Objective: to analyse the correlations between the echo-intensity (EI) of the quadriceps muscle measured by a quantitative ultrasound and the vertical jump in school-age children. Methodology: a cross-sectional, comparative and non randomised study was carried out. An intentional sample composed of 184 schoolchildren aged between 7 and 10 was used. Transversal images of the right quadriceps femoris were obtained by ultrasound to determine the echo intensity of the rectus femoris or anterior, vastus intermedius and vastus lateralis. Explosive power was measured by countermovement jump (CMJ) and repeated jumps for fifteen seconds (RJ15). Results: The EI of the evaluated components of the quadriceps correlates significantly with the measurements of the vertical jump type CMJ and RJ15 in boys and girls aged between 7 and 10 (PP15R vs.Dif 1C, Dif 2C, Dif 3C, r= 0.53-0.59).