Time course of adaptation in heart rate variability and respiratory sinus arrhythmia to intensive endurance training

The weekly time course of adaptation of Heart rate variability (HRV) and respiratory sinus arrhythmia (RSA) characteristics were examined over 4-weeks of intensive endurance training in 17 sedentary subjects (35.8 +/- 10.0 yrs). Five-minute ECG and breathing recordings were obtained at rest at spontaneous breathing (SP) and at paced breathing rates of SP+4 (M4) and SP-4 (P4) breaths/minute. Spectral power components of HRV were computed from the RR interval sequences; amplitude and phase of RSA were computed from the sinusoid fitted to the instantaneous heart rate within each breath. RSA sensitivity was obtained from the slope of the RSA amplitude versus breathing frequency relationship. Four weeks of training caused significantly increased in VO2max (37.3 +/- 4.4 versus 40.0 +/- 5.4 ml/kg/min; (p < 0.01); a significant decrease in DBP (p < 0.001); a slight decrease in SBP (p < 0.08), and max HR slightly decreased (P < 0.06). None of these variables were changed significantly in control subjects. No change in resting HR was found after training (74 +/- 14 versus 74 +/- 7 bpm). Pre versus post-training results in total and high frequency spectral powers (ms2) of HRV taken at SP were as was RSA amplitude. A significant reverse correlation was found between RSA-amplitude and breathing frequency (r = 0.40). Results suggest a trend towards an increased gain in the amplitude of RSA for varying breathing frequencies after training. In conclusion, four weeks of intensive endurance training may significantly increase maximal aerobic power and provide some health related benefit such as a decrease in diastolic pressure, without concurrent changes in resting heart rate or its modulation by cardiac vagal activity