The research aims to identify the effect of intensive decentralized training on the development of some special physical abilities, the rebound force index and the effectiveness of defensive and offensive performance of kumite juniors in karate under 16 years old. The researcher used the experimental method by following the experimental design for the single experimental group using pre- and post-measurements. The research community is represented by the junior kumite players in the sport of karate under (16 years) in the Eastern Karate region and registered with the Egyptian Karate Federation for the 2022/2023 season. The basic research sample was chosen intentionally and included (15) juniors from the Zagazig University Workers Club, in addition to (10) juniors from the same research community and outside the basic research sample to conduct the exploratory study and scientific transactions. The most important results of the research were that intensive decentralized training has a positive effect on the development of some special physical abilities (transitional speed, speed of changing direction, muscular ability, muscular strength, kinetic balance) and the rebound force index for karate juniors under 16 years, and that intensive decentralized training has a positive effect on the development of the effectiveness of defensive and offensive performance for junior kumite players in the sport of karate under 16 years, and there are improvement rates for the post-measurement of the pre-measurement of the research sample members in the tests under study, ranging between (2.09%: 41.65%), and also the effect size values ranged between (0.87: 3.33), which indicates a medium to large effect size significance. The researcher recommends using intensive decentralized training in developing the physical abilities and rebound strength index of karate players of both genders during different age groups and conducting a comparison between intensive decentralized training and other methods of decentralized training and standing training among kumite players of both genders and different age groups.