The Effects of Loaded Plyometrics and Short Sprints in U19 Male Soccer Players in Tunisia

Featured Application: The investigation examined adding 8 weeks of bi-weekly loaded plyometric and short sprints training into standard training for U19 soccer players. We report that including bi-weekly loaded plyometric and short sprints training in standard training during the coronavirus-19 (COVID-19) pandemic improved the physical ﬁtness of young soccer players. Particularly, the ability to repeatedly change direction beneﬁted most from the intervention. Abstract: We investigated adding 8 weeks of bi-weekly loaded plyometric and short sprints (LPaSS) training into training for under 19 (U19) soccer players. An experimental group (EG, n = 18, age: 17.5 ± 0.58 years, body mass: 67.4 ± 4.37 kg, height: 1.76 ± 0.05 m, body fat: 11.4 ± 1.55%), and a control group (CG, n = 16, age: 17.5 ± 0.58 years, body mass: 68.7 ± 3.65 kg, height: 1.78 ± 0.03 m, body fat: 11.6 ± 1.14%) participated. The pre- and postintervention measures were: the squat-jump (SJ); the countermovement-jump with arm swing (CMJA); the ﬁve jump test (5JT); 10 m and 30 m sprint; the ability to change direction (sprint with 90 ◦ turns (S90 ◦ ) and sprinting 9–3–6–3–9 m, involving running both backwards and forwards (SBF); repeated sprint ability (RSA), and balance (Y-balance test). The EG experienced superior jump ( p < 0.001; d range : 1.69–1.89), sprint ( p < 0.001; d range : 1.82–2.56), S90 ◦ ( p < 0.001; d range : 1.64–2.25), RSA ( p < 0.001; d range : 3.90–4.17), and balance ( p < 0.001; d range : 1.11–2.54) improvement. Comparatively, the pre- to postchanges in the CG ranged from d = 0.36 (dynamic balance) to d = 1.00 (10 m sprint). Therefore, bi-weekly LPaSS training improves athletic performance in young soccer players, particularly RSA.


Introduction
Soccer is a sport with many important characteristics comprising technical, tactical, and physical aspects [1], combining cyclic and acyclic movements [2,3]. In youth soccer teams, endurance, speed, and muscular strength are valued physical skills [4], so youth athletes have a greater change of selection when they demonstrate excellence in these abilities [5]. Therefore, applicable fitness traits are generally assessed and monitored, and typically include maximum speed, strength, change-of-direction (CoD) ability, balance, and flexibility [6]. Therefore, fitness training in young soccer players is imperative to achieve success in their soccer careers [7].
Conceiving appropriate training programs for soccer players requires a detailed understanding of the physiological load placed on players of particular age groups and positions [8]. In high level soccer matches, particularly at the elite level, there is a well described requirement to produce high speed intermittently, and repeatedly [9,10]-this ability to recover between each sprint, and maintain performance in subsequence sprints is known as repeated sprint ability (RSA). RSA is therefore considered a specific fitness requirement of soccer [8]. Time and motion analyses indicate youth soccer players cover an average of 8448 m during a match, 501 m at 16-19 km·h −1 and 428 m at >19.1 km·h −1 [11].
Plyometric and sprint exercises have been used as training methods for increasing abilities such as jumping, sprinting, and change of direction performance [12][13][14][15]. Several researchers have discussed the use of plyometric [16][17][18][19] and sprint [20][21][22] training with additional external loading. Recently, it has come to light that a combination of training methods may be more efficacious than standalone approaches [23]. Indeed, some recent studies have reported improved fitness parameters after loaded combined plyometric and short sprints (LPaSS) training in soccer players [24]. Recently, Hammami et al. [24] studied effects of LPaSS training in male U17 soccer players and reported improved sprint performance, jumping, CoD ability, RSA, and balance performance.
To our knowledge, there is a dearth of studies examining the effects of LPaSS training in under 19 (U19) soccer players. As such, the primary aim of the present investigation was to determine the effects of replacing some proportion of habitual soccer training with LPaSS on jumping, sprinting, agility, CoD ability, RSA, and balance in male U19 soccer players. We hypothesized that replacing some of regular training with 8 weeks of LPaSS training twice per week would improve both horizontal and vertical jump performance, sprint performance, and CoD ability when compared with the control group, who maintained habitual training. Specifically, we examined vertical and horizontal jump performance (i.e., squat jump (SJ), countermovement jump with arm swing (CMJA), and the five jump test (5JT)), sprint performance (i.e., 10 m and 30 m sprint), CoD ability (sprint with 90 • turns (S90 • ) and sprint 9-3-6-3-9 m with backward and forward running (SBF)), RSA, and dynamic balance performance (Y-balance test).

Participants
This study was completed during the 2020/2021 soccer season. During the study, Government guidelines were altered as a response to the pandemic in early April 2020, and citizens were prohibited from leaving their homes to visit coastal areas, children's playgrounds, parks, sporting areas, recreation areas, and squares. Moreover, seeing more than two people was prohibited. The government prohibited citizens from leaving their homes and being on the streets without documents evidencing the purpose for their being outside. Police was entrusted to oversee and monitor citizens and ensure citizens followed self-isolation regulations. Breaking these regulations was penalized with a fine or imprisonment. Procedures were granted approval by the national university institutional review board (approval number: KS00000-KS2021) for human subjects and complied with the requirements of the Declaration of Helsinki. Participants (and their guardians, in the case of minors) gave written informed consent (and assent where necessary).
Before recruitment, a power calculation (nQuery Advisor 4.0; Statistical Solutions, Saugus, MA, USA) was performed using previous data [25], which suggested a sample size of 13 participants in each group would provide the study with 80% power to detect a mean difference of 2.90 cm in CMJ height using a two-sided t-test with an alpha level of 0.05 under the assumption of a pooled standard deviation of 2.50 cm [26]. Therefore, to allow for participant drop out, thirty-four males from one soccer team in the first National division participated. Participants had 7.6 ± 0.8 years playing experience prior to enrollment, and before participating they were examined by the team doctor. Participants were randomized between the experimental group (EG, n = 18, age: 17.5 ± 0.6 years, body mass: 67.4 ± 4.4 kg, height: 1.76 ± 0.05 m, body fat: 11.4 ± 1.6%), and control group (CG, n = 16, age: 17.5 ± 0.6 years, body mass: 68.7 ± 3.7 kg, height: 1.78 ± 0.03 m, body fat: 11.6 ± 1.1%). No baseline between-group differences existed in age (p = 0.926, η p 2 = 0.000) or anthropometric characteristics (p ≥ 0.269).
Participants were included on the bases they were over 18 years of age, were without cognitive impairment, had no current injuries, had >3 years of soccer experience, and did not present with coronovirus-19 (COVID-19) symptoms. Asymptomatic COVID-19 cases or discharged cases were considered eligible. Participants abstained from alcohol and pharmaceuticals for ≥24 h before testing and maintained their habitual diet according to self-reporting.
Before the study, participants had completed a training period of 8 weeks (5-6 sessions per week) which included resistance training targeting hypertrophy and muscle strength. In the final three of eight weeks, muscle power was targeted by reducing load and increasing speed of execution. Each weekend, participants took part in a non-competitive training match. Participants maintained participation in five training sessions per week when the league season started.

Experimental Design
Only training with the soccer team was permitted during the experiment. Both the EG and CG trained five times per week for~90 min per session, with a competitive game each weekend. During the first three training sessions of each week, 60% of the total session time was focused on developing technical and tactical skills, and 40% of the time was focused on developing physical abilities. In addition, during the last two training sessions of each week, 75% of the total session time was focused on technical and tactical skills development, and 25% of the time was focused on physical ability development. All participants also participated in weekly 60 min school physical education sessions. The planning of training time (technical-tactical and physical) is frequently used in research that has studied the effects of one type of training program on the athletic performance of participants [27,28]. Strength and conditioning training was performed twice a week. Indeed, the first session of training of the week was focused on the development of aerobic capacity through skill drills/circuit training with low intensity and medium volume (8 to 10 min). The second training session was focused on the development of maximum aerobic power through high intensity interval training (HIIT) and small-sided games [29]. The third second training session focused on developing maximum anaerobic power through resistance training, including dynamic exercises based on bodyweight only and jump exercises and sprinting exercises. The remaining training sessions focused primarily on tactical and technical skills ( Table 1). The soccer players performed the training with the team and under the supervision of a coach. The control group maintained their normal training schedule throughout the 8-week intervention, while each Tuesday and Thursday the experimental group replaced the technical-tactical part of their standard training program with loaded combined plyometric and short sprints training. The loaded combined plyometric and short sprints training program consisted of 4 principal workshops, twice per week ( Figure 1). For the EG, each LPaSS workshop began with plyometric exercises (hurdle jumps, lateral hurdle jumps, bouncy strides, and drop jumps) and finished with short sprints. Details are given in Table 2. The EG executed all workshops with a weighted vest (8% body mass) [18,19]. Plyometric training design was based on from Bedoya et al. [30] and the sprint training protocol was based on recommendations for CoD ability [31,32]. The EG executed all workshops with a weighted vest (8% body mass) [18,19]. Plyometric training design was based on from Bedoya et al. [30] and the sprint training protocol was based on recommendations for sprint distances [31,32].

Testing Schedule
Two weeks before testing commenced (2 months into the soccer season), both the EG and CG underwent familiarization with testing procedures. Tests were conducted at least 3 days after the last competitive match and 5-9 days after the previous training session. All performance tests were conducted on a tartan surface and were integrated into the weekly training schedule, and a standardized warm-up preceded each test. Fitness tests were conducted over three days in an identical order to facilitate similar fatigue or learning or order effects. Anthropometric assessment, SJ, CMJ, and S90 • were conducted on day 1. The Y-balance test, 10 and 30 m sprint, and SBF were conducted on day 2. The 5JT and RSA test were completed on day 3. Fitness tests have been previously described in detail [18,24,33] and are therefore not detailed here to avoid self-plagiarism, but also for brevity.

Reliability
All investigated performance parameters showed an excellent relative reliability (ICC ≥ 0.75). The ICC ranged from 0.90 (SBF) to 0.98 (Y-balance test, right leg, ad; Table 3). The absolute reliability measured by CV was also on a comparable high level. All CVs were below 10% (Table 1) and ranged from 1.5% (S90 • ) to 4.8% (Y-balance test, left leg, pl; Table 3).

Normal Distribution and Variance Homogeneity
The

Effect of Training on Jump and Sprint Performance
For vertical and horizontal jump performance, intervention effects (group × time interaction) were significant for all parameters and ranged from η p 2 = 0.94 (5JT) to η p 2 = 0.99 (SJ). The interaction effects for the sprint parameters were of a similar magnitude (from η p 2 = 0.94 (10 m sprint) to η p 2 = 0.95 (30 m sprint) ( Table 4)). The effect sizes (d) for EG were consistently higher than for CG. The improvements of performance in EG ranged from d = 1.69 (5JT) to d = 2.56 (30 m sprint), whereas the effect sizes for CG were markedly lower (d range : 0.41 to 1.00). No difference between groups was observed for 5JT (η p 2 = 0.14).

Effect of Training on Change of Direction Ability
The S90 • , SBF, and RSA improved in the EG (d range = 1.64-4.17), more than in the CG (d = 0.41-0.60) ( Table 5). The improvements regarding RSA parameters (d fastest time = 3.90, d mean time = 4.17) were the largest for any parameter and were almost twice as large as S90 • (d = 2.25) and SBF (d = 1.64) improvements. Equal preintervention values were observed for SBF (group effect: η p 2 = 0.08).

Effect of Training on Balance
For all balance parameters significant interaction effects were observed as an indication of the effectiveness of the intervention (Table 6). Partial eta squared (η p 2 ) consistently moved on a very high level (from 0.78 to 0.95). Y-balance test right leg pld (p = 0.034, η p 2 = 0.13), left leg ad (p = 0.094, η p 2 = 0.09) and pmd (p = 0.488, η p 2 = 0.02), were not different between groups preintervention. The effect sizes in for improvement in EG ranged from d = 1.11 (right leg pld) to d = 2.54 (left leg pmd) and were greater than in the CG (d max = 0.64, left leg pmd).

Relationships between Anthropometric and Performance Parameters
Meaningful correlations (r ≥ 0.5) were not observed between anthropometric and performance parameters. The largest relevant relationship was detected for height and right leg pl (r = 0.475).

Discussion
In this experiment we investigated the effects of LPaSS training in U19 soccer players. We accept our hypothesis that replacing some habitual soccer training with LPaSS twice a week for 8 weeks improved horizontal and vertical jump performance, sprint performance, and agility. The data presented here demonstrate LPaSS resulted in significant fitness gains (e.g., RSA, balance, power performance) in the EG, but not in the CG.

Effect of Training on Jump Performance
Explosive strength in the form of jumping is perceived as important to optimize football performance and is generally considered when testing for fitness and talent screening in soccer players [44]. Arnason et al. [45] observed that competitive higher level soccer players exhibit superior jumping ability than lower performance level players. Furthermore, vertical jump performance is reportedly associated with lower leg strength and sprint performance in soccer players [46]. The EG experienced improved performance compared to control in both vertical and horizontal jump performance. This corroborates findings of Hammami et al. [24] who studied the effects of a combination of a 10-week LPaSS training in male soccer players (U17s) and noted improved vertical and horizontal jump performance. Moreover, Gil et al. [21] observed increased vertical jump performance following loaded sprints training, in elite senior soccer players. Moreover, both Negra et al. [18] and Rosas et al. [19] reported significantly improved vertical and horizontal jump performance following loaded plyometrics in prepubertal male soccer players. To explicate improved performance with the addition of external load during plyometrics and/or jump training, Kobal et al. [17] reported adding load during jump training increased ground reaction force (GRF) in both vertical or horizontal axes, and ensured the GRF was applied over more time. As instantaneous GRF was greater, and GRF was applied over a longer time, this resulted in increased impulse during jumps [47], thus producing superior adaptations in jumping with additional external load [17].

Effect of Training on Sprint Performance
Elite players demonstrate superior ability in sprint distances over 5 to 10 m [48]. Powerful muscular contractions are required in both attacking and defending situations when goals typically occur [49]. During in-game situations, 500 to 600 m is covered during sprints [2], which occur every 90 s [50]. Data from this study showed LPaSS improved sprint performance over 10 and 30 m (Table 4). Our findings are in accordance with existing literature, which has reported improved sprint performance after several programs, including LPaSS training [24], loaded plyometric training [17,18,28], and loaded sprint training [20,21,47]. Recently, Hammami et al. [24] examined the effects of 10-week LPaSS training in male U17 soccer players and noted improved sprint performance over 5, 10, 20, 30, and 40 m. Moreover, Kargarfard et al. [28] observed improved 30 m sprint times following unloaded plyometric and short sprints training in U19 male soccer players. Indeed, gains in sprint performance as a result of unloaded plyometric and short sprints training in male U15 soccer players have previously been observed [27]. Research has shown that loaded sprint training can increase the eccentric force of the extensor muscles during the ground contact braking phase and increase leg stiffness, increasing the ability of muscles to store elastic energy and enhance power output [51,52].

Effect of Training on Change of Direction Ability
Although linear sprinting is an important ability in soccer, the ability to accelerate, decelerate, and CoD is probably more determinant of soccer ability, and more relevant to in-game situations [53,54]. Match analysis has demonstrated the importance of CoD ability in soccer since an average sprint time of 2.3 s (10 to 12 m sprint) [55] and an average of 50 directional changes per match have been recorded [56]. The data presented herein demonstrated that the EG improved CoD ability more than the CG. This is the first investigation of the effects of LPaSS on CoD ability in male U19 soccer players. We reported improved CoD ability, which corroborates the results of previous recent studies [24,29,30] following LPaSS training [24], loaded plyometric training [16,18], and loaded sprint training [21,22] in male soccer players. Recently, Negra el al. [18] demonstrated significant improvements in CoD ability following 10 weeks of loaded and unloaded combined plyometric and short sprints training in youth soccer players (U17). However, our results contrast with the results of De Hoyo et al. [57] who reported no gain in change of direction ability after loaded sprint training in U19 soccer players. Differences in training protocol, intervention period, tests used, and participant characteristics could contribute to the divergence between studies. The mechanism responsible for CoD ability improvements in our study may be associated with neural adaptations and improved motor unit recruitments [58]. Improved CoD ability after plyometric training may also occur as a result of neuromuscular adaptations such as increased neural drive and inter-and intramuscular coordination. These neuromuscular adaptations may be responsible for, or occur independently but simultaneously with, altered gait parameters (e.g., modified stride length, stride frequency, contact time and horizontal GRF) [59,60], ultimately improving change-of-direction performance [22].

Effect of Training on Repeated Sprint Ability
Soccer players perform many high-intensity runs during an average match [61], with short intervals of recovery between bouts, repeatedly throughout the competition [62]. Thus, recovery ability and RSA is a critical component of soccer player performance [51,63] and is associated with high aerobic power (VO 2max ) [1]. RSA is a significant factor in determining success in soccer [64,65]. Rampinini et al. [65] reported significant correlations between distances covered during competitive matches and RSA test performance in professional soccer players. Our results suggest LPaSS improved RSA; our study, to the best of our knowledge, is the first to have examined the effects of LPaSS on RSA in young soccer players. Our results are confirmed by Hammami et al. [24] who observed improvement in repeated change of direction (RCoD) ability following LPaSS training in male U17 soccer players. Additionally, Rey et al. [66] reported significant improvement in all repeated sprint ability parameters except the fatigue index following loaded sprints training in male senior soccer players. In contrast, our results contradict those of Kargarfard et al. [33] who reported no improvement in RSA following combined unloaded plyometric and short sprints training in U19 male soccer players. The divergent results between our data and previous investigations may result from various experimental design factors such as the competition level of the participants, gender or sex, or differences in the external load applied. The improvement in RSA performance following a plyometric training program is probably related to the improvement in explosive performance, improving the ability to change-of-direction when pivoting during the RSA test [67], or running economy [68], increased efficiency in running [69], and or an enhanced degree of tendon stiffness [70].

Effect of Training on Balance Performance
In a soccer match, players perform multiple actions (primarily with the lower limbs), which include passing, dribbling, and shooting, all with football boots and commonly on a grass playing field [71]. Balance ability is important as it permits players to maintain postural control whilst running, spring, kicking, and changing direction [72]. Moreover, players must exert postural control when they are blocked by opposition [73]. In addition to the stand-alone benefits of balance ability for maintaining postural control, balance performance is associated with other components of fitness. In this context, Pant et al. [74] noted correlations between balance performance and muscular strength around the ankle joint. To the best of our knowledge, only one previous study has investigated the effects of LPaSS training on balance performance in young soccer players [24]. Hammami et al. [24] examined the effects of 10-week LPaSS training on dynamic and static balance performance in male soccer players (U17) whereby balance improved as a result of LPaSS, corroborating the results of the present study. Makhlouf et al. [75] confirmed observations made herein by reporting improved static and dynamic balance following combined plyometric and short sprint with CoD training in young soccer players. EG improvements were likely resultant from enhanced neuromuscular control of the lower limb musculature [76].
In terms of diet in relation to exercise and COVID-19, Ammar et al. [77] suggested that the quarantine itself can be considered a risk factor for the consumption of poor-quality foods, such as ultraprocessed foods, when compared to the standard living condition. Combined with the potential for lower levels of physical activity, impaired nutritional habits could lead to a positive energy balance (i.e., weight gain). There is limited evidence to evaluate the effect of confinement on loaded plyometrics and short sprints and dietary behaviors. Investigating how physical activity and eating behaviors are affected by lengthy restrictions is important to establish appropriate recommendations for lifestyle modifications during this time [78].

Limitations
The results of this study may not be ubiquitously observed in all age groups and all competition levels (i.e., outside of 17-19 year olds, or outside of soccer players) so LPaSS cannot be uniformly prescribed to all soccer teams regardless of age, gender, or competition level. Moreover, sexual maturation and dietary assessment were not evaluated within the present investigation, which may have hampered internal validity. As such, future studies should include an assessment of sexual maturation when examining LPaSS (or any training type) in young soccer players as the level of sexual maturation may influence the adaptive response. Moreover, the addition of laboratory-based tests (i.e., VO 2max tests) could confirm the field-based tests utilized in the present study, although this may not be as ecologically valid. The extension of the present study into other invasion games, genders, and competition levels may provide confirmatory evidence of the effects noted in this study. It is also necessary to compare the gains in test performance with the analysis of performance during competitions to determine whether better fitness translates into improved gaming performance. Otherwise, neuromuscular mechanisms that underlie the improvements reported here can also be an area of future research. Finally, future studies should extend to compare the effects of LPaSS training on the fitness and body composition of young and adult soccer players.

Practical Applications
This investigation demonstrated the efficacy of substituting some typical usual soccer training with twice weekly LPaSS training in male adolescent soccer players. Thus, it appears pragmatic to incorporate LPaSS training into traditional in-season technical and tactical adolescent training sessions to improve performance. Finally, the weighted vests used in this study as additional load are inexpensive and unsophisticated, thereby making them simple for all clubs to access and easy for both coaches and athletes to handle during strength and conditioning exercises.

Conclusions
To conclude, incorporating LPaSS training into standard training improved jump performance, sprinting, CoD ability, RSA, and balance. Therefore, soccer coaches and practitioners should incorporate combined plyometric and short sprints training into soccer training to enhance fitness. Most of the gains associated with LPaSS training seem to be of significant magnitude, so should be of interest for both soccer players and their coaches. Further investigation is warranted to fully optimize LPaSS, including variables such as quantity of external load, and training volume.