A high aerobic capacity is important for success during a 90-minute soccer game (Stolen et al., 2005). However, the ability to produce explosive single-bout effort (i.e. sprinting, jumping, changing direction) is as important as aerobic capacity for success in soccer (Faude et al., 2012; Stolen et al., 2005). Although some evidence show no difference in explosive strength (i.e. jump) between elite and recreational youth soccer players (Chrisman et al., 2012), most studies show that players selected in National teams vs. their non-selected counterparts (Buchheit et al., 2013), and future international and professional players vs. future amateur players (le Gall et al., 2010) had superior explosive characteristics (i.e. speed, power) at youth level. In an attempt to improve these characteristics, and hence the performance and future competitive level of athletes, plyometric training (PT) have been commonly used in young soccer players (Diallo et al., 2001; Meylan and Malatesta, 2009; Michailidis et al., 2013; Thomas et al., 2009), with the advantage of being easy to integrate in soccer practice (space, time, equipment), and replicating the neuromuscular stimulus encountered in explosive soccer activities such as jumping and sprinting (Gehri et al., 1998). Therefore, PT may be advocated as an appropriate approach for enhancing soccer-related performance abilities. However, although the primary governing factors regulating the performance of repeated poweroriented drills are their intensity, duration and recovery duration (ACSM, 2009), the characteristics of between sets recovery of a PT that generates optimal gains are not clear (Saez de Villarreal et al., 2012), especially in young soccer players.
To the best of the authors' knowledge a limited number of studies have established the optimum PT design (i.e. interset rest intervals) for explosive strength enhancement (de Salles et al., 2009). Rest interval can have a significant acute effect on metabolic, hormonal, cardiovascular, and performance responses to strength exercise (Garcia-Lopez et al., 2007; Kraemer et al., 1987; Kraemer and Ratamess, 2005). Power performance is highly dependent on the phosphagen energy system, which requires 4 min for replenishment after an intense exercise set (Harris et al., 1976). An inadequate rest interval may lead to accumulation of inorganic phosphate (Westerblad et al., 2002) or alterations in the concentration gradients of several ions (i.e. [H.sup.+], [Na.sup.+], [K.sup.+], [Ca.sup.2+] [Mg.sup.2+], [Cl.sup.-]) (de Salles et al., 2009). In boys, incomplete recovery between sets can increase blood lactate and acidosis (Ratel et al., 2002b), These alterations may reduce the force or shortening velocity capabilities of skeletal muscle (de Salles et al., 2009; Westerblad et al., 2002). Therefore adequate rest may be vital to ensure the quality of each repetition being performed in exercise sets designed to develop muscle power (ACSM, 2009). For non-athletes, two to five min of rest had been recommended between sets when training for muscle power (ACSM, 2009). However, research to sustain this affirmation is scarce (de Salles et al., 2009). In fact, although 40 or 160 s of rest between sets had the same chronic impact on muscular power (Pincivero and Campy, 2004), the short rest induce greater neuromuscular adaptations (Pincivero et al., 2004). Also, compared to 60 s of rest between sets, a 10 s recovery period was more effective to induce chronic improvements in endurance sprint performance after 8 weeks of training (Saraslanidis et al., 2011).
It has stated that for short duration (i.e. 6 s per sprint) (Abt et al., 2011). In young subjects the recovery capacity from high-intensity plyometric exercises has been reported to be better than in adults (Marginson et al., 2005). For example, 15 s of rest between 10 sets of maximal cycling allow for maintenance of 86% of their peak power (Ratel et al., 2004) and with 30 s the peak power show no decline (Ratel et al., 2002a; 2002b). A high level of flexibility, slower muscle fiber-type composition, and a high level of habitual physical activity in young subjects may help explain their higher recovery ability after high-intensity PT (Marginson et al., 2005). Therefore, as the recovery capacity of young athletes is better than in adults, especially from high-intensity exercise, one may hypothesize that brief (i.e. 30 s) interset rest between PT drills could be viewed as an adequate recovery period to induce training adaptations in this population, especially considering that a typical bounce drop jump takes less than 1 s of maximal effort, with a relatively low metabolic demand, which reduce the time needed to recover from the exercise (Balsom et al., 1992a; Chaouachi et al., 2011). Also, although long and short rest intervals may be equally efective, short inter-set rest may best accommodate the logistical constraints of the training sessions. Thus, the aim of the study was to compare the effects of plyometric training using 30, 60, or 120 s of rest between sets on explosive adaptations in young soccer players.
Subjects and experimental design
Participants with more than 2 years of soccer experience were recruited from an amateur soccer team. Subjects trained 2 sessions per week, in addition to one or two competitive games per week. Athletes also participate in their regular weekly physical education classes. Initially 90 male participants between 8 and 14 years of age fulfilled the inclusion criteria to participate in the study. Participants were randomly assigned to four groups: control group (CG; n = 23), 30s rest interval group (G30; n = 23), 60s rest interval group (G60; n = 22) and 120s rest interval group (G120; n = 22). To be included in the final analyses participants were required to complete >90% of all the training sessions and attend to all measurements sessions. As a result of these requirements 36 participants were removed from the study. Therefore 54 young male soccer players were included for the final analyses. The number of subjects that were included in each group for the final analyses, and their characteristics, are provided in Table 1. None of the participants had any background in regular weight training or competitive sports that involved any of the training methods used in the investigation. To know the soccer-specific weekly training load during the intervention, the session rating of perceived exertion (RPE) was determined by multiply the soccer training duration (minutes) by session RPE as described previously in young soccer players (Impellizzeri et al., 2004). This product represents in a single number the magnitude of training load in arbitrary units (AU). We used the Chilean translation of the 10 points category ratio scale (CR10-scale) modified by Foster et al. (Foster et al., 2001). All groups in the study design had similar soccer-specific weekly training load (Table 1).
Exclusion criteria included participants with 1) potential medical problems or a history of ankle, knee, or back pathology in the 3 months prior to the study, and 2) medical or orthopedic problems that compromised their participation or performance in the study. All participants (and their parents or guardians) were fully informed about the experimental procedures and possible risks and benefits associated with the study. They were then invited to sign an informed consent document before any of the tests were performed. The study was conducted in accordance with the Declaration of Helsinki and approved by the Ethics Committee of the Department of Physical Activity Sciences from University of Los Lagos, Osorno, Chile.
All tests were carried out between 18:00-20:00 h. All participants (and their parents or guardians) were instructed to a) have a good night's sleep ([greater than or equal to] 9 h) before each testing day, b) have a meal rich in carbohydrates and to be well hydrated before measurements, c) use the same sport shoes during the pre and post intervention testing. All participants were motivated to give their maximum effort during performance measurements. In previous studies from our laboratory (Ramwez-Campillo et al., 2013; Ramrnez-Campillo et al., 2014a; Ramrnez-Campillo et al., 2014b) we obtained high intraclass correlation coefficients for the different performance measurements, varying betweens 0.81 to 0.98.
For performance measurements, participants were carefully familiarized with the test procedures during two practice sessions per week during two weeks, performing 20 min of technique training for the testing exercises to be used during measurement. The participants also completed several explosive type actions during these four sessions to become familiar with the exercises used during training. Participants did not report subjective feelings of muscle damage after the familiarization sessions. Considering that the ability to produce explosive single-bout effort such as sprinting, jumping, kicking, or changing direction is as important as aerobic power for success in soccer (Faude et al., 2012; Stolen et al., 2005), the following performance tests were applied in this respective order: countermovement jump (CMJ) for maximal vertical distance (cm); 20 (RSI20) and 40 (RSI40) cm drop jump reactive strength index (mm/ms); maximal kicking distance (m); 20-m sprint time (s); L-run CODS (s). Ten minutes of standard warm-up were executed before each testing day.
Height was measured using a wall-mounted stadiometer (Butterfly, Shanghai, China) recorded to the nearest 0.5 cm, body mass was measured to the nearest 0.1 kg using a digital scale (BC-554 Ironman Body Composition Monitor, Tanita, Illinois, USA) and body mass index (BMI) was calculated (kg-m-2). Maturity was determined by self-assessment of Tanner stage (Weeks and Beck, 2010). The validity of this method for research had previously been reported...