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 ms² and normalized units (nu)] and the status without and with fatigue. The variables not distributed normally were transformed by taking their common logarithm (log₁₀).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': HR_SU (+6.27±0.61 bpm), logTP_SU (-0.36±0.04), logLF_SU (-0.27±0.04), logHF_SU (-0.46±0.05), logLF/HF_SU (+0.19±0.03), HF_SU(nu) (-9.55±1.33). Differences were also significant (P<0.0001) in standing: HR_ST (+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.