Soccer is a very popular sport worldwide and the number of women playing this game has grown extensively in recent years. Currently, there is an aim to reach 60 million female players by 2026 [1] but despite this high projected growth, relatively little scientific evidence has emerged on female soccer players, most particularly youth female players [2].

To increase the potential for success in soccer, players require high levels of physical fitness [3]. The sport is now played at a greater pace than at any time in the past with players requiring a very high standard of physical fitness to meet the demands of competition. Data collected across seven seasons have indicated increased (up to 63%) high-intensity running distances as well as a greater amount of explosive sprints in competitive games [4]. Of relevance to the current study, these trends have also been observed in female players indicating an increased physical demand in women’s soccer [3]. Actions such as linear sprints (i.e., 45%), jumps (i.e., 16%) and change-of-direction sprints (CODS; i.e., 6%) are common actions preceding goal-scoring opportunities [5], indicating their importance in underpinning successful play. Interestingly, lower-limb power and jumping ability have been shown to be significantly related to a team’s final league standing (unstandardized beta = -0.36-0.31, P=0.012-0.009) [6] thus indicating their importance in underpinning success in the sport. Moreover, the maintenance of repeated short-duration maximal and near-maximal actions across a 90-minute game are also considered to be important [3] with increased endurance enabling players to execute a greater number of maximal-intensity actions during play [7]. Furthermore, physical fitness characteristics such as repeated-sprint ability (RSA), linear running speed (e.g., 10-30-m), strength, and muscular power of the lower limbs (e.g., vertical jump height) may discriminate between soccer players of different competitive levels [8], potentially serving as a basis for coaches in the selection of teams and squads.

A plethora of different training methods reported in the scientific literature aimed to improve soccer players’ physical fitness levels. One such method is jump training which offers unique benefits to the player. Jump training is a highly versatile method that can be conducted within a relatively small physical space and with no equipment. This may be an important advantage during certain scenarios in which athletes may be forced to train at their homes [9], as was the case during the recent Covid 19 pandemic. Moreover, jump training may be considered more fun and engaging than other training methods (e.g., flexibility, endurance), particularly among younger soccer players [10]. Importantly, jump-training exercises can mimic the unique short-duration high-intensity actions of soccer, potentially increasing the transfer of performance effects from training to competitive play due to the principle of specificity [11]. On this, jump training has previously been shown to be beneficial for a myriad of physical fitness outcomes such as jumping itself, linear sprinting, agility and CODS, RSA with and without a COD, short-term endurance, long-term endurance, maximal strength, balance, kicking speed, range of motion and coordination [12-14].

Despite the above-mentioned benefits of jump training, the evidence relating to the effects of this type of activity on the physical fitness of youth female soccer players is very limited due to numerous methodological and study design issues. In one study in youth female soccer players (age, 13.4 years) [15], after 12 weeks of jump training, participants improved kicking distance and vertical jump height. However, this study included no randomization procedure. In another study [16], youth female soccer players (age, 16.5 years) undertook a 10-week in-season training program to improve physical fitness. However, the program incorporated jumps, resistance training and high-intensity anaerobic sprints, making the identification of an isolated effect of jump training impossible. Similarly, another study [17] in youth female soccer players (age, 15.4 years) incorporated six weeks of an anterior cruciate ligament injury prevention program which included jumps mixed with resistance training, COD, and core exercises.

With the above limitation in mind, we sought to carry out the first randomized-controlled trial that assessed the isolated effect of a jump training program on physical fitness in youth female soccer players. Our aim was to compare the effects of jump training with soccer training alone. We hypothesized that there would be significant improvements in the physical fitness of youth female soccer players following jump training, with no such changes observed in the control group.

Of the subjects considered for data analyses, all received the treatment as allocated. Neither group reported any changes in their training habits during the four week intervention period, as compared to their training routine prior to the study.

There were no significant differences between the groups in any descriptive- (Table 1) or fitness-based variables (Table 3) at baseline. The group, time, and group by time interaction effects are displayed in Table 3.

No significant changes were noted in the control group in any of the dependent variables although small effect sizes were observed in CMJ (ES=0.33) and 4BT (ES=0.27). In contrast, the jump training group demonstrated significant improvements in CMJ (p=0.001; ES=0.85), 4BT (p=0.002; ES=1.01), and MKV (p=0.027; ES=0.77), with small to medium effect sizes observed in the 20-m sprint (p=0.069; ES=0.59) and Yo-Yo IR1 (p=0.299; ES=0.20) tests.

The aim of this study was to evaluate the effects of a jump training program, as compared to standard soccer training, on measures of physical fitness youth female soccer players. To the best of our knowledge, and based on the characteristics of the extant evidence on this particular topic, our research represents a novel approach to determining the benefits of jump training for young female players. Indeed, in a recent jump training scoping review, which included 420 articles, just three studies undertaken in a youth female soccer player population were identified [13]. Of these studies, none featured a randomized-controlled design that could determine the isolated effect of a jump-training program on youth female soccer player’s physical fitness. Further, in two recent meta-analyses, on the effects of jump training on female soccer players [24, 27], just two studies included youth players both also failed to adopt adequate randomization protocols.

In the context of the above evidence, our study is probably the first randomized-controlled trial which can quantify the isolated effects of a jump program on youth female soccer players’ physical fitness levels in the absence of other forms of strength and conditioning training. Previous studies conducted on youth females [15] combined jumping with other training methods (e.g., traditional resistance training) making it impossible to isolate the effect of the former from the latter. The current findings are therefore novel and offer valuable and scientifically sound data for practitioners to leverage in the field. On this, our results support the use of jump training to induce improvements in a wide array of performance tests, such as CMJ, 4BT, 20-m sprint, MKV, and Yo-Yo IR1, in this population. In contrast, regular soccer training induced only small improvements that were restricted to CMJ and 4BT only. This is important information for soccer and strength and conditioning coaches alike who should collaborate closely so that both technical and fitness-based training can be integrated into players’ programs for maximal performance benefits. The improvements in performance that we observed might be mediated by differing physiological adaptations such as improvements in neuromuscular-related functioning of the stretch-shortening cycle (SSC) [28], neural drive to the agonist muscles [29], optimization of relative force generated per motor unit recruited [28], leg muscle qualities (e.g., stiffness) [30], the direction of force production and application (as a result from physiological adaptations) [31] and running economy [32].

The jump training group improved jumping performance with medium effects size observed in the CMJ and 4BT. The beneficial effects of jump training on jumping ability in youths have previously been reported [33]. However, a lack of clarity in how the aforementioned neuromuscular improvements induced by jump training [29] are exhibited in females of different maturation status, in addition to the confounding effects of maturation-related changes on the SSC function [28], means that explanations of the effects are speculative and more research is needed to elucidate underlying mechanisms. Similarly, the beneficial effects of jump training on sprint performance have been previously reported, most notably after interventions in youths that incorporated horizontally oriented jumping movements [22], as in the current study (~40% of all jump exercises). Improvements of this nature may be related to an individual’s ability to apply greater force in a horizontal direction [31] in addition to other factors [34] such as increased muscular power, muscle composition and neuromuscular adaptations. Indeed, a recent meta-analysis [35] suggested that horizontal jumps could be superior to vertical jumps because there is both a horizontal and a vertical component to the former, but only a vertical component to the latter. Of note, the jump training group demonstrated a borderline small to moderate improvement in sprint time as compared to the control group which demonstrated no change over the four week period. Considering that speed-related athleticism is a key trait for optimal long-term athletic development, its improvement may contribute to youths’ health and fitness, physical performance, injury resistance, confidence and competence [36]. Particularly relevant for soccer players, greater sprinting capability might help individuals to cope with the greater speed demands of modern competitive soccer [4]. Moreover, with linear sprinting being the most common action (i.e., 45%) preceding a goal-scoring scenario [5], its importance in underpinning successful soccer play must be acknowledged by coaches.

Although a considerable number of studies have previously reported increases in MKV after jump training programs, most of these have focused on the kicking ability of male players [37, 38], with the age of the participants ranging from 10.3 to 15.5 years, and the training programs lasting between 6 to 12 weeks. Increases in kicking velocity of ~7.0-16.0% and kicking distance of ~11.0-37.0% have been reported. Larger changes were observed after interventions that incorporated progressive load increases (e.g., volume) [39], a combination of vertical and horizontal and unilateral and bilateral jumps [22], and jumps that were loaded with external resistance [38, 40]. Similar findings were noted in youth female [15], young adult female [41] and young adult male soccer players [42], with significant improvements in kicking velocity (8-13%) and kicking distance (~23%) after eight to twelve weeks of training. For those studies that reported improvements in kicking performance for the dominant (7-12%) and non-dominant legs (8-13%), similar improvements were observed between limbs [42, 43]. Overall, the extant evidence supports our own findings with effect sizes in MKV being substantially larger in youth female soccer players in the experimental group (ES = 0.77) as compared to their control group counterparts (ES = -0.07). Such adaptations may offer advantages during competitive play with an apparent transfer of training effect to soccer-specific skill performance in young female players. Despite this encouraging result, the underlying mechanisms of these adaptations are not clear, with further research required for clarification of this element of training.

In relation to endurance performance in youth male soccer players, some studies have reported no significant changes in Yo-Yo IR1 test results (5.8-6.2%; ES = ~0.3) [44, 45]. This contrasts with other investigations which observed significant improvements in Yo-Yo IR1 test scores after jump training programs, in youth male players (~11-19%; ES = 0.27-0.41) [22, 39], and in adult female players (~10%; ES ~0.3) [41]. Although some contrasting findings have emerged in the literature, jump training seems to improve running endurance performance, particularly in tasks requiring a sudden change of direction with such movements requiring utilization of the SSC (e.g., Yo-Yo IR1 test). The above findings corroborate the observation in our study that jump training can improve youth female soccer players’ performance in the Yo-Yo IR1 test as compared to regular soccer training. However, the observed effect was of only small magnitude (ES = 0.20 vs. 0.03) in our investigation. Larger improvements in endurance may be achieved with jump training programs involving a progressive overload (e.g., volume-based overload) and a combination of bilateral and unilateral and vertical and horizontal jumps [13, 22, 39], all of which are key programing elements in the current study.

The current study does have its limitations. Our analysis is limited by the small sample size (n = 14) though it is important to emphasize that the statistical power analysis revealed that six participants per group provided 80% power (at α < 0.05) to detect an effect size of 0.2 when comparing changes in physical performance in youth soccer players. In addition, we did not incorporate more sophisticated physiological or biomechanical measures to and elucidate the underlying mechanisms related to the physical fitness improvements observed after the jump training program. Future studies should address this limitation with mechanistic measures of performance.

Based on these results, a multidimensional jump training program is effective in increasing different elements of physical fitness in youth female soccer players. The observed fitness changes relate to jump height, sprint speed, kicking ability and endurance. Our jump training program was comprehensive in that it incorporated all the characteristics of a multidimensional approach to enhancing SSC function and muscular power in athletic populations and adaptations were achieved in a very short timeframe. Coaches are advised to prescribe a combination of cyclical, acyclical, horizontal, vertical, unilateral and bilateral jumps using both short ground contact [<250 ms] and long ground contact [>250 ms] times to address the nature of the SSC as it is utilized in sport. With consideration of these components, coaches can prescribe jump training programs that address the diverse demands of soccer which requires players to execute dynamic actions that share many of the same movement characteristics of the aforementioned jump types.

In addition to the above, we emphasize the safety of jump training (i.e. no injuries occurred during the intervention) and the feasibility of carrying out two training sessions per week alongside regular soccer training. Our results demonstrate positive effects on relevant physical fitness measures for youth female soccer players and the various training techniques can easily be integrated into conventional training with minimal resources.