Heart rate variability kinetics during different intensity domains of cycling exercise in healthy subjects

The purpose of this study was to verify the heart rate variability (HRV) and heart rate (HR) kinetics during the fundamental phase in different intensity domains of cycling exercise. Fourteen males performed five exercise sessions: (1) maximal incremental cycling test; (2) two rest-to-exercise transitions for each intensity domain, that is, heavy (Delta 30) and severe (Delta 60) domains. HRV markers (SD1 and SD2) and HR kinetics in the fundamental phase were analyzed by first-order exponential fitting. There were no significant differences in amplitude values between SD1(Delta 30) (8.98 +/- 3.52 ms) and SD1(Delta 60) (9.44 +/- 3.24 ms) and SD2(Delta 30) (24.93 +/- 9.16 ms) and SD2(Delta 60) (25.98 +/- 7.29 ms). Significant difference was observed between HR Delta 30 (52 +/- 7 bpm) and HR Delta 60 (63 +/- 8 bpm). The time constant (tau) values were significantly different between SD1(Delta 30) (17.61 +/- 6.26 s) and SD1(Delta 60) (13.86 +/- 5.90 s), but not between SD2(Delta 30) (20.06 +/- 3.73 s) and SD2(Delta 60) (19.47 +/- 6.03 s) or HR Delta 30 (56.75 +/- 18.22 s) and HR Delta 60 (58.49 +/- 15.61 s). However, the tau values for HR Delta 30 were higher and significantly different in relation to SD1(Delta 30) and SD2(Delta 30), as well as for HR Delta 60 in relation to SD1(Delta 60) and SD2(Delta 60). The kinetics of the autonomic variable (SD1 marker) was accelerated by the increased intensity. Moreover, significant differences were found for the tau values, with faster HRV markers than HR, in both intensities of Delta 30 and Delta 60, which suggests that these variables indicate distinct and specific cardiac autonomic response times during different intensity domains in cycling.