Fig. 3

Correlation analysis between fatigue severity perception and HRV indices among the participants. Total FIS-40 score significantly correlated with a the mean RR, b the RMSSD, and c the HFnu among CFS/ME patients (black squares, upper regression lines) but not for those healthy controls (white circles, bottom regression lines). Significant correlations were found between HRV indices and presence of perceived fatigue among CFS/ME individuals (p < 0.01 for mean RR and HFnu, and p < 0.05 for RMSSD), but not in healthy controls