Endurance (cardiorespiratory capacity) is a relevant physical fitness trait in soccer [1
]. However, jumping, single and repeated sprinting, change of direction, and kicking are also key proxies of soccer performance [2
]. Indeed, these maximal-intensity single-effort physical fitness traits preceded goal opportunities in competitive leagues [2
]. For example, sprinting and jumping actions occur in more than 50% of goal situations [2
]. Moreover, mean values of acceleration >2.26 m·s−2
occur 1.78 times per minute, in addition to sprint distance of 2.87 m per minute, and/or highs-peed running of 6 meter per minute, with greater values during highly-competitive matches [6
]. In addition, maximal-intensity short-duration actions such as jumping and sprinting may be associated with team position in a given tournament and/or players’ competitive level [4
Several training approaches are used among female soccer players to improve jumping, single and repeated sprinting, change of direction and kicking power, as well as endurance attributes [5
]. However, plyometric jump training (PJT) may be particularly effective, offering several advantages (e.g., reduced cost; injury prevention) compared to other methods (e.g., traditional resistance training) [9
]. Additionally, the incorporation of PJT among training practices in soccer might be highly translated into game scenarios. For instance, there is a strong reliance on vertical and horizontal expressions of power during various game scenarios in soccer such as when defending, shooting, and attacking [1
]. In turn, according to the principle of training specificity, soccer players should regularly engage in PJT programs. Indeed, PJT have demonstrated a significant transference effect between jump training exercises and soccer-specific physical performance [12
From a physiological perspective, PJT capitalizes on the stretch-shortening cycle (SSC) where musculotendinous units are eccentrically stretched during the loading or impact phase before concentrically shortening in the push-off or take-off [15
]. In this regard, PJT results in a wide range of distinct physiological and biomechanical adaptations (e.g., increased motor unit recruitment and rate of force development (RFD) [17
]. Among soccer athletes (mixed with other sports), after 8 weeks of PJT, significant changes were noted at the muscle fiber level, including myosin heavy chain isoform composition in type I/IIa fibers, increased cross-sectional area, absolute peak calcium-activated force, maximum unloaded and loaded shortening velocity, absolute and normalized peak power, velocity at peak power, absolute force at which peak power was reached, and increased stiffness [21
]. In a similar study, increased percentage of type I/IIa and IIa fibers were noted, in line with a decrease in IIx fibres from the vastus lateralis [22
]. In another study with soccer players, 8 weeks of PJT combined with resistance training increased electromyography activity (72–110%) in the vastus medialis and rectus femoris muscles during jumping [23
]. Another study with soccer players reported significant increases (4–15%; ES = 0.3–1.3) in leg stiffness after 4 weeks of training [24
Due to the beneficial effects of PJT, several systematic reviews and meta-analysis (SRMA) have been published evidencing the effectiveness of this training mode to improve distinct power-related attributes in athletes from different sports disciplines including handball [25
] and volleyball [26
]. Likewise, there is a growing body of experimental evidence examining the effects of PJT on physical fitness attributes in female soccer players [27
]; however, this evidence has not yet been comprehensively aggregated. Although a recent SRMA examined the effects of PJT on female soccer, only vertical jump height (i.e., countermovement jump) was analyzed [28
]. Another SRMA examined the effects of PJT on male soccer players physical fitness [29
], including measures such as jumping, sprinting and strength. However, considering the differences between female and male soccer players [1
] and their potential different response to PJT [30
], it would be adventurous to extrapolate results derived from male to female soccer players.
Given the increased scientific awareness of PJT relevance regarding physical fitness improvement, it was deemed appropriate to aggregate PJT studies conducted in female soccer players in a SRMA to strengthen the level of scientific evidence [31
] on this topic. This knowledge can guide practitioners to use PJT routines that are effective in female soccer, avoiding the simple transfer from the knowledge related to male soccer players. Thus, the aim of this SRMA was to assess the effects of PJT on female soccer player’s physical fitness (e.g., jump, sprint, kicking ability, change of direction speed; anaerobic performance; endurance).
2. Materials and Methods
This SRMA was conducted and reported in accordance with the PRISMA statement [32
2.1. Eligibility Criteria
A Participants, Intervention, Comparators, Outcomes, and Study (PICOS) design approach was used to rate studies for eligibility [32
]. The respective inclusion criteria adopted in our meta-analysis was as follow: (i) apparently healthy female soccer players, with no restrictions on their playing level or age, (ii) a PJT programme, defined as lower-body unilateral or bilateral bounds, jumps, and hops that commonly utilise a pre-stretch or countermovement stressing the stretch-shortening cycle, (iii) a control group, (iv) at least one measure of physical fitness (e.g., jump, sprint, kicking ability, change of direction speed; anaerobic performance; endurance) before and after PJT, (v) controlled trials. The respective exclusion criteria adopted in our meta-analysis was as follow: (i) female soccer players with health problems (e.g., injuries, recent surgery), (ii) exercise interventions not involving PJT or exercise interventions involving PJT programmes representing less than 50% of the total training load when delivered in conjunction with other training interventions (e.g., high-load resistance training), (iii) absence of active control group, (iv) lack of baseline and/or follow-up data, (v) non-controlled trials. Of note, two previous scoping reviews [27
] indicated that several otherwise high-quality studies in the field of PJT did not include a randomization design. In order to avoid the exclusion of potentially relevant studies, we considered for inclusion non-randomized study designs, as long as baseline values between groups were similar for the main outcome of the study. In contrast, the inclusion of an active control group was considered essential in order to isolate the effect of PJT from the rest of training methods that female soccer players commonly conduct in their regular training schedule.
Additionally, only full-text, peer-reviewed, original studies written in English were considered, excluding cross-sectional, review papers, or training-related studies that did not focus on the effects of PJT exercises (e.g., studies examining the effects of upper-body plyometric exercises). Retrospective studies, prospective studies, studies in which the use of jump exercises was not clearly described, studies for which only the abstract was available, case reports, special communications, letters to the editor, invited commentaries, errata, overtraining studies, and detraining studies were also excluded from the present meta-analysis. In the case of detraining studies, if there was a training period prior to a detraining period, the study was considered for inclusion.
2.2. Information Sources
We considered previous recommendations from the two largest scoping reviews examining PJT to conduct the literature search [27
]. Computerized literature searches were conducted in the electronic databases PubMed (comprising MEDLINE), Web of Science Core Collection, and SCOPUS. The search strategy was conducted using the Boolean operators AND as well as OR with the following keywords: “ballistic”, “complex”, “explosive”, “force-velocity”, “plyometric”, “stretch-shortening cycle”, “jump”, “training”, “female”, “women”, “football”, and “soccer”. For example, the following search was adopted using Pubmed: (((((((((“randomized controlled trial” [Publication Type]) OR “controlled clinical trial” [Publication Type]) OR “randomized” [Title/Abstract]) OR “trial” [Title]) OR “clinical trials as topic” [MeSH Major Topic]) AND “soccer” [Title/Abstract]) OR “football” [Title/Abstract]) AND “training” [Title/Abstract]) OR “plyometric” [Title/Abstract]). After an initial search (April 2017), accounts were created in the respective databases. Through these accounts, automatically generated emails were received for updates regarding the search terms used. The search was refined in May 2019 and updates were received daily (if available), and studies were eligible for inclusion up to January 2020. Following the formal systematic searches, we conducted additional hand-searches (e.g., personal libraries). One author (RRC) performed the only search.
2.3. Study Selection and Data Collection Process
After the exclusion of repeated article titles, a review of retrieved article titles was conducted. Then, examination of article abstracts followed. Thereafter, full articles were assessed. The reasons to exclude full-text articles were recorded. The data extracted from the included articles was recorded in a pre-form created in Microsoft Excel (Microsoft Corporation, Redmond, WA, USA). Two authors (MSG and JSS) conducted the process independently, and a third author (RRC) resolved disagreements regarding study eligibility.
2.4. Data Items
For the current review, physical fitness was chosen as the main outcome. A priori, common measures of physical fitness were considered, but not limited to: (i) jump (i.e., height; distance; flight time; power; reactive strength [i.e., mm.ms; ms.ms]), (ii) sprint (i.e., time; velocity), (iii) kicking ability (i.e., distance; velocity), (iv) change of direction speed (i.e., time; speed), (v) anaerobic performance (e.g., repeated sprint ability mean time; 30-s Wingate test mean power), (vi) endurance (e.g., shuttle-run total time or distance).
In addition to the aforementioned data items, adverse effects were registered, and descriptive characteristics of the PJT interventions (e.g., duration; frequency) and athletes (e.g., age; fitness level) were extracted. A complete description of the additional PJT and athletes’ characteristics have been previously published [33
2.5. Methodological Quality in Individual Studies
The methodological quality of eligible studies was assessed using the Physiotherapy Evidence Database (PEDro) scale, as previously described [34
] and interpreted for PJT literature [28
]. Briefly, studies with ≤3 points were considered of poor quality, 4–5 points as moderate quality, and 6–10 points as high quality. Two of the authors (RRC and MSG) independently scored the articles. Disagreements in the rating between both authors was resolved through discussion with a third author (JSS). Aiming to control the risk of bias between authors, the Kappa correlation test was used to analyse the agreement level for the included studies. An agreement level of k = 0.89 was obtained.
2.6. Summary Measures and Synthesis of Results, and Publication Bias
For analysis and interpretation of results, we followed previous recommendations for PJT meta-analyses [26
]. Briefly, using a random-effects model meta-analyses were conducted only if ≥3 studies provided means and standard deviations for the same pre-post PJT parameter (e.g., sprint time), in order to calculate an effect size (ES; Hedges’ g ES) alongside their respective 95% confidence intervals (CIs). Data was standardized using post-intervention standard deviation score. Calculated ES were interpreted as previously recommended for sport sciences studies: <0.2, trivial; 0.2–0.6, small; >0.6–1.2, moderate; >1.2–2.0, large; >2.0–4.0, very large; >4.0, extremely large [38
]. For studies that incorporated ≥2 intervention groups and only one control group, the sample size in the control group was proportionately divided during analyses [39
]. Heterogeneity was assessed using the I2
statistic, with values of <25%, 25–75%, and >75% considered as representative of low, moderate and high heterogeneity, respectively. The risk of bias was analyzed with the Egger’s test [40
]. To adjust for publication bias, a sensitivity analysis was conducted using the trim and fill method [41
], with L0 as the default estimator for the number of missing studies [42
]. Subgroup analyses to assess the potential influence of PJT duration (number of weeks), training frequency (number of sessions per week), total number of training sessions, in addition to the age, the expertise level of the participants (i.e., moderate-level vs. high-level players), and the athletes years of soccer experience were performed according to the median split technique. Analyses were performed using specialized software (Comprehensive Meta-Analysis; version 2; Biostat, Englewood, NJ, USA). Statistical significance was set at p
3.1. Study Selection
The electronic search process identified 7206 studies (2261 from PUBMED, 2280 from SCOPUS, and 2665 from WOS), plus 24 studies through other sources. Duplicate studies were then removed (n = 4761). Study titles and abstracts were screened with a further 2020 studies removed. Accordingly, full-text versions of 449 studies were screened. From these, 224 studies did not include an appropriate study design (e.g., control group), 188 studies did not include soccer players or female soccer players only, and 27 were excluded for different reasons (e.g., no measure of physical fitness provided). The remaining 10 studies [43
] were included in the SRMA (Figure 1
). The included studies involved 13 individual experimental groups and 140 participants, and 110 participants in the 10 control groups. The characteristics of the participants and PJT interventions are indicated in the Table 1
and Table 2
. Briefly, the age of the participants was between a mean of 13.4 to 26.6 years, with a fitness level that varied from recreationally trained to professional athletes. The PJT interventions lasted between six up to 12 weeks. Almost all studies (except one) incorporated PJT during the in-season period. A complete description of the physical fitness measures used in the meta-analyses is provided in Table 3
3.2. Methodological Quality
Using the PEDro checklist, five studies achieved 4 or 5 points and were classified as being of “moderate” quality, while five studies achieved 6–10 points and were therefore considered as being of “high” methodological quality (Table 4
3.3. Meta-Analysis Results for Countermovement Jump
Seven studies provided data for CMJ, involving 10 experimental and seven control groups (pooled n
= 182). There was a significant effect of PJT on CMJ (ES = 0.71; 95% CI = 0.20 to 1.23; p =
= 62.9%; Egger’s test p
= 0.224; Figure 2
A). The relative weight of each study in the analysis ranged from 6.9% to 13.3%.
No significant sub-group difference (between-group p = 0.188) was found when PJT interventions with ≤6 weeks (5 study groups; ES = 0.40; 95% CI = −0.01 to 0.80; within-group I2 = 0.0%) were compared to PJT interventions with >6 weeks (5 study groups; ES = 1.18; 95% CI = 0.09 to 2.28; within-group I2 = 81.1%).
Similarly, no significant sub-group difference (between-group p = 0.664) was found when PJT interventions with players ≤22.8 years old (4 study groups; ES = 0.62; 95% CI = 0.17 to 1.07; within-group I2 = 0.0%) were compared to PJT interventions with players >22.8 years old (6 study groups; ES = 0.84; 95% CI = −0.04 to 1.71; within-group I2 = 76.6%).
Moreover, no significant sub-group difference (between-group p = 0.080) was found when PJT interventions with high-level players (4 study groups; ES = 1.41; 95% CI = 0.26 to 2.56; within-group I2 = 85.3%) were compared to PJT interventions with moderate-level players (6 study groups; ES = 0.30; 95% CI = −0.14 to 0.75; within-group I2 = 0.0%).
Furthermore, no significant sub-group difference (between-group p = 0.420) was found when PJT interventions conducted on players with ≤5.7 years of soccer experience (4 study groups; ES = 1.42; 95% CI = −0.05 to 2.89; within-group I2 = 83.8%) were compared to PJT interventions conducted on players with >5.7 years of soccer experience (5 study groups; ES = 0.40; 95% CI = −0.01 to 0.80; within-group I2 = 0.0%).
3.4. Meta-Analysis Results for Countermovement Jump with Arm Swing
Three studies provided data for CMJA, involving three experimental and three control groups (pooled n
= 88). There was a non-significant effect of PJT on CMJA (ES = 0.41; 95% CI = −0.34 to 1.15; p
= 0.28; I2
= 65.3%; Egger’s test p
= 0.452; Figure 2
B). The relative weight of each study in the analysis ranged from 25.4% to 37.8%.
3.5. Meta-Analysis Results for Drop Jump
Six studies provided data for DJ, involving nine experimental and six control groups (pooled n
= 154). There was a significant effect of PJT on DJ (ES = 0.79; 95% CI = 0.12 to 1.47; p =
= 73.1%; Egger’s test p
= 0.063; Figure 2
C). The relative weight of each study in the analysis ranged from 6.3% to 14.0%.
3.6. Meta-Analysis Results for Kicking Performance
Three studies provided data for kicking performance, involving four experimental and three control groups (pooled n
= 59). There was a significant effect of PJT on kicking performance (ES = 2.24; 95% CI = 0.13 to 4.36; p =
= 89.4%; Egger’s test p
= 0.040; Figure 3
). After the trim and fill method, the adjusted values remained as the observed values. The relative weight of each study in the analysis ranged from 22.8% to 26.2%.
3.7. Meta-Analysis Results for Linear Sprint
Seven studies provided data for linear sprint performance, involving 10 experimental and seven control groups (pooled n
= 186). There was a significant effect of PJT on linear sprint performance (ES = 0.79; 95% CI = 0.39 to 1.18; p <
= 38.2%; Egger’s test p
= 0.257; Figure 4
A). The relative weight of each study in the analysis ranged from 5.4% to 15.3%.
No significant sub-group difference (between-group p = 0.167) was found when PJT interventions with ≤6 weeks (six study groups; ES = 0.62; 95% CI = 0.24 to 1.00; within-group I2 = 0.0%) were compared to PJT interventions with >6 weeks (four study groups; ES = 1.30; 95% CI = 0.41 to 2.20; within-group I2 = 63.9%).
Similarly, no significant sub-group difference (between-group p = 0.545) was found when PJT interventions with players ≤21.45 years old (five study groups; ES = 1.02; 95% CI = 0.25 to 1.79; within-group I2 = 65.3%) were compared to PJT interventions with players >21.45 years old (five study groups; ES = 0.75; 95% CI = 0.33 to 1.16; within-group I2 = 0.0%).
3.8. Meta-Analysis Results for Change of Direction Speed
Five studies provided data for CODS performance, involving eight experimental and five control groups (pooled n
= 144). There was a significant effect of PJT on CODS performance (ES = 0.73; 95% CI = 0.39 to 1.06; p <
= 0.0%; Egger’s test p
= 0.813; Figure 4
B). The relative weight of each study in the analysis ranged from 6.3% to 25.8%.
3.9. Meta-Analysis Results for Endurance
Five studies provided data for endurance performance, involving eight experimental and five control groups (pooled n
= 150). There was a significant effect of PJT on endurance performance (ES = 0.60; 95% CI = 0.09 to 1.10; p =
= 53.7%; Egger’s test p
= 0.328; Figure 5
A). The relative weight of each study in the analysis ranged from 10.0% to 17.3%.
3.10. Meta-Analysis Results for Anaerobic Performance
Three studies provided data for anaerobic performance, involving five experimental and three control groups (pooled n
= 89). There was a non-significant effect of PJT on anaerobic performance (ES = 0.36; 95% CI = −0.06 to 0.77; p =
= 0.0%; Egger’s test p
= 0.121; Figure 5
B). The relative weight of each study in the analysis ranged from 13.5% to 39.2%. Of note, from a pool of 48 potential moderator analyses, due to a limited number of studies (i.e., <3 per moderator), only six moderator analyses were possible (as indicated above).
3.11. Adverse Effects
None of the included studies reported soreness, pain, fatigue, injury, damage, or adverse effects related to the PJT intervention.