Correlates of Cardiac Autonomic Control in High-Fit Endurance Athletes

PURPOSE: To examine the correlates of cardiac autonomic control, assessed by heart rate variability (HRV) and heart rate recovery (HRR), in a group of high-fit endurance athletes. METHODS: Nineteen endurance athletes participated in this study (age: 29.8±6 yrs; 8 males) and had maximal oxygen consumptions (VO2max) that placed them above the 99th percentile based on American College of Sports Medicine normative values (VO2max: 67.1±2 ml/kg/min). Participants were assessed for VO2max via a graded exercise test (GXT), percent body fat (BF), weekly physical activity (PA), resting HRV, and HRR. HRV indices were obtained from a 5-minute EKG collected during supine rest and were reported in the time domain as the standard deviation of RR intervals (SDNN) and percentage of adjacent RR interval differences >50 ms (pNN50). Spectral power in the total (TP; 0.00-0.4 Hz), low- (LF; 0.04-0.15 Hz), and high- (HF; 0.15-0.40 Hz) frequency power ranges was derived via an FFT and were transformed using the natural log (ln). Nonlinear indices of HRV were reported as Poincare plots (SD1 and SD2) and sample entropy (SampEn). HRR, a marker of vagal reactivation, was reported as the reduction in HR from peak exercise to the HR one minute after the cessation of the GXT. Pearson's correlation coefficients were reported between autonomic and physiological variables and were considered significant at the 0.05 level. RESULTS: Participants exhibited a maximal HR of 186.9±3 bpm, a resting HR of 49.7±2 bpm, a BF of 11.2±1%, and reported a PA of 6558.1±658 kcals/week. Means of the autonomic variables were as follows: SDNN (89.2±11ms), pNN50 (46.0±6%), lnTP (8.7±0.2ms2), lnLF (7.2±0.2 ms2), lnHF (7.3±0.3 ms2), LF/HF (1.4±0.3), SD1 (60.9±10ms), SD2 (108.8±13ms), SampEn (1.6±0.1), and HRR (30.8±3bpm). There were significant correlations between age and lnHF (r= -0.46), LF/HF (r= 0.53), SampEn (r= -0.49), and maximal heart rate (r= -0.65). VO2max was only significantly correlated with PA (r= 0.54) and HRR (r= 0.50). CONCLUSION: These data suggest that the relationship between age and HRV is evident in high-fit endurance athletes, whereas the relationship between fitness and HRV is not. The maintained relationship between HRR and VO2max suggests that HRR may be a better marker of fitness-related differences in autonomic control in this population.