Calculation of approximate entropy (ApEn) requires to select the correct threshold “r”. Previous studies recommended r to be between 0.1 and 0.25 times the signal standard deviation, and now r=0.2 is used in almost all HRV studies. Recently it has been claimed that for fast signal dynamics, r=0.2 may lead to erroneous conclusions, while r maximizing ApEn, r _MAX , correctly assesses entropy. We verified 1) if r _MAX differs from r=0.2 also for HR; and 2) if all r values in the 0.1–0.25 range provide similar ApEn measures. For this aim, we recorded R-R intervals in 10 young subjects for 10’, in supine and sitting positions, and calculated ApEn(r) for r between 0.02 and 1.20, identifying rMAX and ApEn(r _MAX ). r _MAX felt into the recommended range, but it significantly differed from 0.2. At the extremes of the range, the effects of posture change on ApEn were even opposite: ApEn(0.25) decreased while ApEn(0.1) increased. Therefore the choice of r is critical even in HRV studies.