OBJECTIVE: To evaluate the effect of a 3-year after-school physical activity (PA) intervention on aerobic fitness and percent body fat (%BF). METHODS: In total, 18 schools were randomized into intervention or control arms. Measurements were made at the beginning and end of the third, fourth and fifth grades. Fitness was measured with heart rate response to a bench-stepping task.%BF and bone density were measured with dual-energy x-ray absorptiometry. The intervention included 40 min of academic enrichment activities, during which healthy snacks were provided, and 80 min of moderate-to-vigorous PA (MVPA). RESULTS: Data analyses were performed on 206 youths who remained in the same schools for the 3-year period, who were measured at all six time points and, for the intervention group, who attended at least 40% of the sessions in each of the 3 years. The group by time interactions were significant for fitness (p < 0.01) and %BF (p < 0.05). Children in intervention schools improved in fitness and %BF during the school years and returned to levels similar to those in control schools during the summers. Over the six measurement points, the intervention group increased more than the control group in bone density (p < 0.01), fat-free soft tissue (p < 0.01), weight (p < 0.01), height (p < 0.01), and body mass index (p < 0.05). CONCLUSIONS: An after-school program focusing on MVPA had a beneficial effect on fitness and body composition. During the summers, the beneficial effect of the previous year's participation on fitness and %BF was lost. This highlights the importance of year-round programs to promote healthy growth in youths.

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