Heart rate variability: response following a single bout of interval training

We investigated the effect of exercise on heart rate variability by analysing the heart rate power spectrum prior to, and 1 and 72 h following, an interval training session. Subjects initially performed a graded test to exhaustion to determine maximal oxygen uptake (VO(2) max) and the running speed at which VO(2) max was first attained (vVO(2) max). The training session was completed on a separate day and comprised six 800 m runs at 1 km x h (-1) below vVO(2) max. Prior to the training session (pre), 1 h following the training session (+ 1h), and 72 h following the training session (+ 72 h), subjects sat quietly in the laboratory for 20 min whilst breathing frequency was maintained at 12 breath x min (-1). Cardiac cycle R-R interval data were collected over the final 5 min of each 20 min period and analysed by means of autoregressive power spectral analysis to determine the high frequency (HF) and low frequency (LF) components of heart rate variability. Heart rate was higher, and the standard deviation of the R-R intervals was lower, at + 1 h than for pre or + 72 h (p < 0.05). The HF and the LF components of heart rate variability were also lower (p < 0.05) for + 1 h than for pre or + 72 h when the data were expressed in ms(2). However, no changes in the LF:HF ratio were observed, and the changes in the HF and LF components disappeared when the data were expressed as a fraction of the total power. Whilst these findings illustrate the importance of controlling the timing of exercise prior to the determination of heart rate variability, the time course of the post-exercise heart rate variability response remains to be quantified.