Boxing one of the sports which is characterized by specified number of rounds interspersed with short breaks. The boxer performs offensive, defensive and counter-offensive skills from various punching distances; continuously and effectively, taking into account the correct technical performance requirements characterized by being forceful, rapid, proactive and enduring since the beginning of the game until the end in order to win and achieve higher scores. Training should be designed to be compatible with the requirements of boxing. The boxer should have high physical efficiency during the game so that he can strike the greater number of punches and effectively finish the game. Muscle strength is one of the most important physical elements for boxers. Boxing is mainly based on the ability of the muscles to contract and expand, as well as working alternately to perform limb movements. The more powerful are the muscles the more effective are the contractions. Abu Ela Abdel-Fattah (1997) defines muscular balance as the strength of one muscle or muscle group and its correlation to another muscle or other muscle group. Terzis, G., & Al, (2003) believes that speed distinctive strength exercises for the muscles of the arms; particularly the triceps muscle, as they have a positive impact on the level of technical performance. The rapid and powerful movements of the boxer lead to stressing the tissues of the body (tendons-muscles-ligaments) and makes them vulnerable to injury. Therefore, strength training increases the connectivity of the tissues connecting the body parts and help preventing injury.

This research aims to identify the impact of athletic stress on the biomechanics of the movements of the joints in the lower extremity during performing compound skills by the junior soccer player. The researcher used the descriptive method through using kinematographic analysis. The research was applied to a sample of ( 6) junior soccer players selected intentionally from Mansoura Sports Club. The researcher used the following physical tests: Standing Balance Test -Illinois Agility Test -Quadrant Jump Test -Vertical Jump Test Isometric Leg Strength Test, in order to identify the level of stress in addition to skill tests as well (receive&kick). This study used kinematographic analysis by using Simi Motion System. The researcher used the pre-and post-measurement before and directly after the match and before the players could recover. The most important results were that the averages values of the biomechanical variables and the physical measurements confirm a decrease in the post-measurement test than the pre-measurements in all the skill performance phases. Also, the time of performing the two skills under study was shorter in the pre-measurements, which confirms in general that the research sample was significantly influenced by stress. There is a significant correlation between the physical and the biomechanical variables expressing stress. In addition, there is a correlation between physical and biomechanical variables and the skill performance phases in terms of post-measurement, which influences the effectiveness of the skillful performance skills in soccer for the research sample.