The functional disorder of the hamstrings muscles is associated with prolonged sitting in a chair, poor posture, lack of motor activity and others. Maintaining elastic muscle tissue is an important factor for children practicing martial arts; karate in particular. This study has the purpose to examine hamstring muscles and their condition in children practicing karate. Methodology During the period between March - May 2022, we worked with 30 children in total, in the age group between 8-10 years old, who were practicing karate or boxing in clubs “Fighters”, located in the city of Varna, and club “Khan Krum”, located in the city of Shumen, respectively. For most accurate results, we formed two separate groups with 15 children each. The participants in our control group were training boxing, whilst the children in our experimental group were practicing karate. All 30 children had been training for a period of 6 months, practicing 2 times per week, one astronomical hour per session. At the beginning of every training session with our experimental group, we treated the hamstring muscles of the participants with post-isometric relaxation. To conclude our findings, we assessed the condition of the hamstring muscles of all 30 children at the beginning and end of the study period, utilizing the method of Laseg and a Seat Test with outstretched legs on the floor with a slope to the toes. Results The results at the end of the study period show that the functional deficit of the hamstring muscles continues to be present in 11 children training boxing. In comparison, only five out of 11 initially diagnosed children with hamstring muscle imbalance from our experimental group showed signs of no improvement. Conclusion The results of this study prove the hypothesis that practicing karate with the inclusion of PIR leads to improved elasticity of the hamstring muscles. The proper selection of physiotherapeutic methods and their regular application could not only heal hamstring muscle stiffness but it could also prevent it, if applied from an early age.