Identification of metabolic thresholds in fitted heart rate curve

Abstract

The aim of this study was to compare the linear and sigmoid adjustments for the behavior of the heart rate (HR) during graded exercise test and to verify the validity of these models to identify metabolic thresholds from Boltzman's non-linear adjustment during incremental exercise. Ten high level mountain bikers (27.1 ± 7.4 years; 67.0 ± 6.1 kg; 175.1 ± 4.1 cm; 6.1 ± 1.9% estimated body fat) were submitted to graded exercise test in cycle-simulator, with initial load of 100 W and 30 W increments every 3 min until exhaustion. The HR behavior was fitted by Boltzman's sigmoid equation, and HR inflection point (HRIP = fixed value of V50 from equation) and HR deflection point (HRDP = Dmax method) were identified. Blood lactate samples were withdrawn at the end of each stage. The first lactate threshold (LT1) was determined by the calculation of minimum value equivalent of lactate/work. To identify the second lactate threshold (LT2), 1.5 mmol.l-1 was added to the LT1 value. In all athletes the sigmoid model was more appropriate than the linear one (p < 0.0001). The HRIP (161 ± 10 bpm) and HRDP (179 ± 5 bpm) were identified in 8 and 7 subjects, respectively. From the similarity of the results about the intensity in power output and HR, it can be suppose that the HRIP is in the same physiological domain of the LT2, above to LT1 and lower to HRDP. However, were found poor association between lactate thresholds, HRIP and HRDP (-0.08 to 0.37).