Introduction: The impact of stretching on muscle strength encourages conflicting opinions of experts which makes the inclusion of stretching modalities in the warm-up process somewhat controversial despite benefits that include increased range of motion and injury prevention. Although some claim that the application of both static stretching and post-isometric relaxation has a negative effect on muscle performance, others refute these claims. The main goal of this research was to examine the acute impact of muscle stretching on the muscle strength of the hamstrings muscles. Materials and methods: 30 respondents (25 women and 5 men) between the ages of 18 and 24 voluntarily participated in the research. All subjects were measured three times (1x week), before stretching and after applying static stretching and post-isometric relaxation. The range of motion was measured in the position of the modified front split suggested by Sands et al. (2008.) with a centimeter strip from the top of the front upper lateral spine of the pelvis of the hind leg to the base before and after the application of the intervention. Muscle strength was measured with a manual dynamometer and the values of maximum muscle strength, average muscle strength and time needed to generate maximum muscle strength measured without stretching and after applying each of the two previously described modalities were compared. Results: A statistically significant increase in range of motion was found after the application of static stretching (right leg P=0,00; left leg P=0,00) and post-isometric relaxation (right leg P=0,00; left leg P=0,00). There were no statistically significant improvements in muscle strength except for the dominant leg in average muscle strength after static stretching (P=0,02) and speed of achieving maximal muscle strength after post-isometric relaxation (P=0,03). Conclusion: Static stretching and post-isometric relaxation have a significant effect on increasing the range of motion without causing a negative effect on muscle performance and therefore represent a useful tool in preventing injuries and improving the performance of athletes.