Fig. 1

Data records for a single participant at the three intensity levels. r rest, m moderate, v vigorous. (a) Raw RR intervals. The horizontal dashed line depicts the border between moderate and vigorous intensity (60% of HRR) for this participant: HR = 143 bpm, RR interval = 420 ms. (b) Lomb-Scargle PSD estimates. The vertical dashed lines delineate the ultra-low frequency (ULF), very-low frequency (VLF) and low-frequency bands