Higher exercise intensity delays postexercise recovery of impedance-derived cardiac sympathetic activity

Systolic time intervals (STIs) provide noninvasive insights into cardiac sympathetic neural activity (cSNA). As the effect of exercise intensity on postexercise STI recovery is unclear, this study investigated the STI recovery profile after different exercise intensities. Eleven healthy males cycled for 8 min at 3 separate intensities: LOW (40%-45%), MOD (75%-80%), and HIGH (90%-95%) of heart-rate (HR) reserve. Bio-impedance cardiography was used to assess STIs - primarily pre-ejection period (PEP; inversely correlated with cSNA), as well as left ventricular ejection time (LVET) and PEP: LVET - during 10 min seated recovery immediately postexercise. Heart-rate variability (HRV), i.e., natural-logarithm of root mean square of successive differences (Ln-RMSSD), was calculated as an index of cardiac parasympathetic neural activity (cPNA). Higher preceding exercise intensity elicited a slower recovery of HR and Ln-RMSSD (p < 0.001), and these measures did not return to baseline by 10 min following any intensity (p <= 0.009). Recovery of STIs was also slower following higher intensity exercise (p <= 0.002). By 30 s postexercise, higher preceding intensity resulted in a lower PEP (98 +/- 14 ms, 75 +/- 6 ms, 66 +/- 5 ms for LOW, MOD, and HIGH, respectively, p < 0.001). PEP recovered to baseline (143 +/- 11 ms) by 5 min following LOW (139 +/- 13 ms, p = 0.590) and by 10 min following MOD (145 +/- 17 ms, p = 0.602), but was still suppressed at 10 min following HIGH (123 +/- 21 ms, p = 0.012). Higher preceding exercise intensity attenuated the recovery of indices for cSNA (from STIs) and cPNA (from HRV) in a graded dose-response fashion. While exercise intensity must be considered, acute recovery may be a valuable period during which to concurrently monitor these noninvasive indices, to identify potentially abnormal cardiac autonomic responses.