Participants

Twenty-one elite youth soccer players belonging to the academy of a professional soccer club, initially volunteered to participate in this study (13.5 ± 0.7 yrs). The youth players were selected to train and take part in the under-14 age category of the club at the beginning of the season (March). From March to December (when the congested match competition occurred), they participated in the training program without any interruptions, except for a 15-day period of holidays, which occurred during July. Data on anthropometric characteristics (stature, body mass, sitting stature, and leg length), physical performance and salivary testosterone concentration were collected for all the cohort, one week prior to the beginning of the competition (Table 2). All players typically participated in three soccer training sessions per week (strength and conditioning training sessions and technical-tactical sessions) and competed in a weekly single match. They habitually performed one strength training session in the gym per week. This was usually done during the first part of the session followed by soccer-specific technical exercises. Specific conditioning training sessions were composed of high-intensity short running bouts (HIBs) and small-sided-games (SSGs). Players habitually performed HIBs or technical exercises prior to SSG.

Data normalized per minute of on-field playing time (mean ± SD) and within the players coefficient of variation (CV) for total running distance (TRD), high-speed running distance (HSRD), accelerations (ACC), and decelerations (DEC) for the 4 assessed matches.

* different to match 1, 2 and 3 (p = 0.05)

Following collection of this information and data related to match participation, players were, retrospectively separated into two groups: (1) a selected group (SLG) and (2) a non-selected group (NSG). The SLG was composed of players who participated in all four assessed matches and completed a minimum 75% of total match time in every match considered for analysis. Consequently, ten outfield players (SLG) were included. Despite not maintaining rigid playing positions, as can be expected in U14 match-play, running performance data from two full backs, two central defenders, four midfielders, and two forwards were analysed.

Written informed consent was obtained from each player and their parents or guardians, respectively, and the study was approved by the local University Ethics Committee. All players underwent a thorough medical assessment to verify their health status prior to participation and were free from illness or injury at the time of this study.