Does exercise training improve heart rate variability (RR interval and HF power)?
Exercise training increases RR interval and HF power, but the mechanisms of training bradycardia likely involve factors beyond just increased vagal modulation.
Exercise training results in significant increases in RR interval and HF power. These changes are influenced by study population age. The smaller effect size for HF and weak relationship between HF and RR interval suggest factors additional to increased vagal modulation are responsible for training bradycardia.
Sandercock et al. (2005) studied this question.