Effects of a Structured After-School Physical Activity Program on Physical Fitness, Body Composition, and Sleep in Primary School Students: A Quasi- Experimental Study

Background Physical inactivity is a significant public health issue among children. While compulsory physical education (PE) may be insufficient to meet daily activity recommendations, structured after-school physical activity (PA) programs have been proposed as a complementary strategy to promote health. This study aimed to investigate the effects of a 12-week after-school PA program on body composition, physical fitness, physical activity levels, and sleep outcomes in primary school students. Methods Thirty-four fourth-grade students from a primary school in Beijing were nonrandomly assigned to a control group (CG, n = 16, regular PE classes) or an experimental group (EG, n = 18, regular PE classes plus a structured after-school PA program, 5 times/week, 40 minutes/session) on the basis of their willingness to participate. Assessments were conducted at baseline, at mid-intervention, and post-intervention. The data were analyzed via two-way repeated-measures ANOVA, t-tests, and non-parametric tests. Results Body composition: The EG significantly reduced the waist‒to‒hip ratio ( F  = 12.290, P  < 0.05) and prevented the significant increases in body fat percentage and hip circumference observed in the CG ( F  = 6.175, 14.431, both P  < 0.05). Physical fitness and agility: The EG demonstrated significant improvements in sit-ups and sit-and-reach test results and multiple agility tests (e.g., 15-s burpee, F  = 39.194, P  < 0.05), outperforming the CG in several metrics. Physical activity: Time spent in moderate-to-vigorous PA (MVPA) significantly increased in the EG ( F  = 8.270, P  < 0.01). Sleep: No significant within-group changes in any objective sleep parameters (e.g., sleep efficiency, total sleep time, or WASO) were observed in the EG (all P  > 0.05). Conclusion The after-school PA program effectively improved body composition, physical fitness, and daily MVPA levels but did not significantly impact sleep, suggesting that sleep may be more influenced by household routines and baseline patterns than structured afternoon activity. Trial registration: This study was registered prospectively in the Chinese Clinical Trial Registry (ChiCTR) on December 30, 2022 (Registration number: ChiCTR2200067241).