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. (Fosteret al., 2001). All groups in the study design had similar soccer-specific weekly training load (Table 1).