Heart Rate Kinetics and Sympatho-Vagal Balance Accompanying a Maximal Sprint Test

When a maximal sprint starts, heart rate (HR) quickly increases. After the exercise ends, HR keeps high for seconds before recovering with a roughly exponential decay. Such decay and its time constant (τoff) have been widely studied, but less attention was devoted to the time delay (tdelay) between sprint end and HR decay onset. Considering the correlation between sympatho-vagal balance and performance, as well as the occurrence of heart failure in cardiopaths during the post-exercise phase, we evaluated sympatho-vagal balance before and after sprint, trying to correlate it with both tdelay and τoff. R-R intervals, recorded in 24 healthy adults from 5 min before to 5 min after a 60-m sprint-test (from Storniolo et al., 2017, with permission of all authors), were re-processed to extract HR variability power (LF and HF) in the low- and high-frequency ranges, respectively. The sympatho-vagal balance, evaluated in pre-test resting period (LF/HF)REST and at steady-state recovery (LF/HF)RECOV, was correlated with tdelay and τoff. Both (LF/HF)REST and (LF/HF)RECOV had a skewed distribution. Significant rank correlation was found for (LF/HF)REST vs. τoff and for (LF/HF)RECOV vs. both τoff and tdelay. The difference (LF/HF)RECOV-REST had a normal distribution and a strong partial correlation with tdelay but not with τoff. Thus, a long tdelay marks a sympathetic activity that keeps high after exercise, while a high sympathetic activity before sprint leads to a slow recovery (high τoff), seemingly accompanying a poor performance.