The aim of this study was to know the effect of interval training to increase physical fitness. Subject was Student Extracurricular at Senior High School 1 South Indralaya, Palembang, and collected using total sampling (N =30). Through a quasi-experimental research, treatment given to the subject during 8 weeks and the frequency of training 3 times a week with 60% -70% of the maximum heart rate and duration of 30 minutes. Sample do to a 400-meter jogging with 3 replicates, the ratio of 1:3 and

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... ctive recovery. The results showed that there was difference between pretest and posttest data physical fitness (p-value < 0,05). It can be concluded that the interval training can be used to improve physical fitness.