The Fast Fourier Transformation (FFT) and spectral methods let quantify components of the HRV, by classifying the frequencies in three regions: High (HF= 0.15-0.4 Hz), Low (LF= 0.04-0.15 Hz) and Very-Low (VLF= 0.003-0.04 Hz) frequency powers. The most valuable zone is the HF because is modulated exclusively by the SNP. The LF is regulated by both, the PNS and SNS (Sympathetic Nervous System). And the VLF modulation is unclear, but it is though that could be influenced by the PNS and renin-angiotensin-aldosterone system .