HF/LF ratio (HRV frequency-domain)

Frequency-domain HRV analysis decomposes the beat-to-beat interval spectrum into bands: high frequency (HF, 0.15–0.4 Hz) primarily reflects respiratory sinus arrhythmia driven by vagal modulation, while low frequency (LF, 0.04–0.15 Hz) has mixed sympathetic and vagal contributions, with the precise sympathetic share dependent on respiratory rate and posture. The LF/HF ratio (with HF/LF being the reciprocal form) was historically used as an index of sympathovagal balance, but this interpretation is contested; current HRV Task Force guidance and subsequent work note that LF is not a pure sympathetic marker and that the ratio has limited physiological specificity. HF power and RMSSD remain the more validated short-term vagal indices, while LF power and the ratio are best treated as supplementary spectral descriptors rather than reliable autonomic balance readouts.