Abstract
The aim of this study was to compare the heart rate (HR) dynamics and variability before and after high-intensity interval training (HIIT) and moderate-intensity continuous training (MICT) protocols with workloads based on treadmill workload at which maximal oxygen uptake was achieved . Ten participants performed cardiopulmonary exercise testing (CPET) to obtain oxygen uptake . All training protocols were performed on a treadmill, with 0% grade, and had similar total distance. The MICT was composed by 21 min at 70% of . The first HIIT protocol (HIIT-30 : 30) was composed by 29 repetitions of 30 s at 100% of and the second HIIT protocol (HIIT-4 : 3) was composed by three repetitions of 4 min at 90% of . Before, during and after each training protocol, HR dynamics and variability (HRV) were analysed by standard kinetics and linear (time and frequency domains). The repeated measures analysis of variance indicated that the HR dynamics, which characterizes the speed of HR during the rest to exercise transition, was statistically (p < 0.05) slower during MICT in comparison to both HIIT protocols. The HRV analysis, which characterizes the cardiac autonomic modulation during the exercise recovery, was statistically higher in HIIT-4 : 3 in comparison to MICT and HIIT-30 : 30 protocols (p < 0.005 and p = 0.012, respectively), suggesting that the HIIT-4 : 3 induced higher sympathetic and lower parasympathetic modulation during exercise in comparison to the other training protocols. In conclusion, HIIT-4 : 3 demonstrated post-exercise sympathetic hyperactivity and a higher HRpeak, while the HIIT-30 : 30 and MICT resulted in better HRV and HR in the exercise-recovery transition. The cardiac autonomic balance increased in HIIT-30 : 30 while HIIT-4 : 3 induced sympathetic hyperactivity and cardiac overload.
Highlights
There is a wide debate regarding optimal exercise training protocols to improve cardiorespiratory function, glycaemic control, disease prevention and treatment [1] as well as to reduce mortality [2]
When analysing the HRon kinetic variables, a significant difference was found in the τ variable between the moderate-intensity continuous training (MICT) and high-intensity interval training (HIIT)-4 : 3 protocols ( p = 0.041), with MICT promoting a delay in the heart rate (HR) response in the transition between rest and exercise
The same result was obtained in the comparison between the MICT and HIIT-30 : 30 protocols ( p = 0.032), since MICT presented a longer HR response time than the HIIT-30 : 30 protocol
Summary
There is a wide debate regarding optimal exercise training protocols to improve cardiorespiratory function, glycaemic control, disease prevention and treatment [1] as well as to reduce mortality [2]. Among the possible effects and outcomes of an exercise intervention, the study of the cardiac autonomic nervous system (ANS), by the analysis of the heart rate (HR) responses during the transitions from rest to exercise to recovery, provides information that might be used in the future for a better training protocol prescription, avoiding the risks and controlling the onset of fatigue [11,12,13,14]. During the rest to exercise transition, the HR increases initially due to a reduction of the parasympathetic nervous system activation, followed by an increase in the sympathetic tone [15]. The HR initially decreases based on a sudden vagal reactivity followed by a gradual sympathetic autonomic tone reduction [16]. The evaluation of the ANS by characterizing HR responses during the rest, exercise and recovery periods presents an interesting prognostic value in the general population [17,18,19,20] and can be used as a cardiovascular fitness index [15,21]
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