Glossary
HF power
Also called HF, high-frequency power, high frequency band
The share of heart rate variability in fast rhythms of 0.15 to 0.40 Hz, the range of normal breathing. It mainly reflects vagal activity when breathing is 9 to 24 breaths a minute.
When software breaks HRV into rhythms of different speeds, HF power is the variation in the high-frequency band, 0.15 to 0.40 Hz. That matches a breathing rate of 9 to 24 breaths a minute, which is why HF is also called the respiratory band. It is usually reported in ms².
Most of the HF rhythm is respiratory sinus arrhythmia Respiratory sinus arrhythmia The normal rise in heart rate as you breathe in and fall as you breathe out. It is driven by the vagus nerve and makes up much of resting HRV. More about Respiratory sinus arrhythmia : your heart speeds up a little as you breathe in and slows as you breathe out. The vagus nerve Vagus nerve The main nerve of the parasympathetic nervous system, running from the brainstem to the heart, lungs and gut. Its fast signals to the heart drive most beat-to-beat HRV. More about Vagus nerve drives this. The Task Force standards describe vagal activity Vagal tone The ongoing influence of the vagus nerve on the heart, which slows it down. It is usually estimated from HRV measures such as RMSSD and HF power. More about Vagal tone as a major contributor to HF power, based on experiments that blocked or stimulated the vagus nerve. Researchers use HF power, along with RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next. It mainly reflects vagal activity. More about RMSSD , as an index of vagal tone.
The index only holds while you breathe inside that 9 to 24 range. Breathe slower, as in a 6-breaths-a-minute exercise, and the breathing rhythm moves into the LF LF power The share of heart rate variability in slow rhythms of 0.04 to 0.15 Hz, about one cycle every 7 to 25 seconds. Once read as sympathetic activity, which is now disputed. More about LF power band, so HF power stops tracking vagal activity well. Deeper breaths also make the rhythm bigger. RMSSD is less affected by breathing rate, which is one reason researchers often prefer it.
HF power needs at least about 1 minute of recording, and 5 minutes is the research standard. The lesson RMSSD, SDNN and lnRMSSD explained shows how HF relates to the time-domain numbers apps report.