Heart Rate Break Point May Coincide with the Anaerobic and Not the Aerobic Threshold

 

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Abstract

Recently, Conconi et al. (4) proposed that the point where heart rate departs from linearity in an incremental exercise test is a good predictor of the aerobic threshold (AeT, i.e., the exercise intensity at which blood lactate concentration increases systematically above resting levels). We hypothesized that this heart rate break point (BrP) is a better predictor of the anaerobic threshold (AnT, i.e., the exercise intensity at which blood lactate concentration shows a rapid rise during an incremental test). To test this hypothesis, 11 subjects with different levels of conditioning were tested on a cycle ergometer using a progressive incremental exercise protocol. Heart rate from EKG tracings and blood samples for lactate determination were taken every minute. The results showed the following significant correlation coefficients when the variables were expressed in watts: AeT and AnT = 0.92; AeT and BrP = 0.89; AnT and BrP = 0.97. The AeT was significantly lower than the BrP (166.4 ± 52.6W and 234.5 ± 69.5 W). There was no significant difference between the AnT and BrP (240.0 ± 67.1 W and 234.5 ± 69.5 W). Another group consisting of 16 subjects performed two tests to evaluate the reproducibility of the BrP. Although a ventilatory AnT (defined as a consistent decrease in the fraction of expired COj) was noted in all the tests, eight subjects failed to demonstrate a BrP in at least one of the evaluations, even though post-exercise blood lactate levels and peak heart rates were consistent with a maximal effort. We conclude that the BrP may coincide with the AnT and not the AeT; however, the fact that the BrP could not be demonstrated with a documented ventilatory AnT in several subjects suggests that no causal relationship exists between the two, and that the BrP may not be a generalizable physiologic variable.