HEART RATE VARIABILITY ADJUSTMENTS DURING CYCLING EXERCISE AT DIFFERENT INTENSITIES UNTIL EXHAUSTION

Heart rate variability (HRV) is a useful tool to assess the autonomic system modulation during exercise, however to best of our knowledge no one has been verified the responses of HRV during constant workload cycling exercise until exhaustion at two different intensities. Purpose: We sought to assess the HRV and physiological changes during cycling exercise until exhaustion at the first lactate threshold (LT1) and above the second lactate threshold. Methods: After a preliminary maximal incremental test to assess the peak power output (WPEAK), subjects (n = 10) performed a constant workload cycling test until exhaustion at two different intensities. In a randomized and counterbalanced order, cycling until exhaustion either at LT1 or at 25% of the difference between LT2 and the WPEAK (> LT2). HRV, blood samples and ratings of perceived exertion were collected throughout both exercise sessions. Results: A total parasympathetic withdrawal (SD1 < 3ms) occurs since from exercise beginning at exercise > LT2 and just at 60% (p < 0.05). High Frequency (HF) also showed a main effect of time at LT1 exercise and time-intensity interaction from 20 to 80% of total exercise (p < 0.05). Regarding physiological variables, norepinephrine was increased progressively at > LT2 (p < 0.05) but not at LT1 (p > 0.05). Additionally, respiratory ratio was increased significantly (p < 0.05) throughout the exercise at both intensities. Conclusion: Our results suggested that HRV and respiratory rate doesn’t elicit a steady state behavior neither at LT1 not > LT2 and the exercise performed at > LT2 intensity lead to a parasympathetic withdrawal since from beginning of the exercise while at LT1 just from 60% of the exercise.