Heart-rate variability threshold, a valid alternative for ventilatory threshold testing and training purposes?

Background: Although spiro-ergometry is the established ‘gold standard’ for determination of the second ventilatory threshold (VT2), it is a costly and rather time-consuming method. Previous studies suggest that assessing the second anaerobic threshold (AT2) on the basis of heart rate variability (HRV) during exercise may be a more cost-effective, practical and non-invasive technique. Appropriate validation studies, however, are lacking.