LF power

Frequency-domain analysis splits the ups and downs in your heartbeat intervals into rhythms of different speeds, a bit like separating the bass and treble in music. LF power is the amount of variation in the low-frequency band, from 0.04 to 0.15 Hz. Those are slow waves that repeat roughly every 7 to 25 seconds. It is usually reported in ms² and needs at least 2 minutes of recording.

For years, LF power was treated as a marker of 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”) nervous system. Later work does not support that. At rest, LF mainly reflects the baroreflex, the loop that adjusts heart rate to keep blood pressure steady, and it carries both sympathetic and 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 influences. Billman’s 2013 review concluded that LF power does not index sympathetic drive to the heart.

Breathing complicates things further. Breathing slower than about 9 breaths a minute moves the breathing-linked rhythm out of the HF HF 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 band and into the LF band. That is why slow-breathing exercises at around 6 breaths a minute raise LF power. The rise comes from your breathing pattern and does not signal stress.

The lesson RMSSD, SDNN and lnRMSSD explained explains how frequency measures relate to the simpler numbers.