A modified D-max method to estimate heart rate at a ventilatory threshold during an incremental exercise test

The purpose of this study was to design a modified D-max method to determine heart rate at a ventilatory threshold (HRVT) and to investigate whether this method would be valid during incremental exercise tests. The HRVT was estimated from a new parameter defined as HR at the maximal difference point between linearly- and quadratically approximated HR trends (modified D-max method). HR and ventilatory gas data for 105 subjects (53 males and 52 females; 38.26 ± 12.06 years; 166.62 ± 8.21 cm; 65.31 ± 11.10 kg) were simultaneously collected during an incremental treadmill test to evaluate the validity of the modified D-max method. Reference HRVTs were manually identified from the ventilatory gas data by an experienced sports physiologist and compared with those estimated by the HR parameter. A strong positive correlation (r = 0.71, p < 0.01) and a low HR difference of 9.94 ± 7.10 bpm between the reference and estimated HRVTs were obtained. The results indicate that the modified D-max method outperforms the conventional D-max method (r = 0.53, p < 0.01), the three-piece linear regression lines method (r = 0.42, p < 0.01), and the parallel straight line slope method (r = 0.57, p < 0.01). Furthermore, the modified D-max method improves the predictive accuracy of HRVTs by combining its result with subject's age. The combined parameters have a strong positive correlation with the reference HRVTs (r = 0.74, p < 0.01) and a lower HR difference of 9.40 ± 6.91 bpm. The results suggest that the modified D-max method is highly applicable to predicting HRVTs during incremental exercise tests and also improves HRVT detection accuracy by combining its result with the subject's age.