RESUMO
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.