OBJECTIVE: The beat-to-beat variability in electrocardiogram intervals (RR, ie, heart-period variability) provides information on cardiac autonomic activity that predicts arrhythmias and mortality rate in animals and adults. We determined the effect of physical training on heart-period variability in obese children. METHODS: Thirty-five subjects were randomly assigned to physical training and control groups. The training involved 4 months of exercise, 5 days per week, 40 minutes per day. Cardiovascular fitness was measured with submaximal heart rate during supine cycling; percentage of body fat was measured with dual-energy absorptiometry; and resting heart-period variability parameters were measured in a supine position. A pretraining to posttraining change score was computed for each variable. The effect of the training was determined by comparing the changes of the training and control groups. RESULTS: Compared with the control group, the trained group (1) reduced submaximal heart rate and percentage of body fat (p < 0.01); (2) increased in the root mean square of successive differences, a time-domain parameter reflective of vagal tone (p < 0.05); (3) decreased in low-frequency power expressed as a percentage of total power, a frequency-domain index of combined sympathetic and vagal activity (p < 0.03); and (4) decreased in the ratio of low- to high-frequency power, an index of sympathetic-parasympathetic balance (p < 0.01). CONCLUSIONS: In obese children, physical training alters cardiac autonomic function favorably by reducing the ratio of sympathetic to parasympathetic activity.

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