should be able to squat 1.5 to 2 times their body weight (2). Therefore, before the PT, all players participated in strength training in general for 2–3 days per week and special preparation sessions for 2 months. The aim of this kind of training used some basic functional exercises for upper and lower body (e.g., push-up, crunch, front and lateral lunges, back extension, bench press, squat, leg press, single leg band–assisted squat, barbell squat, hip abduction-adduction, knee flexion-extension, ankle circuit, lat pull-down, Nordic hamstring curl, standing cable wood chop exercises, standing cable row) to prepared the players before attempting a plyometric program for the demands of more advanced training by improving their overall strength. Study was implemented for duration of 2 months covering the second term of competitive season (March–April). The experiment was performed 4 months later, the beginning of the competitive season.
During the training time, all groups continued their warm-up, technical and tactical training, and games together accompanied by the same coach. General soccer training consists of 4-day practices (Monday, Tuesday, Thursday, and Friday) and 1 game (Sunday) per week. Practices typically lasted for 2 hours and focused on soccer-specific skills: dribbling, throwing, passing, heading, tackling, trapping, and small side games. Additionally, the PG participated in an 8-week training program. The PT performed 1 session (60 minutes) per week on Wednesday, at the same time (16:00–17:00 PM) of day throughout the intervention period. Plyometric training group performed a variety of plyometric exercises designed for the lower extremity, whereas the control group did not participate in any plyometric exercises. No injuries occurred during the 8 training sessions. Both groups stayed away from participating in other physical activities during this study.
Plyometric program was created for developing explosive power of lower extremity, and 10–15 minutes of standard warm-up (jogging, multidirectional movements, sprints, dynamic stretching exercises and ball drills) was performed before the main training session. The main session lasted for 30–40 minutes, which consisted of plyometric exercises, and cool down lasted for 5–10 minutes. The PT protocol designed 4–5 sets, 5–15 repetitions of 4–5 exercises, a total of minimum 90 “touches” and maximum 220 “touches” per session. A touch is defined as a foot contact with the ground.
The 8-week training set off 4 exercises of low to high intensity with low to high volume of total touches that also progressively increased each week. There was 1-minute active rest period between each exercise repetitions, and 3–5 minutes for each set during which the subjects walked around the field. The training program of PG is outlined in Table 2.
The descriptive statistics were expressed as mean values, SDs, and value ranges. The test-retest reliability of the speed and strength tests was evaluated using intraclass correlation coefficients. All tests were reliable and reliability of these tests for intrarater correlation coefficient ranged between 0.89 and 0.96 with no significant differences found between mean values for tests vs. retest. Test of normal distribution (Shapiro-Wilk) were conducted on all data before analysis. When groups are assigned at random, analysis of covariance (ANCOVA) is considered an adequate method for comparing changes between groups (15). Single factor ANCOVAs were used to pretest for differences between groups (PG and CG) for the dependent variable (20-m sprint, triple jump, CMJ, and standing board jump tests) as a covariate. Therefore, statistical analyses of data were performed using Mann-Whitney U-test to compare differences between PG and CG. The Wilcoxon signed rank test was used to analyze pretest and posttest difference of the players within their group in all test variables. Significant was established at p ≤ 0.05. Data analysis was performed using SPSS (version 14.0; SPSS Inc., Chicago, IL, USA).
The groups' differences of the anthropometric and training characteristics are summarized in Table 1. There was no statistically meaningful difference (p > 0.05) among the pretest results related to physical (height, weight, BMI) (Table 1) and motoric properties (Table 3) of both groups and these results were determined to be homogeneous of groups. Tests of normality indicated that dependent variables were normally distributed.
Significant differences (p < 0.05) were observed between all the pre- and posttest measurements of both groups. However, no significant differences (p > 0.05) were observed in the speed time decrease in 20-m sprint test between pre- and posttests for the control group. Also, differences between pretests and posttests within each group were stated and percentages were found in the order of significant increase in PG's and CG's dominant leg triple hop distance test (+12.1%, +4.3%), nondominant leg triple hop distance test (+15.7%, +6.6%), standing broad jump (SBJ) (+5.2%, +2.1%), CMJ (+17.6%, +6.9%), and PP (+10.8%, +4.4%) values (p ≤ 0.05). The PGs showed significant decrease in (p ≤ 0.05) 20-m sprint time (−8.1%) from pretest to posttests.
There were significant differences between the groups for pre- and posttest results of dominant triple hop distance test, nondominant triple hop distance test CMJ, PP, 20-m sprint (s) parameters, and SBJ in favor of PG (Table 3).
The main finding of this study was that the supplementary 8 weeks of low-impact and high-volume PT on 1 day per week significantly increased CMJ, triple hop distance, SBJ, PP, and 20-m sprint performance in female soccer players.
Training volume and frequency are very important parameters to be taken into account for an optimum PT program design. In the present study, training containing PT technique and condition was implemented in addition to regular training. Plyometric training was done once a week in our study; however, it deems suitable to implement this training at least twice a week (10). The main reason for this situation is that the team was in competitive season, and control group consists of players from first 18. It was foreseen that prolonging the study for a longer duration would create conditional imbalance in the team.
Plyometric training was suggested as an alternative to strength training oriented at lower extremity for female athletes (29). Results of 4 meta-analysis showed that PT increases vertical jumping distance for both athletes and nonathletes and for both men and women. However it was reported that height of vertical jump did no increase and yet it decreased at the least (8,10,23,24).
According to studies, there are also suggestions that positive effect of PT on vertical jumping can be developed at different periods of time such as 4–10 weeks (2,6,10,20,30). Although implemented PT is of 8-week low frequency, high volume, low intensity (nondeep jump), and 1 day per week in our study, it created positive effects on CMJ performance of PG female soccer players (17.6%), which is more than control group players (6.9%). This development rate, which is specified as 4.7–15%, is much more than meta-analysis of Villarreal et al. (8) on the effects of vertical jumping performance of PT.
The results of present study show that vertical jump (18.6%) and kicking distance shows an increase at adolescent female soccer players doing PT low-frequency in 12 weeks and on 1 day a week, low-impact (nondeep jump) model with respect to control group, and the results show similarity to Rubley et al. (32). In addition, our studies are compatible with Siegler et al. (34) and Campo et al. (4), who used 10-week (3 d·wk−1) and 12-week (3 d·wk−1) PT programs for female soccer players, respectively, and found a meaningful increase at jumping ability. However, our study is not compatible with Chimera et al. (6) study, which says that 6 weeks of plyometric program, which is implemented at 2 days a week and 20–30 minutes a day, creates a 5.8% increase at jumping ability and this is not statistically meaningful. The different results on our studies may be because of different frequency, intensity, time, type of plyometric exercises, and training level of female athletes (6,8,23).
Triple hop distance, CMJ, and SBJ are functional tests and they reveal leg strength from different points of view. Especially, THD can reveal asymmetry in strength of the legs, because it tests 2 legs separately in contrast to vertical jumping (13). Standing broad jump tests horizontal jumping property. Another result is that SBJ (5.2%) and THD (dominant leg 12.1%, nondominant 15.7%) increase at both legs in PG female athletes are more than CG (THD dominant leg 4.3%, THD nondominant leg 6.6%, SBJ 2.1%) sets forth the effect of PT from different points of view. It is observed that PT created more difference at jumping distance of nondominant leg with respect to dominant leg at especially THD test. The meaningful performance increase at all jumping parameters at control group may be related to common technical and conditional exercises done at other days.
Studies about the effect of PT on horizontal jumping performance are scarce. In the study of Markovic and Mikulic (24), it is stated that PT increases horizontal jumping performance by 1.4–7%. This increase is found less than vertical jumping (24). More studies are needed for observing the effect of PT on the increase in long distance jump from standing still. Increase in vertical jumping performance (17.6%) in PG is found to be more than SBJ (5.2%), and it shows similarity with the results of the study of Lehnert et al. (18). This result might be explained by having less drill at lateral and horizontal direction than vertical direction in our study.
It was stated that PT, which implemented 3 times a week and for duration of 6–10 weeks, was found to have more positives than other programs. Also, it is suggested that treatment with more than 15 sessions increases strength performance, whereas performance of more than 40 repetitions per session seemed to be the most beneficial volume (10). However, low training frequency (1 d·wk−1) but high jump training volumes and low-intensity (nondeep jump) training model used in our study, and which is increased gradually according to weeks, resulted in a meaningful increase at some of the strength and speed parameters of elite female soccer players. These results show similarity to the study of Rubley et al. (32), which shows the effects of PT on vertical jumps in children with 1 day a week program. Having high speed in soccer can provide advantages in actions such as making a move to balls during defense and attack and gaining ball from the opponent. There are several studies showing the increase (5,7,9,31) or no change (14,21,23) in sprint performance after PT. At the last meta-analysis study (9), the view that PT is an effective training method for improving sprint performance is supported. Plyometric training results in an increase in sprint performance, especially between 10- and 40-m distances (9). Although there is no increase at 20-m sprint time performance of CG, there is a 8.1% performance increase in PG. Skipping jumps with horizontal displacement, combined high-intensity training including sprint and jumping in our training program, results in an increase in horizontal acceleration and (9) supports these positive views about sprint time performance. The fact that the increase in the PP performance of legs of PG (+10.8%) is bigger in proportion to the CG (+4.4%) can be explained by neuromuscular adaptation. Although the effects of neural adaptation was not studied in our study, positive effects of PT on especially speed and jump performances shows similarity to the study of Wilkerson et al. (37).
As a result, our study was found to support the view that PT increases speed, PP, horizontal and vertical jumping performance of female soccer players significantly.
Our study shows that, in addition to technical training, well-planned PT with duration of 8 weeks, 1 day per week, influences multidimensional development of muscular performance. Despite the numerous benefits associated with high-impact and high-intensity PT, the possibility exists for this type of training to induce acute muscle soreness, muscle damage, or even musculoskeletal injuries (25). There is a lack of information concerning the effects of low-frequency, high-volume, and low-intensity PT program in the muscle performance especially on female players. The results of present study shows that short duration (a day per week) PT may result in the development horizontal and vertical jumping strength, PP, and speed performance of elite female soccer players who have a good level of basic and PT. The results indicate that safe, effective, and alternative PT can be useful to strength and conditioning coaches, especially in competition season where less time is available for training. Short duration PT program implemented in our study might have been suggested for increasing physical capacity, which is one of the dominant parameters of elite female soccer players, during competitive season.
The authors thank Marmara University, Female Soccer Team players who participated in this study.
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