Aims: To compare the effects of 2 and 5 min of passive static stretching (SS) on stiffness and blood flow in the rectus femoris in adolescent athletes using shear wave elastography (SWE) and superb microvascular imaging (SMI). Material and methods: This prospective study included 20 male athletes with median age of 14.5 (12.5-16.5) years. The subjects were divided into two groups based on the SS duration as follows: 2 min (n=10) and 5 min (n=10). At rest and after 2 and 5 min of SS, stiffness and blood flow values were compared in the rectus femoris for each group. Inter-operator reliability was also analysed. Results: There was no significant difference between resting and 2 min of SS in terms of stiffness. The stiffness values decreased significantly from resting to 5 min of SS. The blood flow increased significantly from resting to 2 and 5 min of SS. Inter-operator reliability was moderate to perfect for SWE and SMI measurements (ICC: 0.52-0.83). Conclusions: SWE and SMI can be used to acquire reliable quantitative data about muscle stiffness and blood flow in adolescents. While stiffness parameters significantly decreased from resting after only 5 min, blood flow significantly increased both after 2 and 5 min. For physical rehabilitation protocols, 5 min of SS may be chosen to reduce stiffness. For competitions, 2 min of SS may be sufficient for warm-up exercise because it increases the blood flow optimally. Five min of SS may be preferred for the cool-down exercise to enhance recovery.