Background & Aims: Optimal performance of sports skills is due to the complex interaction of physiological, anthropometric, psychological and biomotor factors. The most important factors for success in various sports include bodybuilding, tactical, technical skills, mental abilities and athletes' experience (2). The performance of elite athletes who are active in various sports can be generally attributed to two factors: one is related to the level of training of these athletes and the adaptations made by them, and the other is related to genetic factors and the potential talents are the individual. Among these, their individual (genetic) characteristics can be partially identified by examining anthropometric characteristics and functional adaptations resulting from exercise using functional tests. Sports coaches, on the other hand, are always looking for ways to bring athletes to the highest levels of athletic performance(3). Sports experts and coaches have acknowledged that optimal performance depends on a number of factors, including physical, psychological, and skill factors. Anthropometric characteristics and physical condition, although they may be only part of the factors influencing sports activities, but it is important for coaches to identify these characteristics and be able to understand its value in their athletes (4). However, due to little research in the field of improving selected biomechanical variables in adolescent athletics and also the lack of research on the effect of physical characteristics on the effectiveness of training and the lack of research on combined training on performance and performance factors of adolescent athletes, conduct research The present is necessary and it is hoped that this research can take an effective and small step to reduce the research gaps in the field of athletics as an Olympic-oriented sport. Therefore, the present study seeks to answer the main question whether eight weeks of combined training (plyometric and resistance) have a significant effect on selected biomechanical variables (flexibility, balance, power and speed) of ectomorphic and mesomorphic runners? Methods: The method of the present study was quasi-experimental and was done with pre)test, post-test and control group. The statistical population of the study consisted of adolescent athletes aged 12 to 17 years. The sample consisted of 15 ectomorphic adolescents, 15 mesomorphic adolescents and 15 adolescents as a control group who participated in the adolescent training of active teams in Tehran province leagues and were purposefully invited to participate as a statistical sample. In this study, the training program consisted of 8 weeks of specific training for each of the experimental groups that performed combined exercises for 8 weeks. The Sargent test was used to evaluate the power, the 30 m sprint test was used to evaluate the speed, the Star Test Equilibrium test (SEBT) was used to evaluate the dynamic balance, and the Flex board test was used to evaluate the flexibility. In this study, the training program included 8 weeks of specific training for each group as follows. The control group performed normal exercises with evidence, and the experimental groups performed selected plyometric or resistance exercises for 30 minutes, including 10 minutes of warm-up, and 20 minutes of special exercises. Also, the selected resistance training program was in three periods with 12 repetitions in each period, during which the subjects performed the mentioned exercises with 60% of maximum strength. To increase the speed or speed of movement, a training load with a weight of about 60-30 should be Percentage of the athlete's maximum strength. One minute between periods and two minutes between stations, active rest (flexibility and stretching movements) was considered and included the following movements: foot press, knee extension, knee flexion, lifting on the toe with weights, squats and lifting the pair was performed with weights (15). The subjects then performed plyometric exercises for 30 minutes. The selected plyometric exercise program was designed based on the principle of overload and each movement was performed in three fixed repetitive winds in each period in all weeks, between each stage. A minute of rest was considered. The exercises offered included speed jump with one and two legs, jumping from a squat position, bending the knee with a jump, jumping with the open leg forward, scissor jumping, and so on To analyze the data along with descriptive statistics (mean and standard deviation, etc.), regression test, and analysis of covariance and Bonferroni post hoc test in SPSS software version 24 were used. Results: Covariance test showed that the value of F related to covariance in all four research variables was not statistically significant (P≤0.05), on the other hand, the value of F related to the group in all four variables of research was statistically significant (P≥0.05).That is, after removing the pre-test effect, there is a significant difference between the mean scores of the two groups in the post-test of selected biomechanical variables in the ectomorphic group, so eight weeks of combined training on selected biomechanical variables (flexibility, balance, power and speed) Ectomorphic work is impressive. The covariance test in Table 3 showed that the value of F related to covariance in all four research variables was not statistically significant (P≤0.05), on the other hand, the value of F related to the group in all four variables of research was statistically significant (P≥0.05). is significant. That is, after removing the pre)test effect, there is a significant difference between the mean scores of the two groups in the post-test of selected biomechanical variables in the ectomorphic group; Therefore, the null hypothesis that the mean difference between the two groups in the post-test is not significant is rejected after eliminating the possible effect of the pre-test. Therefore, eight weeks of combined training affects the selected biomechanical variables (flexibility, balance, power and speed) of Mesomorphic runners. By comparing the effect sizes (ETA coefficient) in both ectomorphic and mesomorphic hands, it is found that the ectomorphic measurements have better performance in all four biomechanical variables. The results of the present study showed that eight weeks of combined training (plyometric and resistance) had a significant effect on selected biomechanical variables (flexibility, balance, power and speed) of ectomorphic and mesomorphic athletics boys. These exercises also have a greater effect on selected biomechanical variables of ectomorphic boys than in mesomorphic athletes. Conclusion: In this regard, it can be said that combined training improves muscle weakness and deficiency in ectomorphs in two ways, which first causes muscle hypertrophy through resistance training and as a result, plyometric training increases muscle well-being. However, mesomorphs have reached their optimal physical level due to adequate muscle volume and may require higher training volume and pressure to increase capabilities, so it can be concluded that the combination exercises used in the present study are suitable for ectomorphic individuals, but individuals Mesomorphs need more pressure and a higher volume of training to improve their physical performance, and this issue can be considered in the design of training and future research on this issue. Use combined exercises in athlete training programs. [ABSTRACT FROM AUTHOR]