Heart rate recovery fast-to-slow phase transition: Influence of physical fitness and exercise intensity

Background: Postexercise heart rate (HR) recovery presents an exponential decay, with two distinct phases: a fast phase, characterized by abrupt decay of HR, and determined by parasympathetic reactivation; and a slow phase, characterized by gradual decay of HR, and predominantly determined by sympathetic withdrawal. Although several methods have been proposed to assess postexercise HR recovery, none of those methods selectively assesses the time of transition from the fast to the slow phase of the HR recovery curve (HRR_PT ), and the magnitude of decay prior to (HRR_FP ) and after this point (HRR_SP ). Therefore, the aim of the present study was to propose a method to identify HRR_PT , HRR_FP , and HRR_SP and to verify the effects of exercise intensity and physical fitness on such parameters.

Methods: Ten healthy young participants (24 ± 3 years; 23.6 ± 1.7 kg/m² ) randomly underwent two exercise sessions (30 min of cycling), at moderate (MI) and high intensity (HI); followed by 5 min of inactive recovery. HR was continuously recorded during the sessions. The algorithm for HRR_PT analysis was written in Python and is freely available online.

Results: HRR_PT and HRR_SP were increased in HI session compared with MI (81 ± 24 vs. 60 ± 20 s; 8 ± 10 vs. 1 ± 5 bpm; p = .04), and there was no difference in HRR_FP between sessions (49 ± 15 vs. 46 ± 10 bpm; p = .17). In addition, HRR_PT for MI exercise session was significantly and negatively associated with VO_2max (r = -0.85, p < .05).

Conclusion: The method herein presented was sensitive to exercise intensity, and partially responsive to aerobic fitness. Next studies should perform the pharmacological and clinical validations of the method.