The analysis of blood lactate concentration and pulmonary gas exchanges are methods traditionally employed to identify the transition in the muscle metabolism energy production. However, the analysis of heart rate variability has been recently suggested as an alternative method. The objective of the present study was to compare the heart rate variability threshold (HRVT) with the ventilatory threshold (VT) in a sample of adolescents. Forty-one subjects (22 boys and 19 girls) with age between 15 and 18 years were submitted to a maximal exercise test in a treadmill (modified Bruce protocol). The VT was identified using the ventilatory equivalent of oxygen by means of ergospirometry resources. The heart rate variability was analyzed from the R-R intervals, through the Poincaré plot, which provides information with regard to the standard deviation of the instantaneous beat-to-beat variability (SD1), the standard deviation of the long-term continuous R-R intervals (SD2) and the SD1/SD2 ratio. The HRVT was identified according to three criteria: (1) difference lower than 1 ms in the SD1 between two consecutive exercise levels; (2) SD1 lower than 3 ms; and (3) occurrence of both criteria simultaneously. Through the analysis of the results it was verified that the R-R intervals and the SD2 reduced progressively at each 10% interval of VO2peak until the end of the physical effort (0.05 < p < 0.01). The SD1 reduced significantly since 20% until 50% of VO2peak. From 60% until the VO2peak there was no significant difference in the SD1. The SD1/SD2 ratio began to increase at 60%. The VT occurred at 54.4 ± 8.8% of VO2peak, while the HRVT occurred at 52.4 ± 12.5%, 57.0 ± 14.1% and 57.8 ± 13.8% of VO2peak, for criteria 1, 2 and 3, respectively. No significant differences between the VT and the three criteria used for HRVT identification were observed. Significant product-moment correlation coefficients were found between the HRVT identified by the three criteria and the VT, when VO2 absolute values were analyzed. However, there was no statistically significant correlation between the HRVT and the VT when expressed in proportion of VO2peak. In short, it seems early to apply the HRVT as an alternative method in the adolescents VT identification.
Heart rate variability; Poincaré plot; Ventilatory threshold; Anaerobic threshold
