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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 (τ off ) have been widely studied, but less attention was devoted to the time delay (t delay ) 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 t delay  and τ off . 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 t delay  and τ off . Both (LF/HF) REST  and (LF/HF) RECOV  had a skewed distribution. Significant rank correlation was found for (LF/HF) REST  vs. τ off  and for (LF/HF) RECOV  vs. both τ off  and t delay . The difference (LF/HF) RECOV–REST  had a normal distribution and a strong partial correlation with t delay  but not with τ off . Thus, a long t delay  marks a sympathetic activity that keeps high after exercise, while a high sympathetic activity before sprint leads to a slow recovery (high τ off ), seemingly accompanying a poor performance.

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