The article is devoted to the study of the peculiarities of the development of flexibility and strength of schoolchildren aged 13-14 under the influence of martial arts fitness. The purpose of the study is to determine the effectiveness of using fitness exercises with elements of martial arts for the development of flexibility and strength abilities of secondary school children. The research involved 20 boys and 20 girls who studied in the 8th grade of a secondary school. The following methods were used to solve the research tasks: studying and analyzing literary sources on the research topic; pedagogical experiment; pedagogical testing of the level of physical abilities development, methods of mathematical statistics. To conduct the pedagogical experiment all participants of the experiment were divided into two groups: experimental (10 girls and 10 boys) and control (the same composition). The essence of the experiment was that for children who were included in the experimental group during 5 weeks during physical education lessons an experimental variable module on fitness with elements of martial arts was introduced. When developing the variable module on martial arts fitness, we adhered to the requirements for variable modules in the current physical education curriculum for schools. Girls and boys in the control group attended physical education classes based on the educational material of the variable module gymnastics. The study proved that the use of a variable fitness module based on the use of classical aerobics exercises and martial arts elements during physical education lessons in middle school contributes to a statistically significant improvement in flexibility (forward bending) and strength (flexion and extension of the arms in a lying position and lifting the torso from a lying position in 30 seconds). At the same time, such exercises are emotional and can have a positive impact on adolescents' functional fitness, well-being, and mood.