The autonomic nervous system recovery after exercise is differently affected by incremental vs. all-out tests

INTRODUCTION: By investigating steady-state exercises at different percentages of maximal aerobic power, it has been reported that the time required for the autonomic nervous system to recover grows with exercise intensity (Michael et al., 2017; doi: 10.3389/fphys.2017.00301). However, little is known about this recovery after exhausting protocols in which the load changes over time, nor is it clear whether training to a specific aspect of the exercise (e.g., power vs. endurance) may differently contribute to the autonomic recovery. Therefore, we aimed to compare incremental and all-out exercises, starting with a population of athletes mainly trained for power output. METHODS: ECG was recorded in fourteen semi-professional rugby players (male Italian League, second and third divisions) who accomplished two tests on the cycle ergometer, randomized and separated by at least 72 h. One was an incremental test (INC) requiring a regular load increase of 30 W every minute till exhaustion; the other was an all-out effort (AOU) against the subject’s maximal cycling load. Tests were preceded by a 10 min resting period and followed by a 10 min recovery period. In the last 5 min of each period, the sympathovagal balance was assessed by measuring HRV power in the low (LF) and high-frequency (HF) bands, together with the mean value of heart rate (HR). Moreover, the post-exercise exponential decay of HR was evaluated to calculate the time constant Tau. RESULTS: A treatment x exercise ANOVA confirmed that HR increased after both exercises, but to a larger extent for INC than AOU (+35% and +24%, respectively; interaction p = 0.007). In parallel, a paired t-test indicated a longer Tau after INC than after AOU (109.8 vs. 65.7 sec; p = 0.002). From the sympathovagal balance standpoint, the ratio LF/HF (in log units) increased similarly after both protocols (about +13%; exercise p = 0.013; no interaction). CONCLUSION: The higher HR value and the longer Tau observed after INC suggest that this exercise is more stressful for the autonomic nervous system. However, the parallel increase in log LF/HF after both tests suggests that INC and AOU induced similar changes in both the sympathetic drive and the vagal tone. The different response to INC vs. AOU exercise might be due to the higher external work exerted in the former test (about 90 vs.21 kJ). Indeed, despite the significantly lower peak power (on average 306 W in INC vs. 927 W in AOU), the INC protocol had a longer duration than AOU (563 vs. 23 sec). A second possibility is that this difference may stem from the training program, which mainly targeted power output in rugby players. It will be of interest to complete these results with those obtained in endurance-trained athletes.