High-intensity endurance training increases nocturnal heart rate variability in sedentary participants

The effects of endurance training on endurance performance characteristics and cardiac autonomic modulation during night sleep were investigated during two 4-week training periods. After the first 4-week training period (3 x 40 min per week, at 75% of HRR) the subjects were divided into HIGH group (n = 7), who performed three high-intensity endurance training sessions per week; and CONTROL group (n = 8) who did not change their training. An incremental treadmill test was performed before and after the two 4-weektraining periods. Furthermore, nocturnal RR-intervals were recorded after each training day. In the second 4-weektraining period HIGH group increased their V0Zmax (P = 0.005) more than CONTROL group. At the same time, nocturnal HR decreased (P = 0.039) and high-frequency power (HFP) increased (P = 0.003) in HIGH group while no changes were observed in CONTROL group. Furthermore, a correlation was observed between the changes in nocturnal HFP and changes in V0Zmax during the second 4-week training period (r = 0.90, P < 0.001). The present study showed that the increased HFP is related to improved VO2max in sedentary subjects suggesting that nocturnal HFP can provide a useful method in monitoring individual responses to endurance training.