The Effects of Resistance Training on Muscular Fitness, Muscle Morphology, and Body Composition in Elite Female Athletes: A Systematic Review

Background Well programmed strength and conditioning training is an indispensable part of the long-term training process for athletes in individual and team sports to improve performance and prevent injuries. Yet, there is a limited number of studies available that examine the effects of resistance training (RT) on muscular fitness and physiological adaptations in elite female athletes. Objectives This systematic review aimed to summarize recent evidence on the long-term effects of RT or combinations of RT with other strength-dominated exercise types on muscular fitness, muscle morphology, and body composition in female elite athletes. Materials and Methods A systematic literature search was conducted in nine electronic databases (Academic Search Elite, CINAHL, ERIC, Open Access Theses and Dissertations, Open Dissertations, PsycINFO, PubMed/MEDLINE, Scopus, and SPORTDiscus) from inception until March 2022. Key search terms from the MeSH database such as RT and strength training were included and combined using the operators “AND,” “OR,” and “NOT”. The search syntax initially identified 181 records. After screening for titles, abstracts, and full texts, 33 studies remained that examined the long-term effects of RT or combinations of RT with other strength-dominated exercise types on muscular fitness, muscle morphology, and body composition in female elite athletes. Results Twenty-four studies used single-mode RT or plyometric training and nine studies investigated the effects of combined training programs such as resistance with plyometric or agility training, resistance and speed training, and resistance and power training. The training duration lasted at least 4 weeks, but most studies used ~ 12 weeks. Studies were generally classified as ‘high-quality’ with a mean PEDro score of 6.8 (median 7). Irrespective of the type or combination of RT with other strength-dominated exercise regimens (type of exercise, exercise duration, or intensity), 24 out of 33 studies reported increases in muscle power (e.g., maximal and mean power; effect size [ES]: 0.23 < Cohen’s d < 1.83, small to large), strength (e.g., one-repetition-maximum [1RM]; ES: 0.15 < d < 6.80, small to very large), speed (e.g., sprint times; ES: 0.01 < d < 1.26, small to large), and jump performance (e.g., countermovement/squat jump; ES: 0.02 < d < 1.04, small to large). The nine studies that examined the effects of combined training showed significant increases on maximal strength (ES: 0.08 < d < 2.41, small to very large), muscle power (ES: 0.08 < d < 2.41, small to very large), jump and sprint performance (ES: 0.08 < d < 2.41, small to very large). Four out of six studies observed no changes in body mass or percentage of body fat after resistance or plyometric training or combined training (ES: 0.026 < d < 0.492, small to medium). Five out of six studies observed significant changes in muscle morphology (e.g., muscle thickness, muscle fiber cross-sectional area; ES: 0.23 < d < 3.21, small to very large). However, one study did not find any changes in muscle morphology (i.e., muscle thickness, pennation angle; ES: 0.1 < d < 0.19, small). Conclusion Findings from this systematic review suggest that RT or combined RT with other strength-dominated exercise types leads to significant increases in measures of muscle power, strength, speed, and jump performance in elite female athletes. However, the optimal dosage of programming parameters such as training intensity and duration necessary to induce large effects in measures of muscular fitness and their physiological adaptations remain to be resolved in female elite athletes.


Introduction
Success in modern sports is the ultimate goal for both athletes and coaches. Adequate programming of resistance training (RT) regimens is needed complementary to sportspecific training to develop the physical fitness and athletic performance of youth and elite athletes [1,2]. During longterm athlete development, there should be specific focus on the promotion of muscular fitness, irrespective of the individual's maturational status and sex [3]. Sufficient levels of muscular fitness are the foundation for motor skill learning and enable athletes to tolerate the demands of long-term training and competition [4,5]. Smith and colleagues [6] introduced muscular fitness as an umbrella term for muscle strength, muscle power, and local muscular endurance. The effectiveness of various RT types (e.g., free-weight training, machine-based training, plyometric) on measures of physical fitness and sport-specific performance is well established for youth [5,[7][8][9] and elite athletes [10,11]. However, most of the available research has been conducted with males and less with females [12,13].
Currently, most systematic reviews of RT variables have been performed only on male or mixed-sex samples [10,14]. Hagstrom et al. [15] stated that there were no reviews conducted specifically for females, so the main purpose of their review was to quantify the effects of RT in females and summarize the existing literature by gender. Because males and females differ in circulating anabolic hormones (e.g., testosterone), anatomy (e.g., limb length and pelvic angles, muscle size, and body composition), and physiology (e.g., cardiorespiratory fitness, metabolic factors, fatigability, and inflammatory responses), there may be differences in how females adapt to RT [16].
The review by Hagstrom et al. [15] provided evidencebased estimates of women's fitness for RT. In their review, an average lean body mass gain of 3.3% was observed, which equates to approximately 1.45 kg (range 0.4-3.3 kg) following a full body program. Muscle strength increased approximately 25% (range 4-40%) in the upper body and 27% (range 6.5-54%) in the lower body. These adjustments occurred after participation in a program with an average duration of 15 weeks. Typically, prescribed parameters include a frequency of three training sessions per week and three sets of approximately ten repetitions per exercise. When intensity is expressed as a percentage of 1RM, the average exercise intensity was 70%.
In the form of original research, Montalvo-Pérez et al. [13] examined the effects of 6 weeks of short-term velocitybased RT compared with traditional RT (i.e., three sets per exercise interspersed by 120-s rest periods, with intensity progressively increasing from 80 to 90% of 1RM from the start to the end of the intervention period) on body composition, muscle strength/power, and endurance in competitive female cyclists. The results showed marked improvements in muscle strength/power as well as a slight increase in time trial performance (∼3-5%), with no differences between interventions but with short-term velocity-based RT inducing greater increases in maximum strength/power for the hip thrust exercise [13]. Moreover, Lesinski et al. [7] examined the seasonal effects of strength endurance training versus power training on measures of physical fitness and body composition in young female soccer players. While strength endurance training showed significantly better ventral core strength (plank test) and change-of-direction performance (T-test), power training resulted in significantly better 1RM leg press, vertical jump, and linear sprint performances. The authors concluded that strength endurance training and power training complement each other and can be implemented in young female soccer players.
Using a meta-analytical approach, Moran and colleagues [8] described the effects of RT on muscle strength in female youth. Based on 11 included studies, the authors reported small overall effects. Effect sizes were larger in girls aged > 15 years. In another meta-analysis, the same authors [9] examined the effects of plyometric training (jump training) on measures of muscle strength in youth females. Fourteen studies were included, and the authors showed overall small training effects for vertical jump performance. For plyometric training, effect sizes were larger for girls aged < 15 years. While these studies provide helpful but preliminary information on the effectiveness of RT with (youth) females, the main body of strength and conditioning research focuses on males (athletes) [10,16].
When systematic reviews on RT were performed in female cohorts only, they primarily dealt with clinical outcomes such as breast cancer lymphedema [10] or bone mineral density [17]. Not surprisingly, the main finding of the review performed by Hagstrom et al. [15] was that RT had a significant effect on muscle strength and hypertrophy in untrained healthy adult women. However, the literature included in the meta-analysis was of moderate quality [15]. Therefore, a review of the existing but limited literature on the effects of RT on muscular fitness and sport-specific performance in women seems timely and necessary, especially for elite female athletes.

Search Strategy
This systematic review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) Statement [18] and the Cochrane Handbook for Systematic Reviews of Interventions [19]. The study protocol was registered in the Open Science Framework (OSF) platform (https:// doi. org/ 10. 17605/ OSF. IO/ 83KD6). The PICOS approach (Population, Intervention, Comparator, Outcomes, Study design) was followed to identify inclusion criteria (Table 1).

Eligibility Criteria
Only randomized controlled trials and controlled trials that examined the long-term effects of RT or combinations of RT with other strength-dominated exercise types on muscle strength and power, muscle morphology, and body composition in highly trained female athletes were eligible for inclusion. Studies were included in the current systematic review if they were in accordance with the following criteria: (i) published in peer-reviewed journals; (ii) included elite female athletes and highly trained women according to the definition of McKay et al. [20] (i.e., highly trained athletes that exercise at least four times per week and compete at national or international level in their sport); (iii) used validated methods of monitoring training load; (iv) used RT or combinations of RT with other strength-dominated exercise types lasting a minimum of 4 weeks; (v) applied measures of muscular fitness or muscle morphology as outcome parameters. Studies were excluded if they (i) did not meet the minimum requirements of an experimental study design (e.g., case reports); (ii) did not meet the minimum requirements regarding study design (e.g., lack of information on training methodology or testing sessions); (iii) did not apply RT or combinations of RT with other strength-dominated exercise types; (iv) were not written in English; (v) involved untrained participants; or (vi) included male participants. Moreover, review articles of any type were not included in the current systematic review.

Literature Search Strategy
This systematic literature search was conducted in nine electronic databases: Academic Search Elite, CINAHL, ERIC, Open Access Theses and Dissertations, Open Dissertations, PsycINFO, PubMed/MEDLINE, Scopus, and SPORTDiscus, from database inception until March 2022. The following key terms (and synonyms searched for in the MeSH database) were included and combined using the operators "AND", "OR", "NOT": (("strength training" OR "resistance training" OR "plyometric training" OR plyometrics OR "resistance exercise" OR "weight lifting" OR weightlifting OR "strength exercise" OR "weight-bearing exercise" OR "resistive exercise" OR "resistive training") AND ("muscular fitness" OR "muscle fitness" OR "muscle strength" OR "muscular strength" OR strength OR "muscle power" OR "muscular power" OR power OR "muscular endurance" OR "muscle endurance" OR "local muscular endurance" OR endurance) AND ("muscle hypertrophy" OR "muscular hypertrophy" OR "muscle mass" OR "muscle fiber" OR "muscle size" OR "muscle fibre" OR "muscle thickness" OR "cross sectional area" OR "computed tomography" OR "magnetic resonance imaging" OR "pennation angle") AND ("lean body mass" OR "fat-free mass" OR "body mass index" OR "BMI") AND ("trained women" OR "trained female" OR "elite female athletes" OR "elite women athletes")). In addition, the reference lists and citations (Google Scholar) of the identified studies were explored in order to detect further relevant research papers. Since the scope of this review article is broad in terms of outcome measures (e.g., muscle strength, muscle power, muscle morphology, and body composition), a systematic review and not a metaanalysis were performed since a large number of outcome parameters would have produced substantial heterogeneity.

Working Definitions
-RT is a collective term that refers to methods of physical conditioning that involve the progressive use of a wide range of resistive loads, different movement velocities, and a variety of training modalities [21]. -Plyometric training consists of quick, powerful actions that involve muscle lengthening, immediately followed by rapid shortening of the same muscle in the stretchshortening cycle [22]. -Muscular strength can be defined as the maximal force or tension a muscle or a group of muscles can generate at a specified velocity [5].
-Elite female athletes refer to well-trained athletes, i.e., highly trained athletes that trained at least four times per week and competed at the national or international level in their sport [20].

Study Selection and Data Extraction
The final screening was done by two investigators (AZ and MD) based on the relevance of the inclusion and exclusion criteria and the identified items for assessing the long-term effects of RT on muscular strength and power, muscle morphology, and body composition in female elite athletes using PICOS criteria. If the title showed any potential relevance, it was screened at the abstract level. When abstracts indicated potential inclusion, full-text articles were reviewed. Furthermore, the researchers independently analyzed the full texts and determined the eligibility of the studies, and disagreements were resolved by consensus. The agreement rate between reviewers was 93% for the eligibility criteria of the study. A third-party consensus meeting was held with a third author (HZ) if the two reviewers were not able to reach an agreement on the inclusion of an article.

Assessment of Risk of Bias and Quality Assessment
The methodological quality of the included studies was assessed using the Physiotherapy Evidence Database (PEDro) scale (https:// PEDro. org. au/), which has been shown to have good reliability and validity [23][24][25]. The PEDro scale has 11 possible points that examine external validity (criterion 1) and internal validity (criteria 2-9) of controlled trials and whether there is sufficient statistical information for interpreting results (criteria [10][11]. The items of the scale are (i) eligibility criteria were specified; (ii) subjects were randomly allocated to groups; (iii) allocation was concealed; (iv) groups were similar at baseline; (v) subjects were blinded; (vi) therapists who administered the treatment were blinded; (vii) assessors were blinded; (viii) measures of key outcomes were obtained from more than 85% of subjects; (ix) data were analyzed by intention to treat; (x) statistical comparisons between groups were conducted; and (xi) point measures and measures of variability were provided. The first criterion is not included in the final score. Moreover, because of the nature of physical activity interventions, patient and therapy blinding and allocation is unlikely, therefore the total score a trial could receive was 8 points. A cut-off point of 6 on the PEDro scale was used to indicate high-quality studies, as this has been reported to be sufficient to determine high quality versus low quality in previous studies [25]. Two independent researchers (AZ and MD) assessed the quality of the studies; if any ambiguity arose, a third researcher (HZ) was contacted and a unanimous decision was achieved.

Data Analysis
Absolute changes (final value versus initial value) in performance indicators of RT on muscular strength and power, muscle morphology, and body composition in female elite athletes were reported as differences between arithmetic means before and after the interventions. The results of intention-to-treat analyses were always used when available in the selected studies. Calculations were performed using a random-effects model. The results of the exercising groups were compared with the results of the control group (non-exercise). For all variables, in the comparisons between exercising groups (e.g., two or more training modes), the studies were stratified by considering the presence of equalization. A study was considered equalized when the design was adopted for different training programs with similar energy expenditures or similar workloads. This information should be mentioned in the article or presented in the results. When this information was not presented, the study was classified as non-equalized. The level of significance was set at 5%. For statistical heterogeneity of the treatment effect between the studies, a threshold p-value of 0.1 estimated by the Cochran Q test Measures of muscle strength (e.g., 1-repetition maximum [1RM], maximum voluntary contraction), proxies of muscle power (e.g., vertical and horizontal jump performance), muscle morphology (e.g., muscle mass, muscle size, muscle fiber), and body composition (e.g., body mass, body fat, lean mass) Study design nRCTs, nRnCTs, and RCTs was rated as statistically significant. For heterogeneity, values > 50% in the inconsistency I 2 test were considered indicative of high heterogeneity. Because some studies had more than one training group with a single control group, this shared control group was divided into two or more groups with smaller sample sizes and was weighted in relation to the different exercise interventions. This procedure was performed to obtain independent comparisons and overcome a unit-of-analysis error for studies that could contribute to multiple and correlated comparisons, as suggested by the Cochrane Handbook for Systematic Reviews of Interventions [19]. Transformation methods were used for studies that presented results as the standard error of the mean, confidence intervals, or interquartile ranges [26]. Data not available and not made available by the corresponding author were imputed. In those situations, the weighted average of all available studies for the variable in question was considered. To conduct the multiple comparisons (different exercising groups vs control), a network model was adopted. For this, the weighted average of all available studies was considered for group imputation.
To strengthen the robustness of the findings, sensitivity analyses were performed by deleting each study separately to analyze the influence of each study on the overall results. All analyses were performed using REVIEW MANAGER software, version 5.3 (Cochrane Collaboration, London, UK).
The percent change (Δ%) was calculated (if not available in the study) for each study to evaluate the magnitude of the effects using the following equation: where M post represents the mean value after long-term training and M pre the baseline mean value.
Effect size (ES) was computed to present standardized effects of long-term training on the outcome variables (e.g., strength performances, muscle power, muscle morphology, and body composition). The ES was calculated with Cohen's d [27,28], by dividing the raw ES (difference in means) by the pooled standard deviations: Values for ES were defined as trivial (< 0.2), small (0.2-0.6), moderate (0.6-1.2), large (1.2-2.0), and very large > 2 [23,29]. Results for each outcome variable are presented with the number of observations (N), Δ%, and ES. Data analyses were processed using SigmaStat 3.5 software (Systat, Inc, USA). The ES and Δ% were analyzed in studies where sufficient data were available.

Study Selection
Our search identified 181 studies related to the effects of long-term RT or combinations of RT with other strengthdominated exercise types on muscular strength and power, muscle morphology, and body composition in female elite athletes (Fig. 1). After the screening of titles, abstracts, and full texts, 33 studies were selected for inclusion in our final analysis, and the characteristics of these long-term studies are summarized in Table 2. The 33 studies were carried out in different countries from five continents (Africa, North America, Europe, Asia, and Australia). A total of 738 participants aged 14-31 years performed RT and 220 participated in the control groups and completed the studies. Athletes from different sports were involved (athletics, cycling, basketball, handball, volleyball, softball, soccer, field hockey, swimming, cross-country skiing, and water polo), but the most represented sport was soccer, with 13 studies.
The 33 studies used different resistance exercise training protocols, with 24 studies using single-mode RT or plyometric training programs, and nine studies investigating the effects of combined training programs such as RT with plyometric or agility training, RT with speed training, and resistance and power training. Training duration ranged between 5 and 12 weeks with a mean duration of 9.4 weeks. According to the PEDro score, the selected studies were classified as 'high-quality' studies with a mean PEDro scale score of 6.8 (median 7) ( Table 3).

Effects of Long-Term Resistance Training on Body Composition and Muscle Morphology in Elite Female Athletes
Eleven studies investigated the long-term effects of RT or combinations of RT with other strength-dominated exercise types on body composition and muscle morphology in elite female athletes (Table 5). Four out of six studies observed no changes in body mass or percentage of body fat (ES: 0.026 < d < 0.49, small to medium) after 10 weeks of resistance, plyometric, and agility training in soccer players [41], or after 12 weeks of RT in basketball players [40], or 10 weeks of a plyometric training program in elite female soccer players [41], and/or after 11 weeks of RT in endurance-trained athletes [42]. However, Skattebo et al. [43] reported an increase in body mass (2.5 ± 1.2%) after 10 weeks of RT in well trained young cross-country skiers. Siegler et al. [44] reported a decrease in mass fat (− 1.40 ± 1.47 kg, ES = 0.313) of 17 female soccer players who completed 10 weeks of in-season, plyometric, resistive training and a high-intensity anaerobic program. Five out of six studies [37,42,[44][45][46][47][48] showed significant training-induced changes in muscle morphology (e.g., muscle thickness, muscle fiber cross-sectional area) (ES: 0.23 < d < 3.21, small to very large). However, one study could not detect any changes in muscle morphology (i.e., muscle thickness, pennation angle) [32].

Discussion
To the best of our knowledge, this is the first systematic review that reports the long-term effects of RT or combinations of RT with other strength-dominated exercise types on muscular fitness, muscle morphology, and body composition in female elite athletes. Findings from this systematic review show that RT alone or in combination with other strength-dominated exercise types leads to significant changes in measures of muscle morphology, muscle power, strength, speed, and jump performance in elite female athletes. The question pertaining to the optimal programming parameters (e.g., frequency, intensity, volume) is still open and should be addressed in future studies.

Muscular Strength Improvements With Resistance Training
Despite the general agreement that prescribing various types of RT, alone or combined with other exercise types, could improve parameters, especially strength, power, linear sprint speed, and jump performance in athletes [22], only a few studies have explored whether such interventions and types of RT were beneficial for elite female athletes. While RT has previously been shown to be effective in improving measures of muscle strength [13,31,37,49,50], power [13,31,37,50], and speed [31,50], such results were obtained by including studies with intervention durations ranging from 4 to 12 weeks. Interestingly, we observed that RT induced large gains in measures of maximal strength (i.e., 1RM, leg press, half squat, and bench pull) in elite females. The maximum strength gain ranged from 8% to 18.9% in response to heavy resistance (lower body push, upper body push, and circuit of abdominal and low back exercises) after 8-12 weeks [50,51].
Initial training status plays an important role in the rate of progression during RT. Training status reflects a continuum

RM bench press
Effect size (Cohen's d) of adaptations to RT such that level of fitness, training experience, and genetic endowment each contribute. Quantification of strength gains appears to occur within a few months of training. Changes in strength are most pronounced early in training when the 'window of adaptation' is greatest [54]. Short-term studies (i.e., 6-24 weeks) have shown that the majority of strength gains occur within the first 4-8 weeks [55]. A limited number of studies have examined different models of progression during long-term RT. However, little is known about the adaptations and improvements in strength in response to prolonged training in elite female athletes. The rate of strength gains varies considerably between untrained and trained individuals, with trained individuals showing much slower rates of improvement [56]. A review of the literature suggests that muscle strength increases by approximately 40% in 'untrained', 20% in 'moderately trained', 16% in 'trained', 10% in 'advanced', and 2% in 'elite' individuals over periods ranging from 4 weeks to 2 years [57].

AƩene et al. [60]
Chimera et al. [46] KrisƟčević et al. [62] Maciejczyk et al. [30] Pacholek et al. [36] Shalfawi et al. [52] Usman and Shenoy. [67] Vikmoen et al. [48] AƩene et al. [60] Fischeƫ et al. [63] KrisƟčević et al. [62] Maciejczyk et al. [30]  Therefore, longer-term RT studies are needed to determine the upper limits of the dose-response relationship between training volume and muscular adaptations [58]. Discrepancies between studies remain unclear, but it appears that the dose-response relationship is more pronounced in resistance-trained individuals. It is not clear whether regular training at maximal loads promotes a superior strengthrelated response to this metric and, if so, how much loading should be incorporated into a comprehensive training program to optimize results. In contrast, trained individuals may require a greater stimulus (e.g. heavier load or greater intensity of effort) to continue to make positive adaptations [58].
Research in highly trained individuals on this topic is lacking, but it seems likely that continued improvements in maximum strength will become increasingly dependent on training closer to an individual's 1RM as they approach their genetic ceiling. Indeed, there is evidence to suggest that the principle of specificity (also known as a specific adaptation to imposed demands) becomes more relevant as the level of training experience increases [59]. Further study is warranted in elite athletes to better understand how training experience impacts the acquisition of strength with respect to the magnitude of the load. It is not clear whether regular training with maximal loads promotes a superior strength-related response on this metric and, if so, how much loading should be integrated into a comprehensive training program to optimize results.
Many sports performance coaches use periodized programs when preparing athletes for muscular anatomical adaptations prior to competition. Thus, future studies should evaluate muscular adaptations in periodized programs, particularly in trained athletes over a long duration. This type of study would simulate adaptations that occur as athletes progress through various phases of competition, such as inseason, post-season, and off-season.

Muscular Power Improvements with Resistance Training
With regards to the outcome of muscle power, our analyses including 15 studies revealed larger training-induced adaptations following plyometric training compared with RT in elite females. Examples of plyometric exercises included explosive jumps, hops, bounds, and skips. Possibly owing to the demand for higher force production at higher velocities, plyometric training has been shown to exhibit a large advantage over RT for improvements in power (7.2-15.4%) [41,60], speed (2.9-13.0%) [41,46,60,61] [35,46,51,[60][61][62][63][64][65][66][67][68]. Plyometric training may elicit adaptations in a wider range of physical qualities across the strength and power continuum in comparison with RT. Plyometric training appeared to be more effective in improving jump performance, whereas free-weight RT was more advantageous in improving maximum strength (where the stretch reflex is not involved). Typically, plyometric training is conducted over a period of several days or weeks (6-12 weeks), at a training frequency of 1-3 sessions per week, and a maximal to near-maximal intensity [68]. Despite the available evidence on the effectiveness of RT in female athletes, there is a need for further research [69][70][71]. This is because of the small number of studies (in comparison with the amount of research conducted on male athletes) and the heterogeneity of the applied study protocols which vary in duration, frequency, intensity, and volume.
Ramirez-Campillo et al. [71] indicated that, in future research, specific dose-response relationships following plyometric training should be identified. An interesting direction would be to determine the minimum duration of plyometric training in highly trained women. Meta-analyses conducted to date indicate that longer training durations (> 10 weeks) yield greater improvement in jump performance [67], and shortening the duration of plyometric training may be crucial in developing jumping performances.
It has been shown that plyometric exercise can also be effective when performed for only 4 weeks, instead of the typically applied periods of 6-12 weeks. Due to the usually short pre-season and long regular soccer season, coaches can prepare their training plan better and more effectively with shorter durations of plyometric training.
When comparing improvements in strength from RT and plyometrics, it may be suggested that RT results in superior strength gains. Despite these findings, there is no reported consensus highlighting the magnitude of differences between RT and plyometrics.
On the other hand, to establish practical applications and guidelines for researchers and practitioners employing and investigating these training methods, current recommendations for exercise prescription suggest combined RT because the benefits may provide an overall synergistic effect, and each intervention has overlapping and unique benefits [72]. Ten studies dealing with combined training, such as resistance, plyometric, and agility training; resistance and sprint training; resistance and power training; resistance and plyometric training [31][32][33][34][35][36][37][38][39]44], showed significant increases in maximal strength (lower and upper body strength, full squat 1RM), peak power, jump performance (squat jump and CMJ), and linear sprint speed (5-, 10-, 20-m and the Loughborough Intermittent Shuttle Test (LIST).
Several studies [33,34,37,38,44] seem to indicate that a combination of resistance and plyometric training is likely to elicit the greatest improvements (i.e., greater improvements in jump height due to plyometrics, as well as greater improvements in strength and sprint measures due to RT), especially if multiple outcome parameters are tested, and it is therefore recommended from a practical standpoint. Only one study [39] observed no change on the maximal isometric strength of knee flexion and extension (ES: 0.13 < Cohen's d < 0.26, trivial to small) after 10 weeks of combined strength and plyometric agility training in female soccer players. The purpose of the noted study was to determine the extent to which an off-season combined resistanceplyometric-agility training program would affect V O 2peak and running economy in collegiate female soccer players. Accordingly, a significant increase in V O 2peak (10.5%) was found, in the absence of a consistently significant increase in the RE at 9 km/h, after a 10-week training program. Furthermore, there was no significant change in maximal isometric strength of knee flexion or extension, which was likely due to the lack of testing specificity when associating isometric measures with dynamic functional athletic performances [73]. Such a relationship has been previously reported by several authors [73,74]; for instance, McKay et al. [20] found that absolute isometric peak torque was not correlated with jump height. These results revealed that plyometric/ sprint training-related jumping improvements were not due to increased maximal knee extension torque production.
Some controversial results exist concerning the 'transfer of training effect' from different methods of RT programs to various athletic performance parameters. A specific strength training method needs to be chosen based on the variables to be influenced. Knowledge of the effects of each method is crucial for the success of training with respect to physical and sports performance and in terms of preventing injuries [70].
Specifically, Harris et al. [75] showed that the combination model of training was effective for improving most of the strength tests compared with models of training-based force or high power. The characteristics of the combination model are not the same in these studies, as they are a variable combination of different methods for developing strength abilities, using their advantages to achieve the best force gradient.
For athletes who have experienced long-term RT, their power may increase to a higher level. However, it becomes complicated to increase the sizes and strength of other muscles. Adding a chain or elastic band to the free weights or changing the state of the body movement can provide a new stimulus for the muscles and improve the coordination between the muscles in the fight against unfixed resistances, thus improving the development of strength [76]. Future research should refine the training load, such as distinguishing between different-level trainers, and the proportion of variable resistance in the total load.

Body Composition Improvements with Resistance Training
Resistance training has several positive impacts on the bodies of athletes. The most obvious effect is related to the amount of muscle mass, which is represented by the muscle cross-sectional area. Adequately applied RT can lead to increases in maximal strength of > 20% after a 21-week training program [77][78][79]. A similar effect on muscle morphology (e.g., muscle thickness, muscle fiber cross-sectional area) has also been described for a RT program in females after 10-12 weeks of progressive RT [32,47,48]. Progressive RT in females can lead to a significant increase in strength and muscle mass. This may primarily be due to the observed adaptations in the central nervous system (efficient motor unit recruitment, firing frequency), especially during the first weeks of training [79,80]. In this context, it is plausible that neural adaptations in female athletes are more pronounced during the first weeks of RT for upper-body muscles [81,82] or lower-body muscles [83], resulting in higher relative strength gains.
Various types of RT may elicit acute and, in some cases, chronic hormonal changes, which appears to play an essential role in mediating hypertrophic signaling reactions [84]. The three most often studied hormones are insulin-like growth factor (IGF-1), testosterone, and growth hormone (GH). Accordingly, we regard these as practically and clinically relevant in terms of anabolic reactions and responses to RT [85,86].

Muscle Morphology Improvements with Resistance Training
Some studies [39,84], while monitoring relative muscle adaptation to different types of training, have shown that the ratio of the cross-section of muscle and neural adaptation provides the best indicator of force-velocity fitness, which focuses on explosiveness, maximum power, and velocity strength. Many studies have shown that the most effective method of enhancement of muscle power is the application of plyometric and/or free weight training consisting of maximal, submaximal, and light weights, as well as a combination of these methods [85,86].

Study Limitations
The results of the current systematic review have some limitations to note. In fact, in the current review, a systematic review approach was used with no meta-analysis performed, which should be considered by future investigators. The scope and approach of our review were broad, which is reflected by the large variety of training and testing protocols and the parameters measured. Moreover, some studies do not precisely report the training program (e.g., frequency, the load used, recovery periods, etc.) and as such that may have influenced the measured performance or muscle morphology. The heterogeneity of the methodological approach between studies is also represented by the different collection methods (application of apparatus) used to measure the outcomes. There is also a lack of data on some muscle morphology (e.g., muscle thickness, muscle fiber cross-sectional area) in many studies. Finally, there is great contemporary interest in the role of reproductive hormonal changes across the menstrual cycle to impact a women's training adaption and/or performance [87]. We did not address this point in this review and chose to approach the topic more from a sex comparison perspective. However, we acknowledge the role of the menstrual cycle on the responses to RT in women is a viable topic in need of further study.

Scientific Conclusion/Clinical and Practical Application
Findings from the current systematic review indicate that resistance or combinations of RT with other strength-dominated exercise types induce significant improvements in muscular fitness, speed, and jump performance in elite female athletes. However, the optimal dosage of RT intensity and duration necessary to produce the most effective results in this population remains unclear.

Declarations
Data Availability Statement All data supporting the findings of this study are available in this published article.
Funding Open Access funding enabled and organized by Projekt DEAL. The authors acknowledge the support of the Deutsche Forschungsgemeinschaft (DFG) and Open Access Publishing Fund of the University of Freiburg, Germany.

Conflicts of Interest
The authors declare that they have no conflicts of interest relevant to the content of this review. All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript. The authors have no financial or proprietary interests in any material discussed in this article.
Ethics approval and consent to participate Not applicable.

Consent for publication Not applicable.
Author Contributions Amira Zouita, and Hassane Zouhal were involved in the conceptualization of the study, data analysis, and the writing of the manuscript. Manel Darragi, Zouita Sghaeir, Mariem Bousselmi, Urs Granacher and Cain C.T. Clark were involved in the data assessment, data analysis, and the writing of the manuscript. Anthony C. Hackney and Urs Granacher were involved in the writing of the manuscript. All authors approved the final version of the manuscript.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.