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The purpose of this study was to investigate the effects of different durations of resistance training on body composition and power in children. The study was a 24-week longitudinal design involving 31 preadolescent children who were overweight or obese (ages 7-12 years), divided into 3 groups and resistance trained 3 times per week. Group 8 (G8) trained for 8 weeks, group 16 (G16) trained for 16 weeks, and group 24 (G24) trained for 24 weeks. All participants were measured at weeks 0, 8, 16, and 24 for body composition and power. Subjects in G8 and G16 continued to be tested during the testing weeks after cessation of their training programs. Body composition and bone mineral content were measured by dual-energy X-ray absorptiometry. Significant improvements in body composition were seen in the initial 8-week training phase, and these improvements were maintained for the subsequent 16 weeks. Significant changes in percent body fat (~5-7%) were observed at 8 weeks in all 3 groups. Total fat mass decreased significantly at week 8 in G16 and G24 (5.9%). By week 24, total fat mass was significantly reduced by 8.1% in G24. Significant improvements were observed in static jump power, which improved by 10.5% at week 16 in G24. These results suggest that an 8-week resistance training program is sufficient time to significantly change body composition, strength, and power measures in children who are overweight or obese. However, further improvements are realized with longer-duration resistance training programs. On cessation of the training programs, the G8 and G16 groups maintained the benefits of the exercise program until the end of the study period. KEY WORDS strength training, obesity, exercise