Fatigue Shifts and Scatters Heart Rate Variability in Elite Endurance Athletes

Purpose: This longitudinal study aimed at comparing heart rate variability (HRV) in elite athletes identified either in 'fatigue' or in 'no-fatigue' state in 'real life' conditions. Methods: 57 elite Nordic-skiers were surveyed over 4 years. R-R intervals were recorded supine (SU) and standing (ST). A fatigue state was quoted with a validated questionnaire. A multilevel linear regression model was used to analyze relationships between heart rate (HR) and HRV descriptors [total spectral power (TP), power in low (LF) and high frequency (HF) ranges expressed in ms(2) and normalized units (nu)] and the status without and with fatigue. The variables not distributed normally were transformed by taking their common logarithm (log(10)). Results: 172 trials were identified as in a 'fatigue' and 891 as in 'no-fatigue' state. All supine HR and HRV parameters (Beta +/- SE) were significantly different (P < 0.0001) between 'fatigue' and 'no-fatigue': HRSU (+6.27 +/- 0.61 bpm), logTP(SU) (-0.36 +/- 0.04), logLF(SU) (-0.27 +/- 0.04), logHF(SU) (-0.46 +/- 0.05), logLF/HFSU (+0.19 +/- 0.03), HFSU(nu) (-9.55 +/- 1.33). Differences were also significant (P < 0.0001) in standing: HRST (+8.83 +/- 0.89), logTP(ST) (-0.28 +/- 0.03), logLF(ST) (-0.29 +/- 0.03), logHF(ST) (-0.32 +/- 0.04). Also, intra-individual variance of HRV parameters was larger (P < 0.05) in the 'fatigue' state (logTP(SU): 0.26 vs. 0.07, logLF(SU): 0.28 vs. 0.11, logHF(SU): 0.32 vs. 0.08, logTP(ST): 0.13 vs. 0.07, logLF(ST): 0.16 vs. 0.07, logHF(ST): 0.25 vs. 0.14). Conclusion: HRV was significantly lower in ' fatigue' vs. ' no-fatigue' but accompanied with larger intra-individual variance of HRV parameters in ' fatigue'. The broader intra-individual variance of HRV parameters might encompass different changes from no-fatigue state, possibly reflecting different fatigue-induced alterations of HRV pattern.