The purpose of this study was to determine if changes in heart rate variability during incremental exercise could be used to estimate lactate threshold and ventilatory threshold in healthy adults. Twenty-four adults performed graded maximal cycle ergometry to volitional fatigue. Blood lactate, heart rate, RR interval, and respiratory gas exchange were monitored. Heart rate variability was analyzed using time domain indices (standard deviation and mean successive difference). A marked RR interval deflection point was found in the region of lactate threshold and ventilatory threshold, and was identified as the heart rate variability threshold. Mean differences between heart rate variability threshold, ventilatory threshold, and lactate threshold ranged from 0.06 +/- 0.3 to 0.12 +/- 0.2 L.min (-1). Correlations between the different measures ranged from 0.82 - 0.89. A small but significant difference was found between ventilatory threshold and lactate threshold (p < 0.05). There was not a significant difference between heart rate variability threshold and lactate threshold (p > 0.05), nor between heart rate variability threshold and ventilatory threshold (p > 0.05). The data suggest the heart rate variability threshold coincides with lactate threshold and ventilatory threshold during graded exercise and can be used for the detection of lactate threshold in healthy adults.

These data demonstrate the robustness of the lactate, ventilatory and heart rate variability thresholds when challenged by a physiological dose of caffeine.

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