Heart rate variability dynamics during early recovery after different endurance exercises

Since heart rate variability (HRV) during the first minutes of the recovery after exercise has barely been studied, we wanted to find out HRV dynamics immediately after five different constant-speed exercises. Thirteen sedentary women performed two low-intensity (3,500 m [3,500_LI] and 7,000 m [7,000_LI] at 50% of the velocity of VO_2max [vVO_2max]), two moderate-intensity (3,500 m [3,500_MI] and 7,000 m [7,000_MI] at ∼63% vVO_2max) and one high-intensity (3,500 m at ∼74% vVO_2max [3,500_HI]) exercises on a treadmill. HRV was analyzed with short-time Fourier transform method during the 30-min recovery. High frequency power (HFP) was for the first time higher than at the end of the exercise after the first minute of the recovery (3,500_LI and 7,000_LI, P < 0.001), after the fourth (3,500_MI, P < 0.05) and the fifth (7,000_MI, P < 0.05) minute of the recovery and at the end of the 30-min recovery (3,500_HI, P < 0.01). There were no differences in HRV between 3,500_LI and 7,000_LI or between 3,500_MI and 7,000_MI during the recovery. The levels of HFP and TP were higher during the whole recovery after 3,500_LI compared to 3,500_MI and 3,500_HI. We found increased HFP, presumably caused by vagal reactivation, during the first 5 min of the recovery after each exercise, except for 3,500_HI. The increased intensity of the exercise resulted in slower recovery of HFP as well as lower levels of HFP and TP when compared to low-intensity exercise. Instead, the doubled running distance had no influence on HRV recovery.