Thirty-nine regional-level youth football players from three different teams, competing in the first division of the U-17 Portuguese football league, were analysed in this study. Only outfield players were included. Their age, stature, mass, percentage of body fat and mean peak heart rate (HR) were (mean±SD) 15.6±1.4 years, 169.5±7.1 cm, 61.5±12.5 kg, 14.9%±4.1% and 197±9
b·min−1, respectively. During games, one player suffered an injury and eight players were substituted. Those players were not considered for further analyses. Therefore, the final sample included 30 players: fullbacks (n=7), central defenders (n=7), midfielders (n=8) and forwards (n=8). All players were interviewed to provide information concerning the number of years of football practice and hours of regular training per week. Players were regularly involved in football for 4.0±2.6 years. Players and their parents provided informed consent, and ethical consent was approved by the Scientific Committee of the Faculty of Sport of the University of Porto and by the club officials. All procedures were conducted according the declaration of Helsinki