Intervals between two consecutive cardiac beats (R-R intervals) and the subsequent analysis of heart rate variability (HRV) obtained simultaneously from the Polar S810 heart rate monitor (HRM) and an electrocardiogram (ECG) in a supine position were compared in twelve children (age 9.6 +/- 0.9 years) before and after protocol correction. R-R intervals were significantly different between the ECG and the HRM uncorrected and corrected signal (p < 0.001, effect size [ES] = 0.005, and 0.005, respectively). However, the bias (95 % confidence interval) was 0.80 (- 124.76 - 123.16) ms and 0.80 (- 12.76 - 11.16) ms, respectively. HRV parameters derived from both signals were not different (p > 0.05) and well correlated (r > 0.99, p < 0.05), except SD2 (p < 0.05, ES = 0.000; r = 0.99). These data support the validity of the Polar S810 HRM to measure R-R intervals and make the subsequent HRV analysis in a supine position in children.