LF/HF ratio

The idea behind the LF/HF ratio was neat. If LF power LF power 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 came from the sympathetic sympathetic Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system (“fight or flight”) system and HF power HF power HF power 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. More about HF power from the parasympathetic parasympathetic Parasympathetic nervous system The "rest and digest" branch of the autonomic nervous system. It slows the heart, mainly through the vagus nerve, and drives most of the beat-to-beat variation in HRV. More about Parasympathetic nervous system (“rest and digest”) system, dividing one by the other would show which side was winning. A high ratio would mean stress; a low ratio would mean calm. The phrase “sympathovagal balance” comes from this idea.

The first half of that assumption does not hold. LF power is not a pure index of sympathetic drive: at rest it mostly reflects the baroreflex, and parasympathetic activity contributes a large share of it. In a 2013 review, the physiologist George Billman concluded that the LF/HF ratio cannot accurately quantify sympathovagal balance, in health or in disease. Breathing adds another problem. Slow breathing pushes the breathing rhythm into the LF band, which raises the ratio even when you are relaxed.

So treat any LF/HF number with care. A rise does not show that you are stressed, and a fall does not show that you have recovered. If an app or report leans on it, look at RMSSD 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 or HF power instead, which have a clearer link to vagal activity 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 .

The lesson The nervous system behind HRV explains how the two branches interact, and What does unbalanced HRV status mean? covers a different kind of “balance” label you may see in apps.