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