The mean interval time (ms) between heart beats (IBImean or RRmean) is represented for Test (A), ReTest (B) and RF-RFE comparison (C). “RF” represents the optimal resonance frequency, “Other Rates” represents the mean of the other breathing rates, “Lowest RSA” represents the lowest RSA amplitude, “6 b/m” represents the rate of 6 breathes per minute, “Free” represents the free slow breathing session only in retest, and “RFE” represents the expanded RF that consists on taking the IBImean of the nearest below and above rates from the RF. (Non-parametric Wilcoxon test has been performed when Friedman test showed significant differences.). * (p < 0.05) and ** (p < 0.01) mean differences with respect to RF in the same session. ## (p < 0.01) means differences with respect to Free Respiration in the same session.