Effects of combined plyometric and sled training on vertical jump and linear speed performance in young soccer players Efectos de la combinación de entrenamiento pliométrico y de arrastres sobre el rendimiento en salto vertical y la velocidad lineal en jugadores jóvenes de fútbol

The objective of this study was to evaluate the effects of a combined plyometrics and sled training carried out two days a week for 8 weeks. 60 young male soccer players were distributed in 4 groups (Under U16) [Experimental Group (U16 Exp, n=15) and Control Group (U16 Cont, n=15); and (Under19) [U19 Exp, (n=15) and U19 Cont, (n=15)]. The jump with countermovement (CMJ) was evaluated with Chronojump-Boscosystem® and the time in seconds (sec) in the 10, 20, and 40 m split with FitLight Trainer®. Was calculated a level of significance of pd»0.05, the effect size (ES) and % change. Both experimental groups improved from pre-to pots-test in CMJ [U16Exp (p<0.02) and U19Exp (p<0.01)] and the 10, 20, and 40 m splits [U16Exp (p<0.02, p<0.03, and p<0.01, respectively) and U19Exp (p<0.02, p<0.02, and p<0.01, respectively)]. Therefore, the combination of football and plyometric and resisted methods can be used for a general development of the neuromuscular capacities of young soccer players.


Introduction
Soccer requires a lot of high intensity actions that include sprinting with periods of acceleration and deceleration, jumps, sprints, and changes of direction (Benítez-Jiménez, Falces-Prieto & García-Ramos, 2020;Gómez-Álvarez et al., 2020). Concerning to soccer, it also need to be mentioned that it is an acyclic and intermittent sport in which performance is highly influenced by actions realized at a high intensity (González-Millán et al., 2014;Sáez de Villarreal, Suárez-Arrones, Requena, Haff, & Ferrete, 2015). The sports are characterized by high intensity actions are explosive 2021, Retos, 42, 228-235 © Copyright: Federación Española de Asociaciones de Docentes de Educación Física (FEADEF) ISSN: Edición impresa: 1579-1726. Edición Web: 1988-2041 and essential in young and senior soccer players (Bouguezzi et al., 2019;Meylan & Malatesta, 2009). Nevertheless, the intermittent nature of the game implies mixed metabolic involvement (Bangsbo, Mohr, & Krustrup, 2006), and the decisive actions depend on the anaerobic energy systems (Raya-González & Sánchez-Sánchez, 2018). The intervention of anaerobic systems in soccer player training must materialize through strength training (Hoff & Helgerud, 2004;Raya-González & Sánchez-Sánchez, 2018), since there is a great relation between sprint, vertical jump and change of direction with the levels of strength, power and ratio of strength production (Swinton, Lloyd, Keogh, Agouris, & Stewart, 2014). Moreover, a review of current literature demonstrates that soccer players require high muscle strength levels, especially in lower body Suchomel, Nimphius, & Stone, 2016), to carry out the previously mentioned high intensity actions. In fact, it is well known that strength and speed are very important physical manifestations for soccer players (Gissis et al., 2006;Reilly, Bangsbo, & Franks, 2000), for this, they are considered predictors of success (González-Millán et al., 2014;Kaplan, Erkmen, & Taskin, 2009;Mujika, Santisteban, & Castagna, 2009). Finally, and accordingly with the literature, we can suggest that two of the procedures to improve both qualities properly are plyometric training and resisted methods (García-Ramos & Peña-López, 2016). Traditionally, plyometrics training was damaging for young population, since it increased injury risk and growth delay . However, recent studies have shown that plyometric training could be beneficial effects for young soccer players when the appropriate training rules for their age are followed (Bedoya, Miltenberger, & López, 2015;Falces-Prieto et al., 2018;Fonseca et al., 2020). Actually, plyometrics training has been recommended to improve vertical and horizontal jump ability (Asadi, Ramírez-Campillo, Arazi, Sáez de Villarreal, 2018;Zisis, 2013). In addition, this training improves acceleration and sprint ability (Bouguezzi et al., 2019) and enhances muscle-tendon ability to produce maximum strength levels in short periods of time (<250 ms) (Bouguezzi et al., 2019;Sáez de Villarreal, Requena, & Cronin, 2012). The improvement of vertical jump is a common objective, both for researchers and for coaches and strength and conditioning coaches of different sport disciplines (Martínez-Rodríguez, Mira-Alcaraz, Cuestas-Calero, Pérez-Turpín, & Alcaraz, 2017). The possibility of jumping more than the opponent could be a reason of success in athletic competitions, as well as an advantage over an opponent in team sports (Sánchez-Sixto & Floría-Martín, 2017).
On the other hand, resisted methods are those training ways in which the specific technique of sprint is imitated adding an overload on the athlete (Rumpf et al., 2015). Resisted sprint training consists of executing sprinting exercises with additional overload, which is supposed to more effectively transfer to sport performance . Some authors recommend some resisted methods for the improvement of the acceleration phase (distance 0-20 m) and others for the maximum speed phase (distance 20-50 m), on the basis of the principle of specificity of training (Cahill et al., 2019;Cronin & Hansen, 2006). These methods include sled, parachute, vests or belts ballast, beach sand runs, and uphill runs (Alcaraz, Elvira, & Palao, 2009a). Regarding the use of dragging with sleds and the load to be moved in football players, some authors recommend from 5-7% (Alcaraz, Palao, & Elvira, 2009b) to 69-96% of the body weight (Cross, Brughelli, Samozino, & Morin, 2017). The implementation of this methodology seeks that the muscles used in the sprints work in overload, since it causes a greater neural activation, and a greater recruitment of fast contracting motor units (Faccioni, 1994).
Due to the great number of factors that affect the performance of both the strength (Cronin & Sleivert, 2005), and the sprint (Radford, 1990), and the unawareness regarding the most optimal method of strength and speed training (Cronin & Sleivert, 2005), it is necessary to optimize the knowledge about the athlete's preparation and the training's effect on the performance (Jiménez-Reyes, Cuadrado-Peñafiel, & González-Badillo, 2011). Although numerous researchers on football have proved that both the plyometric training and the dragging, independently, can produce improvement on the vertical jump height and the speed respectively (Alcaraz et al., 2009a;Bouguezzi et al., 2019;Sáez de Villarreal, González-Badillo, & Izquierdo, 2008), we doubt about the combination of both as an effective and optimal method for the enhancement of young football players. Therefore, the aim of the present study this research was to examine and evaluate the effects of a combined plyometrics and sled training carried out two days a week during 8 weeks in young football players, on the improvement of the vertical jump and linear speed ability.

Material and Method
Participants A group of sixty young male soccer players belonging a High-Performance Soccer Academy and competed in federated competition at the regional level, voluntarily participated in the study, which was performed according to the ethical standards established by the Helsinki Declaration of 2017. 172.64 ± 6.80 cm, weight: 63.21 ± 7.06 Kg]. All participants had been playing soccer for more than five years and at the time of the present research they were recruited in High-Performance Soccer Academy. In fact, all participants were recruited via flyers and they reported normal or corrected to normal vision, had no history of neurological of physical disorders, and gave informed consent prior to the start of the experiment. The players practiced 5 times per week, 90 min per session, with an official competition every weekend. All players participated in 16 proposed sessions (100%). All participants and parents were carefully informed of the experimental procedures and possible risk and benefits associated with participation in the study.
All the participants complied with inclusion and exclusion criteria. Inclusion criteria: (1) being active players of the high-performance academy, (2) not presenting any injury during the last 3 months, (3) having the consent and (4) carrying out at least 80% of the training protocol. Exclusion criteria: (1) not complying with 80% of the training protocol and (2) presenting any ailment or acute disease at the time of the intervention.

Procedure
The intervention was carried out during the months of October and November of the 2019/2020 season. One-hour sessions were scheduled two days a week on non-consecutive days (Tuesday and Thursday), over a period of 8 weeks (see table 1). In addition, to the plyometric and sled training, the experimental group continued with its training program that included 5 technical-tactical sessions per week and a competition match on the weekend. For its part, the control group only carried out the normal dynamics of technicaltactical training and competition matches. The combined training program was performed after a warm-up, as it has been recommended as the best time to perform plyometric training (Bedoya et al., 2015), in addition to ensuring that players were rested and obtained better benefits of the program (Meylan & Malatesta, 2009). Regarding the organization of training, a circuit organization was followed for the benefits of this type of distribution (Orquín-Castrillón, Torres-Luque, & Ponce de León, 2009).
The 10-minute warm-up was based on dynamic movements of both the upper and lower body, performed in rows and with a displacement of 15 meters, under the direct command of the strenght and conditioning coach (Falces-Prieto et al., 2018). As seen in the studies by Falces-Prieto et al. (2020) and Sáez de Villarreal et al. (2015), plyometric exercises consisted of exercises performed at maximum voluntary intensity using the player's body weight or body weight with light resistance. Regarding sled training, the material used was a galvanized sled (RANKING©, Navarra, Spain) with a 54x38 cm platform, total length of 94 cm and weight without load: 4.5 kg and discs of different weights of the DOMYOS© brand (Lille, France) to carry the trailers with the correct load. The load was the weight of the sled itself and 5, 12.5 and 20% of the player's body weight (weight of the sled included), as in the study by Bachero-Mena & González-Badillo (2014).
For their evaluation, the participants visited the laboratory on two separate occasions (before and after the intervention), always at the same time of the day (U16 group at 15:30 and U19 group at 16:30) and evaluated by the same technical specialists. Both for the pre and post-evaluation of the CMJ and the speed, the players first performed a 10-minute warm-up based on: free joint and muscle mobility (3 min), skipping (2x30 seconds), gluteal heel (2x30 seconds), squats with extended arms (2x10 repetitions), continuous vertical jumps (6 jumps with the CMJ execution technique) and two progressive high-speed runs of 10 m. In the first place, the CMJ was evaluated in the laboratory and immediately afterwards, the speed evaluation was carried out in the field. For speed evaluation, 2 attempts were performed with a recovery time of 2 min between repetitions and an average of the two repetitions for subsequent analysis. Times were measured in seconds (sec) at distances of 10, 20 and 40 m, respectively. The evaluation system was carried out through FitLight Trainer® sensors (Ontario, Canada). This system is used to calculate agility, reaction time, speed and coordination in football (Rauter et al., 2018;Reigal-Garrido et al., 2019). For the evaluation, 4 Led sensors were placed on a bar 1 m high and in a straight line at distances 10, 20 and 40 m. At the light signal, the player sprinted in a straight line down an 80-cm lane from the sensor. The recorded time for each of the players was stored in a portable tablet with an Android system and its subsequent analysis in the Microsoft Windows® Excel program (Redmond, Washington, USA).

Statical Analyses
Descriptive statistics are represented as mean and standard desviation (SD). Test of normal distribution (Shapiro Wilk test) was conducted on all data before analysis. The sample was normal and additionally parametric T-Student test of related samples was performed. The statistical analysis was completed with the effect size (ES) using the Cohen's d (1988) to evaluate the magnitude of the differences and the % change from pre to post-test is presented. The criteria for understanding the effect size was: trivial = 0.00-0.19; small = 0.20-0.59; moderate = 0.60-1.19; large = 1.20-1.9; very large = 2.0-4.0; and almost perfect >4, (Hopkins, 2002). The level of significance was pd»0.05. Data analysis was performed using JASP® statistical program (version 0.11.1, Amsterdam, The Netherlands).

Results
The changes in the variable height (cm) of the CMJ and the time (sec) in the distances of 10, 20 and 40 m are shown in Table 2.

Discussion
The main purpose of this study was to evaluate the effects of a combined plyometric and sled training for 8-weeks and carried out two days a week in young soccer players for improving the ability of vertical jump and linear speed. This study highlights a new approach that combine plyometric and sled training methods in young soccer players.
Through the present study, it is concluded that our protocol produces improvements in variable height in CMJ and sprint time in the speed sections of 10, 20 and 40 m respectively in both categories (see table 2 for more information). On the one hand, the results obtained are in line with the research of Alcaraz et al. (2009b) and Sáez de Villarreal et al. (2008), which have shown that plyometric and sled training produce improvements in jump height and speed respectively (p<0.001). We can suggest that the duration of our study has been optimal to produce neuromuscular adaptations Table 1. Combined plyometrics and sled training protocol.
A deep review of the literature reveals that a cross of plyometric and sled training in the same intervention process in young soccer players is justified by the study by Fatouros et al. (2000), who agree that in order to optimize the results of a plyometric training, it must be combined with other training methods. We can indicate that after combining plyometric and sled training, we have found improvements in CMJ in U16 Exp (p<0.02; 2.09 %) and U19 Exp (p <0.01; 5.11%) and sprint time in the distance 0-40 m in U16 Exp (p<0.01; 4.03%) and Exp U19 (p<0.01; 2.11%). Our data is in agreement with Buchheit, Méndez-Villanueva, Delhomel, Brughelli. & Ahmaidi, (2010), who planned a cross training (jumps, coordination and speed stairs; 4-6 series 4-6 exercises; 45 seconds of recovery between series and 3 minutes of recovery between exercises) of 10 weeks, achieving improvements in CMJ (14.68%) and in the sprint time in 30 m (1.96%). On the other hand, a study of Chelly et al. (2010), proposed an 8week plyometric training program (2 weekly sessions) during the season, finding significant improvements (p<0.01) of 9.75% in maximum speed (m/s) and 10% in acceleration (m/s). Furthermore, they obtained statistically significant improvements in CMJ (p<0.01; 2.5%). Finally, we must highlight the work of De Hoyo et al. (2016) with elite Spanish soccer players (n = 11) who carried out an 8-week with a plyometric training program during a season (twice a week), which consisted of the combination of jumping exercises, coordination ladder and sprint. After the completion of the program, the results obtained showed improvements in CMJ (7.2%), in the sprint time of 30-50 m (s) of 0.3% and in the maximum speed of 0-50 m (s) of 1.5%.
The literature found until the present recommends that for a resisted training to be effective, it should not produce losses greater than 10% (Alcaraz, 2010;. The proposed loads on sled training have generated a significant performance increase of 4.03% in U16 Exp and 2.11% in U19 Exp in the 0-40 m distances. Therefore, we can indicate that our protocol offered overload control by allowing no more than 10% reductions in sprinting speed. The relationship between load and the improvement of time in the 10, 20 and 40 m sections in U16 Exp (ES: 0.46, 0.51 and 0.55) and U19 Exp (ES: 0.49, 0.48 and 0.47), are close to the results obtained by the studies mentioned. In this sense, these authors shown improvements in performance of 5 and 20 m (ES: 0.60) in amateur soccer players (26.3 ± 4.0 years), after performing sled training with 80% of body weight (10 x 20 m sprints), carried out two days a week before training, for 8-weeks. Thus, our results of CMJ and linear speed results using sled training coincided with the study by Spinks, Murphy, Spinks, & Lockie, (2007), which showed that with a resisted training carried out 2 days a week over 8-weeks, there were improvements in CMJ (p<0.001) time in 0-5 m (p<0.001), distances 5-10 m (p<0.05) and distances 10-15 (p<0.05) respectively in soccer players (n = 8). Furthermore, the results obtained in our study differ from those obtained by García-Ramos & Peña-López (2016), who demonstrated that after the combination of a plyometric training and a resisted training through the dragging of sleds with loads of between 10 and 20% of body weight, performed two days a week for 8-weeks, did not show significant improvements in CMJ (p<0.28) and sprint time in 10 and 30 m (p<0.33; p<0.14), respectively, in amateur soccer players. This study had some limitations, including that do not provide a comparison between the combined plyometric and sled training with both training methods in isolation, a comparison of the results of this study (combined plyometric and sled training). Future studies should extend the data available in the scientific literature of the effects of isolated plyometric and sled training should be made to improve the knowledge on the topic explaining why it is better or not to combine both methods, especially considering the importance of this type of training on soccer.
Finally, we must bear in mind that training stimuli influence differently according to category and age, and may significantly affect jump performance and speed in young football players (Quagliarella et al., 2011), either by level of biological development, maturation of their musculoskeletal structures or maladjustment to training stimuli, although they improve as the age of the subjects increases (Benítez-Sillero, Da Silva-Grigoletto, Muñoz-Herrera, Morente-Montero, & Guillén-Castillo, 2015). In this sense, plyometric method significantly enhances speed and speed and velocity-force in male soccer players in the short term (Meylan & Malatesta, 2009). To emphasize this point, other study of Thomas et al., 2001, shown that short plyometric training demonstrates significant improvement in vertical jump and agility time in young men. These data are endorsed in our study, when it can be observed that, in the performance variables evaluated, performance has been higher in the U19 category compared to the U16 category.

Conclusion
In conclusion, the present study demonstrated, that implementing two strength training methods (plyometric and sled training), carried out two days a week on non-consecutive days and for 8 weeks, in addition to normal soccer training and can be recommended as an optimal form of physical conditioning to improve vertical jumping and sprints in young football players. Moreover, it must be considered which is the most appropriate plyometric and sled training for each age range, and for this, we can rely on movement patterns, technique, volume, frequency, intensity and required energy systems. However, coaches and / or strength and conditioning coaches due dominates the load control for applicate on soccer training. Thus, should take into account and evaluate each player for adapted the training individually. It should therefore like to highlight the results obtained in this study, when prescribing an appropriate training based on age and sport category.