Objective:To evaluate the differences in quantitative ultrasound of the quadriceps in a sample of schoolchildren who practise different sports. Material and method: A transversal, comparative and non-randomised study was carried out with an intentional sample of 184 schoolchildren, aged between 7 and 10 years. The sample was divided into three groups based on the sport practised: a) only soccer; b) volleyball, basketball, swimming, gymnastics and cheerleading; and c) no sport practised. The eco-intensity, thickness and pennation angle of some components of the quadriceps (rectus femoris, vastus lateralis and vastus intermedius) were measured using B-mode ultrasound imaging. Results: There were significant differences between the 3 groups of children in the variables that measure the quality of the muscle: eco-intensity and fat percentage of the rectus femoris (p < 0.05), muscle thickness of the anterior (p< 0.05) and lateral (p< 0.01) vastus intermedius, and in the pennation angle of the rectus femoris (ARF) (p< 0.05) and of the vastus lateralis (AVL) (p< 0.01). Between groups b and c there were differences in the ARF in boys (p< 0.01) and in the AVL in girls (p< .05). Conclusion: The way in which sport initiation is carried out in children between 7 and 10 years of age may cause differences in the quadriceps muscle, which can be demonstrated through the use of quantitative ultrasound. Moreover, such differen-ces are related to the duration and frequency of the stimulus, which becomes a qualitative aspect to be considered when programming the training contents at these ages.