Martín-San Agustín, R, Medina-Mirapeix, F, Alakhdar, Y, and Benítez-Martínez, JC. Sex differences in the velocity of muscle contraction of the hamstring and quadriceps among recreationally active young adults. J Strength Cond Res 33(5): 1252-1257, 2019-This study determines sex differences in the velocity of contraction (VC) of 5 isolated muscles (biceps femoris, semitendinosus, rectus femoris, vastus medialis, and vastus lateralis) and in the relationships between them. Thirty-six female and 34 male recreationally active young adults participated in the study. The VC was measured by tensiomyography using normalized response velocity (Vrn) to perform comparisons. Sex comparisons were adjusted by height and mass. The study of relationships was carried out by comparing and calculating means and ratios. Sex differences were observed in the VC of rectus femoris (mean difference = 6.20 mm·s; p < 0.001). Conversely, the biceps femoris only showed sex differences in the unadjusted analysis (mean difference = 6.66 mm·s; p = 0.002; d = 0.73. Both sexes showed lower VC values of the hamstring with respect to the quadriceps. Female participants showed differences greater than 15% relative to male participants between biceps femoris and quadriceps ratios and in ratios in the hamstring. Thus, our findings in the VC ratios indicate different mechanical contractile properties between sexes in the relations between the hamstring and quadriceps. Our analysis of the VC at these muscles supposes a new possibility to establish the relationships between knee agonists and antagonists, which allow monitoring the changes in the balance of the VC among the muscle groups.