# HF/LF ratio (HRV frequency-domain)

Frequency-domain HRV analysis breaks your heartbeat-interval signal into frequency bands. The high-frequency band (HF, 0.15 to 0.4 Hz) mostly reflects your vagus nerve's influence (via breathing). The low-frequency band (LF, 0.04 to 0.15 Hz) is a mix of sympathetic and vagal input. Its sympathetic share depends on your breathing rate and posture. The LF/HF ratio (or its reciprocal, HF/LF) was historically read as a measure of 'sympathovagal balance'. But that interpretation is contested. Current HRV guidance, and later work, note two things. LF is not a pure sympathetic marker. And the ratio has limited physiological specificity. HF power and RMSSD remain the better-validated short-term vagal measures. So LF power and the ratio are best treated as supplementary spectral descriptors. They are not reliable readouts of autonomic balance.

## Sources

- Malik M, Bigger JT, Camm AJ, et al.. (1996). Heart rate variability: Standards of measurement, physiological interpretation, and clinical use. Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. European Heart Journal. https://doi.org/10.1093/oxfordjournals.eurheartj.a014868
- Shaffer F, Ginsberg JP. (2017). An Overview of Heart Rate Variability Metrics and Norms. Frontiers in Public Health. https://doi.org/10.3389/fpubh.2017.00258
- Billman GE. (2013). The LF/HF ratio does not accurately measure cardiac sympatho-vagal balance. Frontiers in Physiology. https://doi.org/10.3389/fphys.2013.00026

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